— This article by Jerry Cates, first published on 12 September 2010, was revised last on 12 August 2013. © Bugsinthenews Vol. 11:09(01).
- The following exposition covers an infestation of non-biting midges and tetragnathid spiders at Walden Marina, on Lake Conroe, in southeast Texas. Populations of these concomitant organisms at many Texas lakes have been on a steady incline for at least a decade; in August 2007, for example, the simultaneous bloom of chironomid midges and tetragnathid spiders produced a gigantic spider web along a section of Lake Tawakoni State Park, at Wills Point in northeast Texas, that attracted worldwide scientific and media attention (Quinn & Garde, 2007). Two years later and some 200 miles to the south at Lake Conroe and Lake Houston, populations of these organisms — which had been increasing each year since at least 1997 — reached unbearable proportions. That’s when Allison Harpold, the manager of Walden Marina, contacted me to see if her midge/spider infestation could be resolved without applying toxic pesticides. As a charter member of the Certified Clean Texas Marina program, she insisted on following not just the letter, but the spirit proclaimed by that program.
- This report describes the midges and spiders that were involved, the initial palliative measures that were taken until a workable, non-toxic solution could be developed, and the ultimate approach that is slowly, but successfully, returning Walden Marina to the pristine condition that existed before the midge population began its steady incline. Today, at Walden Marina only localized pockets of the midge and spider infestation observed in 2009 remain, while along the surrounding shoreline of Lake Conroe populations of both organisms continue to soar unabated. Most important — as Allison insisted — not a single drop of toxic pesticide has been applied at Walden Marina in the process.
It is difficult to imagine a more idyllic setting.
Before you stretch the placid waters of Lake Conroe, a large, inland, freshwater lake in southeast Texas. It is a warm, sunny day, and the breeze smells sweet. You’re ready — to the point of excited impatience — to take your watercraft out for a spin on the water.
To you, the waterways are more than a source of recreation; they’re also a precious natural resource. Like most boaters, you do your part to protect them, and you knew that Allison Harpold, the conscientious, hard-working manager at Walden Marina, does everything in her power to make it as clean and green as possible.
The honors she’s garnered over the years for Walden Marina, as a long-standing, charter Certified Clean Texas Marina, testify to her competence. Over the years she has managed to win the coveted Clean Texas Marina of the Year award several times.
In fact, in 2011, she won that award for the fourth time, a record that no other Texas marina can boast. Then, in 2012, she won it for the fifth time, adding to her laurels. Allison engages firms like EntomoBiotics Inc. to help keep Walden Marina as pesticide free as humanly possible, a factor that 2011 Marina Association of Texas president Jodi Looper, of Lakeway Marina, and Clean Texas Marina project chairman Dewayne Hollin, of Texas A&M University, took into account when making the award.
In the photo at left, Allison’s son (middle) accepts the award from Jodi Looper, MAT President (right) and Dewayne Hollin, CTM Chair, Texas A&M University (left) at the annual Marina Association of Texas banquet. Allison could not attend, as the low lake levels produced by the drought of 2011 — which wreaked havoc at all of the lakes and waterways in Texas during 2011 — had created such monumental challenges at the marina that she had to stay behind to deal with them.
So, here you are, at Walden Marina… It’s a great feeling to know you, your boat, and this lake are in such good hands. Seriously, one poorly managed marina can produce a lot of pollution in a year’s time. Imagine what a few decades can cause…
But not here. Not on Allison Harpold’s watch.
A few days before, you’d made a special trip to check your boat’s hull and superstructure, tidy up your part of the dock, and stock up on provisions. Now you look forward to enjoying the fruits of those labors.
As you approach the slip where your boat is berthed, you notice that things have changed since your last visit. It’s hard to believe it — you were here just a few days ago — but thick spider webs are everywhere. Not only that, but a huge bunch of annoying flying insects swarm like a cloud around your boat and your face.
You raise your boat out of the water for a closer inspection of the hull, and find streamers of gooey, snot-like gel hanging from parts that had been underwater.
While daydreaming a little about your upcoming stint on the lake, you hadn’t paid much attention to the dock and the other boats on the way down. Now you look around, and discover that all the other boats are just like yours. Covered with flies and spiders.
Immediately you realize what has happened. Since your last visit the cold days of late winter and early spring have gone, only to be replaced by balmy breezes spawned by a warm, summer-like sun. Along with that transition have come a few unwelcome changes in the local fauna: The lake flies are back, and this year they’ve come back early…
Depending on where you live, you might call these insects sand flies, muckleheads, muffleheads, blind mosquitoes, or chizzywinks — if you call them anything at all. Regardless, if they don’t bite, and under close scrutiny some (the males) are seen to have plume-like antennae, they’re one or more species of non-biting midges in the Chironomidae family.
Scientists generally refer to them as chironomids, as though the moniker refers to a very specific kind of distinctive animal. In a way, it does, as the various species are all so much alike that many cannot be distinguished solely by their outward appearances. Still, worldwide some 5,000 species of these flies have been described to date, 700 or so unique to North America. Most likely, as with similar infestations elsewhere, a number of species of chironomids are represented at Walden Marina. Along with them, seemingly overnight as well, have come several species of spiders.
One genus of spiders (Tetragnatha) in particular, the longjawed orb weavers of the Tetragnathidae family, seems determined to enmesh everything in sight in the webbing that the spiders found here are busily spinning. These spiders thrive on the abundant midges, chowing down with gusto on every fly that gets caught in their webs. They mate early and often, and lay eggs in specially constructed casings plastered to boats, posts, canvas coverings, and anything else that lies still for the indignity. In that sequence, they quickly blossom into huge, semi-social spider colonies housed within the protective confines of expansive, unkempt silken snares.
Their webs are so large, and are woven with such abandon, that they threaten to wrap even you into their domains if you hesitate in one spot for even a moment.
You think this as you flick a spider off your clothing and unwind a long sash of webbing from your midsection.
Gad… they’re everywhere!
These flies and the spiders that feed on them can get out of hand quickly. You saw it happen last year, and it wasn’t pretty. The underlying cause — the water conditions that spawn these flies and, as a consequence, attract so many spiders — isn’t any prettier. As you think this, you peer into the water around your boat and confirm with your eyes what your mind already expected; the water is brackish, glazed with a surface layer of dead flies, egg masses, and a bunch of other, indescribable gunk.
Chironomid midges result from lake pollution of one kind or another, the kind that results in brown-colored, turbid water. Deep below the water surface, at the lake floor itself, they require a nutrient-rich muck to hatch into and feed on while developing as larvae, and to pupate in while awaiting the changes that transform their bodies from larvae to adult flies.
Often the source of lake water pollution is strictly a function of ordinary, natural events that don’t involve humans. Mother Nature is not above polluting her own waterways, under certain circumstances, often with a seeming careless insensitivity that overshadows the pollution caused by man. It is a well-know fact that some of the most severe chironomid fly infestations in the world are endemic to what might well be described as “pristine” lakes. Pristine, because they are located in the middle of perfectly natural settings, unsullied by human habitations and industry, where only native animals, birds, and marine fauna live, breed, and — yes — pollute. A good example is Midge Lake, on Livingston Island, Antarctica, which — along with a number of similar lakes on the island — is home to some of the largest midge infestations ever recorded on earth, yet lies thousands of miles from the nearest human-caused pollution sources (Toro, et al. 2006).
In other locales, though, humans easily — and often unwittingly — have an important, if not overarching part in helping these midges thrive by contributing to natural pollution sources and making things worse. Everything used on the water — or even within miles of the water, where rain-water runoff washes everything that floats or dissolves into creeks and streams that carry them on to the lake — used to wash, paint, and maintain our automobiles, lawns, homes, boats and docks should be considered a potential water pollutant.
It takes serious thought and conscientious effort to make sure nothing you do creates conditions that are hurtful to the water, the myriad forms of wildlife that depend on it, or your fellow boaters. Pesticides, in particular, around lakes and other bodies of water almost always cause more problems than they solve, and tend — in almost every case we have examined — to make bad situations worse.
Because Allison reminds every boater at Walden Marina regularly about their pledge not to use pollutants here, the source of the pollution these chironomids thrive on is almost certainly not the marina itself. Chances are the original source was something as seemingly innocent as the chemical fertilizers used to create and maintain the beautiful, emerald green lawns at the residential homes and apartment complexes ringing the marina’s northwestern perimeter.
Only a thin layer of nutrient-rich matter, easily produced by fertilizers and similar chemicals that are washed into the marina by a few heavy rains, is needed to start a chironomid midge population going. They initially concentrate their populations where the shadows of the marina’s docks and slips block the streams of cleansing sunlight from reaching the lake floor. This allows the nutrient-rich muck to thicken even more, while the growing chironimid population’s life-cycle activities, and those of their natural predators — the tetragnathid spiders — start to turn the water a deepening brown that eventually blocks the sunlight even in the open water. Once that state is reached, the chironomids can develop anywhere on the lake floor.
As the midge population develops, its life stages create its own pollution. When introduced into the water, millions of jelly-like egg masses, midge larvae, pupal casings, and bodies of expired adults, provide all the pollutants needed to support a life cycle that is destined to explode upward toward more and more midges as the years go by. It may take years for midge populations to surge from their small beginnings to the point that they become major polluters of the waterways they spring from, but once the cycle gets started, it slowly, but surely, spirals out of control.
By September of 2009 the midge population at Walden Marina had reached the breaking point.
So, from a biological perspective, what are these lake flies? And what kinds of spiders do their infestations tend to attract, at least in the subtropical climes of Texas? Why are they associated with our freshwater and coastal lakes? How should they be dealt with, not only to relieve boat-owners, swimmers, and fishing enthusiasts of their annoying presence, but in ways that avoid adding to the pollution of the environment, or harming the fish and other aquatic wildlife, not to mention the water, at such locations?
These and other questions will be discussed below. This narrative and its accompanying photos describe how this beautiful marina became the scene of an ugly infestation of chironomid midges and longjawed orb weaver spiders. Details are also provided on the research we carried out to determine what caused the infestation, and the corrective measures that were eventually charted to successfully bring that infestation under control.
Chironomid lake flies are non-biting midges, in the insect order Diptera (Greek δι-, “dye-” = two + -πτερον, “-(puh)TARE-on” = wings, thus “two wings”) a reference to the fact that true flies, such as houseflies, midges, and mosquitoes, have only one pair of wings.
These midges are further classed under the suborder Nematocera (Greek νεμα-, “NEE-ma-” = thread- + -κερας “-SEHR-as” = -horns, a reference to their thin, segmented antennae which, in the males, tend to look like plumes and thus are described as plumose).
Though flies that bite (such as mosquitoes) are almost always important vectors of disease, these lake flies — in the Chironomidae family (from the Greek expression χειρονομεω, “keer-AWN-oh-moh” = ‘I gesticulate,’ a reference to the exaggerated, plumose antennae possessed by some of the males in this family, which are displayed to seduce the females of their species — are non-biting midges. Their larvae hatch from eggs — laid in the water by adult females — that float on the surface for a time before sinking to the lake bottom. On hatching, they first feed on the nutritious gel of the egg mass, and then, on the bottom of the lake, they burrow into the lake floor’s oxygen-poor, nutrition rich muck to feed on sediment nutrients until they are ready to pupate. The pupated fly transitions to the adult form while still buried in the muck, then — once it fully matures inside its pupal casing — breaks out and floats upward to the surface of the water, where — if it doesn’t get gobbled up first by a minnow, a tadpole, or a fish — it emerges as an adult fly.
Both the larvae and pupae of flies in the Chronomidae family serve as food for game fish. Some feed on them while they are still in the muck, and others feed on them while they move toward the surface after emerging as adult flies. In this regard the midges are useful, even critical, members of the freshwater marine ecosystem. Exterminating them from Lake Conroe entirely would not be a good thing. Any control measure adopted at Walden Marina would have to bear that in mind.
Adult flies are not dangerous to humans or other animals, though their swarms can be quite disruptive of human congregations. Their swarms are annoying when the flies buzz around our heads, land on our shirts and trousers, and drop into our drinks and onto exposed food. They are also destructive when they stain walls, boat hulls and covers, and articles of clothing, with fecal excretions. Accumulations of dead midges are known to clog drains, obscure windshields, and mar the surfaces on which they collect. Under certain circumstances they reportedly produce allergic, asthmatic reactions in humans who aspirate them into their lungs or get them into their eyes. Worst of all, they attract the host of previously mentioned predatory spiders that erect nasty sheets of webbing to ensnare and feed on them. And, unlike the midges, the spiders bite. And though their venom isn’t considered deadly to humans, the bite is painful, and the angry, swollen wounds they leave behind are — to say the least — unsightly.
The precipitating cause of the chironomid midge population that began to increase at Lake Conroe in the mid-to-late 1990’s is open to conjecture. Regardless of the underlying cause, those early population increases set the stage for continued development. Over a period of at least ten years of slow but steady growth, the initially small number of midges grew into large swarms, producing a plague of near-Biblical proportions that — each year — generally peaked in the months of August and September.
As the midge populations grew, so did those of their natural predators. All of the solitary spiders in the Araneidae family (the common orb weavers that spin the webs commonly seen in forest, woodland, meadow, and residential yards) tend to be found in larger than usual numbers wherever large numbers of flies can be found. Jumping spiders in the Salticidae family are beneficiaries of large fly populations as well. So long as these solitary spiders are not challenged by large numbers of predatory spiders from other families, they thrive, albeit in densities that fall short of those found in the more rarely found social spiders. The populations of solitary spiders remain at relatively low densities because of their intrinsically competitive natures, which make them fiercely antagonistic toward one another. In close quarters, conflict inevitably results in the death — and consumption — of one by the other.
Certain other spiders, however, particularly certain species of longjawed orb weavers in the family Tetragnathidae, tend to leave one another alone, to the point that they willingly build and share what may be considered communal webs. Often such spiders even cooperate in capturing and consuming prey.
- The genus Tetragnatha derives its name from the Greek word τετρας- “tetras-” = four + -γναθος, “-gnathos” = jaw, which means, of course, “four-jaws,” and thus seems to refer to a Solpugid rather than a spider.
- Because the jaws of tetragnathid spiders are paired like those of all other spiders, the etiology of the word τετραγναθος, as applied by early writers — as early as the 2nd century BC by Aelian, and the 1st century BC by Strabo and Pliny, then much later, by the French arachnologist Latrielle in 1804 — to these spiders in particular, is somewhat obscure; Ubick et al., 2005, p. 323, sheds some light on the mystery, and the reader may wish to consult that resource for more details.
- Tetragnathids sharing communal webs may be able to ensnare more prey than would be possible with less expansive webbing; the presence of several spiders, together, may confer added advantages such as the ability to incapacitate large prey more quickly. In any case, populations of tetragnathids sharing communal nests tend to increase rapidly in the presence of large numbers of flying insects.
- Another spider, the southeastern social cobweb spider (Quinn: Theridiidae: Anelosimus studiosus Hentz 1850) — that is native to a large area stretching from New England to Argentina, is prone to creating large social groupings when abundant prey makes spider sociality propitious. However, though this species was initially suspected of being involved in the spider infestations involving chironomid midges at Lake Tawakoni State Park, assays of the spiders there and at Lake Conroe have not found many of them present at either location.
It is common for the female spiders from most other families outside the Tetragnathidae to fiercely defend their territories. Under ordinary circumstances such pugnacity contributes to survival of the spider, but in the presence of multiple tetragnathids that engage in cooperative attacks on antagonistic species, it becomes a detractor. Solitary, pugnacious orbweavers are soon either devoured or displaced by more sociable spiders, some of whom — though not organically ill-tempered — often bring rather formidable weapons to whatever fight they are invited to. Interestingly, the males of those other families spend much of their time wandering about in the territories of others, seeking mates of their species, which may explain why most of the non-tetragnathid spiders found at Walden Marina tended to be males.
The dynamics that lead to large populations of tetragnathid spiders in lake, pond, and marina settings, are more complicated than this brief exposition suggests, but suffice it to say that tetragnathid spiders and chironomid midges tend to go together, at least in the subtropical climes found throughout much of Texas, along the Gulf coast to Florida, and westward to southern California.
At the height of the seasonal infestation that took place during the summer of 2009, literally millions of newly emergent adult midges took flight each night, just from the waters of the marina itself. Then, besides leaving ugly spots on boats, docks, and furnishings with their fecal matter, they polluted the lake water with their feces, pupal casings, dead bodies, and the huge masses of sticky, jelly-like clumps of eggs laid by female midges after they mated. Large quantities of such impurities were added to the water each day by this process, increasing its turbidity and making it impossible to peer into the water’s depths with any degree of clarity.
Secchi disk measurements of the clarity of the water inside Walden Marina, in September 2009, revealed that sunlight — even on a bright, sunny, cloudless day — could not penetrate beyond a few inches below the water’s surface anywhere in the marina. Sunlight is one of nature’s most important purifying cleansers. The rays of the sun break down pollutants like nothing else, but wherever sunlight cannot penetrate pollutants decay so slowly that they accumulate and produce habitat for organisms, like chironomid midges, that feed on the pollutants and, later, add to it.
The tetragnathid spiders attracted to Walden Marina by the chironomid midges, besides festooning docks, boats, and surfaces of the water with their extensive webbing, produced large numbers of bright white egg cases that were glued to the hulls of boats, the exposed surfaces of the docks, and everything else their webs came into contact with.
Obviously, the need to find a means of eradicating both the lake flies and the spiders became a high priority for Allison.
Eradicating the midges and spiders from this marina was not destined to be an easy task, though. Even the most innocent kinds of pest management here would have to be carried out in consonance with the highest standards of the Clean Texas Marina (CTM) program.
As with all “environmentally friendly” programs, the challenges presented by the situation at Walden Marina were daunting. Fortunately, though, the various parts of the puzzle fit together neatly, so corrections in one area tended to produce corresponding corrections elsewhere.
Chironomid midge infestations are not separate from, but rather the underlying cause of, the tetragnathid spider infestation. Wherever the midge infestation was resolved, the population of tetragnathid spiders immediately dropped below the threshold of annoyance. The ultimate goal, therefore, was to bring the midges under control. Since EntomoBiotics Inc. is a pioneer in the use of bio-rational PestAvoidance methods that eradicate pests through habitat modification rather than with pesticides, we viewed the constraints of the CTM program as a positive good.
Besides charting a long-term solution to the midge infestation, immediate stop-gap measures were instituted to mitigate the effects associated with the tetragnathid spiders. Pesticides being out of the question, we supplied Walden Marina with 5-gallon pails of the non-toxic plant oil based cleansers that we manufacture to be applied to boats and docks to remove spider webbing, egg masses, and fecal spotting. Though these products are not pesticides, their use made conditions more bearable for the boaters at Walden Marina until the core problem — the midges themselves — could be dealt with.
Important modifications were made to the formulas for these natural plant-oil cleansers, first to ensure that they would not mar the plastics and fabrics of the boats, as well as to minimize the amount of d-Limonene the cleansers contained. Eventually all traces of d-Limonene were removed from all our cleansers, making them imminently suitable for use in an aquatic environment. Scientific studies suggest that d-Limonene, in large quantities, besides being toxic to fish and other aquatic organisms, may have an adverse effect on certain aquatic ecosystems due its mediation of water nitrogen levels. Such effects are generally temporary, and involve high concentrations. Though the quantities we used at Walden Marina were never expected to come close to the thresholds cited in those studies, we preferred to err on the side of prudence and eliminate that group of terpenes altogether.
From September through November of 2009 we focused on cleaning up the docks and boats at Walden Marina with a reformulated essential plant oil cleanser (without d-limonene) while carrying out basic research on the biological implications of severe chironomid midge infestations within marina environments. Throughout 2010 our research on these issues continued while, behind the scenes, we held a number of discussions with industry experts, measured the temperature and oxygen levels of the the water and lake floor of the Walden Marina estuary, and documented the marina basin’s bathimetrics.
For a number of months our discussions with industry experts appeared quite positive. Though the remedies these experts recommended were expensive to buy, install, and service, the quoted prices of several remained within Allison’s budgetary limits. It was discouraging, but not surprising, to learn that none of these approaches promised a quick fix to the midge infestation. Much like the cyclic process that preceded it, the decline these methods offered would have to be gradual and measured, provided they worked at all.
But would they work?
Bio-rational control measures targeting chironomid midge populations elsewhere in the U.S., Canada, Australia, and New Zealand, among others, have traditionally sought to disrupt the life cycles of the midge’s larval and pupal stages through the use of one or several mechanical devices. Those devices were developed to enable marina, lake, and pond managers to alter conditions in the bodies of water under their control to make them less attractive and nurturing to midges, eventually reaching the point where the essential ingredients needed for the midges to thrive were no longer present.
Any hope of disrupting the chironomid midge’s life cycle must consider the nutrients, temperatures, and oxygen levels at the lake floor itself. At Walden Marina, that meant, in general, the oxygen levels existing 18-24 feet below the surface of the water. Low oxygen and high nutrient levels at the lake floor offer the highest potential for midge propagation. Traditional eradication methods use mechanical processes that increase the amount of dissolved oxygen, and reduce the quantity of nutrient, at depth, as a first step.
One highly successful approach to disrupting the midge’s life cycle works to circulate the water in order to mix upper levels with lower levels, tending toward a more or less uniform oxygenation level throughout. A second approach artificially oxygenates the water at depth without physically mixing the various water levels, usually by infusing the water with a steady stream of air bubbles, to accomplish much the same objective.
Increasing the oxygen levels at the lake floor leads to the oxidation of nutrients that would otherwise be available to the midges. In the face of such nutrient losses, the larvae are starved before they can pupate. Over time, then, the midge population declines. Being environmentally friendly, non-pesticidal approaches to the midge problem, both methods have received well-deserved acclaim.
But they don’t always work, particularly if certain conditions apply where they are to be used. In other cases, even where they have a fighting chance of working, the necessary energy and equipment required may exceed all bounds of practicality.
Before choosing which of these solutions to recommend to Walden Marina, we first needed to make a few important calculations.
Deciding how many of each of these mechanisms would be needed to achieve the desired life cycle disruptions depended on the size the body of water involved. Walden Marina, being an ancillary part of a much larger body of water (Lake Conroe), any evaluation done here must also take into consideration the manner in which the waters of the two associated bodies mix. Ideally, from the standpoint of these oxygenation devices, Walden Marina should have the character of a closed basin whose deepest water consists of an entirely captive sub-pool that does not mix with the water of Lake Conroe at all. If, contrariwise, all the water in Walden Marina mixes regularly with Lake Conroe, any effort to enrich the oxygen levels of the marina would fail simply because all that oxygenated water would quickly flow out into the much larger lake.
On measuring water temperatures in the marina at one-foot intervals we discovered that, beyond the first few feet of surface water, the temperatures were surprisingly constant. This was a puzzling development. It seemed to imply that none of the the water in the marina is captive, but instead — in its entirety — is a dynamic extension of Lake Conroe proper. To test this hypothesis — with help from one of the marina’s boat owners — we took soundings throughout the marina basin, including the inlet to Lake Conroe. The results confirmed what the uniform water temperatures implied. Walden Marina is blessed with an inlet channel that is as deep as its deepest interior floor. No portion of its estuary is captive, with an internal circulation separate from that of Lake Conroe. That discovery ruled out the use of oxygenators and water circulators as a means of achieving control of local populations of chironomid midges. As one of our consultants put it, “You simply cannot oxygenate the whole of Lake Conroe.”
At first blush, that seemed to leave few options beyond the use of pesticidal toxicants. One of the options that many marinas have found quite attractive involves a system of intermittent misters, fed through tubes emanating from a central mist generator that periodically pressurizes the tubing, ejecting a pesticidal mist from a series of nozzles placed aloft and regularly spaced throughout the marina’s environs. This system works well as a means of exterminating the tetragnathid spiders, but it has little or no effect on the chironomid midges. Several marinas, resorts, and similar venues that use such systems have informed me that, while generally free of spiders, they continue to experience a steady increase in chironomid midges with each passing year.
One other pesticidal option remained — at least provisionally — on the table for Walden Marina. It involved the application of insect growth regulators (IGRs). Though some IGRs are toxic and others are not, the EPA regulates even the non-toxic IGRs under the same basic rubric used to regulate toxic pesticides. Two IGRs presently on the market are known to be effective against insects that exhibit, in their life cycles, the complete metamorphosis that occurs in dipteran organisms such as chironomid midges and mosquitoes.
One of these, the mildly toxic and somewhat exotic IGR pyriproxifen (4-phenoxyphenyl (RS)-2-(2-pyridyloxy)propyl ether 2-[1-(4-phenoxyphenoxy) propan-2-yloxy] pyridine), is extremely effective against both chironomid midges and mosquitoes. However, because pyriproxifen is a toxic carbamate that degrades slowly in water, has the potential to bio-accumulate in aquatic organisms, and is not labeled for applications to waterways, we never considered using it at Walden Marina.
Another, an IGR which happens to be essentially non-toxic to mammals, has a long history of performing well against dipteran organisms. It’s a rather ho-hum non-exotic synthetic terpenoid — specifically an acyclic sesquiterpene — that was initially derived from botanical sources. It has a pleasant, floral fragrance and a well-established mode of action.
This IGR molecule is known as methoprene (11-methoxy-3,7,11-trimethyl-2,4-dodecadienoic acid 1-methylethyl ester), and functions in arthropod biology strictly as a juvenile growth hormone analog. It is not otherwise toxic, rapidly biodegrades in water, and does not bio-accumulate in aquatic organisms.
Furthermore, its manufacturer, Wellmark International, markets methoprene under the brand name Altosid® for use in bodies of water, primarily against mosquitoes. This relatively expensive, but efficacious formulation binds the methoprene, in a proprietary process, to heavy granules. When released in water, the granules carry it downward, directly to the lake floor. There, in a slow-release process, the methoprene acts to prevent dipteran larvae from transitioning from larva to adult. As such, methoprene is widely touted as being a safe, effective means of protecting large populations of humans from mosquito-borne diseases such as West Nile Virus.
After considerable research into the utility and safety of methoprene when applied as Altosid® granules for mosquito control, an experiment was carried out with it in March 2011 at Walden Marina. Though in earlier years mosquitoes and midges did not begin to emerge from Lake Conroe before the month of May, in 2011 we found large numbers of adult flies at Walden Marina in early March. This suggested that, unless stringent efforts were taken early that year, the marina faced an onslaught of these pests in greater proportions than in previous years. Accordingly, that month, the first methoprene application was conducted. Soon afterward, the populations of these dipterans noticeably declined.
An April 2011 survey of the docks at Walden Marina observed small swarms of live chironomid midges at three isolated locations. Elsewhere they were found in small numbers if a thorough search was carried out, but nowhere — beyond the three locations noted above — were they abundant. A second application of methoprene IGR was carried out that month, with emphasis on the locations where the isolated swarms had been found previously. However, only half the total amount of methoprene was applied in comparison with the application carried out the previous month, in consonance with our commitment to using the least amount necessary to meet the Marina’s objectives.
A subsequent survey of Walden Marina, conducted in May 2011, found isolated specimens of chironomid midges. No swarms were found anywhere on the docks or anywhere along the shoreline. Very few tetragnathid spiders were observed as well, indicating that these arachnids were not catching abundant prey. Their webs were clear of dipteran carcasses, supporting that conclusion. A third application of methoprene was carried out, reducing the total amount applied to half the amount applied in the month of April.
Once the mosquitoes and chironomid midges stopped emerging from the waters of Walden Marina in large numbers on a nightly basis, the clarity of the water improved remarkably. This was partly unexpected, as a number of long-time residents at the marina had expressed the opinion that the lake water circulating from Lake Conroe itself was perennially turbid and had been so for as many years as they could remember.
However, during the survey conducted in May 2011 one resident pointed out that, for the first time in the past six years, he now could clearly see the underwater light that is submerged about ten feet below the surface of the dock where his boat is berthed. He recalled that when he first arrived at the marina several years before that, the light had been perfectly visible. A few years later it could no longer be seen, though at night a dull glow betrayed the fact that it was still there.
Our research at Walden Marina was discontinued at the end of 2011. The assistance provided by EntomoBiotics Inc., was geared toward helping Allison Harpold maintain Walden Marina in full compliance with the Clean Texas Marina program. It accomplished that objective, but at a high cost that eventually made it too expensive to continue. .
Had it been possible to continue this program, we would have focused on the optimal mix and quantity of safe and effective habitat modification procedures and materials needed to keep the chironomid midges at Walden Marina under control. Execution of control protocols must, however, avoid disrupting the ecosystem in the marina and Lake Conroe in a negative way. Lake flies provide food for game fish. Elimination of all the midges on the periphery of Walden Marina is inconsistent with good environmental stewardship, though total cessation of midge activity within the marina itself is a worthwhile, environmentally sound, and — judging from our experiences in 2011 — a realistically achievable objective.
The Itch Effect
One of the lessons learned during the 2011 phase of this project was that eliminating 90-95% of the chironomid midges at Walden Marina is unacceptable. Total elimination within the marina itself is mandatory. Because of what we have come to call the Itch Effect, allowing even 5% of the midge populations of past years to remain elicits a severe, and unacceptable, annoyance response in boat owners who have experienced swarms of these midges in the past.
Think how it would be if you had a serious, chronic itch of some months’ or years’ duration:
Question: What good would it do to reduce that itch by 95%?
Answer: very little.
It all has to go, or the little that remains will remind you so much of the old, chronic itch that you don’t feel much better. On the way up, things are not so bad (the famous “Boiling the Frog” effect) until the breaking point is reached. On the way down, after crossing back over the breaking point, it all has to go.
What the Itch Effect means is that an extended chironomid midge abatement program at Walden Marina would have to be even more aggressive in the future than it had been in the past. For reasons explained below, however, that cannot mean increasing the amount of methoprene applied here. Our challenge, instead, was to find new ways to modify the underwater habitat at Walden Marina, despite the realization that no part of the marina basin is captive, while simultaneously avoiding pesticidal products of all kinds, including all non-toxic juvenile hormone analogues.
The Postulated Methoprene/Methyl Farnesoate Connection
The methoprene molecule — an acyclic sesquiterpene analogous to the dipteran juvenile hormone JHB3 (a.k.a. JH III bisepoxide) — though non-toxic to humans and other mammals, has the potential to affect non-target aquatic organisms. It has been found, for example, that methoprene is chemically similar to the crustacean juvenile hormone, methyl farnesoate, which is secreted by certain species of lobsters and crayfish.
The crayfish species Procambarus clarkii Girard 1852 is widespread in this region. We have not found it in the waters at Walden Marina, where its usual habitat preferences are lacking, but it seems likely to be close by, plying the bottoms of nearby streams, creeks, and ponds. While famous as a crucial ingredient in Cajun cuisine, P. clarkii is often considered an invasive pest, is nowhere regarded as endangered, and practically everywhere is infamous for wreaking havoc on the habitat it occupies. For example, its burrows damage water courses and rice crops, and its voracious feeding antics disrupt native ecosystems. Worse, it is a known vector for infectious fungal, viral, and helminthic agents, including the crayfish plague fungus Aphanomyces astaci, the crayfish virus vibriosis, and a long list of parasitic worms that attack vertebrates, including man.
Regardless of this litany of negatives, P. clarkii remains a prized food item for some people. And, by happenstance, it is also known to secrete the juvenile hormone methyl farnesoate as part of its molting process. That chemical, while similar to methoprene, differs from it in lacking an epoxide group (a cyclic ether with three ring atoms). Many authorities assert that this disparity, by itself, is enough to prevent methoprene from posing a risk to crustaceans, like P. clarkii, that are exposed to it. Some (e.g., Felterman & Zou, 2011) have even concluded that, for many if not most crustaceans, the presence of exogenous methyl farnesoate in the environment does not negatively impact their development.
A thorough review of extant literature suggests, however, that a measurable potentiality of such a risk, while evidently quite low, still remains. As mentioned above, that risk, even if confirmed, has no effect on our work at Walden Marina, as no crustaceans have been observed there. Still, the program we carried out at Walden Marina was intended to be applicable to as many other locales as possible, including those with thriving populations of crustaceans. That, coupled with the imperative of erring on the side of caution, made the postulated methoprene/methyl farnesoate connection problematic.
Accordingly, we embarked on a serious research and development program to substitute the use of methoprene with more strenuous mechanical methods of habitat modification, in concert with the insertion of non-toxic, essential plant oils, surgically introduced in ultra-low concentrations into the beds of lakes and watercourses. This approach requires the development of proprietary substrates that allow us to target limited portions of the lake or watercourse bed directly, without actually introducing the plant oils into the water itself. In the process of cleansing the underwater floor, this approach will have no impact whatever on the water that covers it or the nearby, but un-touched, lake or watercourse bed. In this manner it will enable us to produce — in select portions of the lake or watercourse bed itself — an environment that neither nurtures or attracts pollutant-related organisms, such as chironomid midges, that thrive by subsisting on pollution-mediated nutrients. And it does so precisely as other ordinary cleansers and soaps do, wherever they are used, but without introducing foreign chemicals or pollutants into the water itself:
- Unlike most other cleansers and soaps, the proprietary architecture of the micro-cleansers being transported, and the substrates that transport them, ensure that the intended cleansing action is strictly limited to the microhabitat into which it is placed. There it metabolizes, disperses, oxidizes, and/or washes away the pollutant-mediated nutrients with which it comes into contact. These micro-cleansers are not pesticides; they do nothing more than cleanse the micro-habitat into which they are introduced, without exterminating, repelling, or mitigating the organisms occupying that habitat.
As we’ve observed in the process of the 2009-2011 experiment carried out at Walden Marina, the positive effects produced by this micro-cleansing action extended far beyond the micro-habitat itself, and included such things as reducing overall water turbidity, eliminating the flotsam associated with the aquaculture of chironomid midges, and eliminating the jetsam produced by the latter’s natural predators, the tetragnathid spiders.
Droughts and Other Challenges
Conditions at Lake Conroe — which was built in 1973 on the West Fork of the San Jacinto River as a reserve water reservoir for the Houston metropolitan area — are not static, either from month to month, year to year, or even decade to decade. At no time has that been more obvious than during the 2009-2011 drought, during which time all of Texas, including the host watershed for every drop of water that flows into Lake Conroe, suffered under drought conditions more dire than ever before recorded in history.
The two side-by-side photos of the Walden Marina dock ramp, shown above, were taken on 7 October (left) and 9 November (right) of 2011. Ordinarily, all of “land” shown in the foreground should be under at least four feet of water, but the continuing drought has brought lake levels down, exposing more and more of the lake bed. Notice how much more of the lake bed has been exposed in the span of only one month and two days.
The photos directly at left, of the lake water depth gauge, were taken on those same dates as the photos of the dock ramp. They reveal the rest of the story. On 7 October lake levels were down about 6½ feet from normal, but by 9 November the lake had nearly dropped to almost 8 feet below normal.
Walden Marina is blessed with the deepest marina on Lake Conroe. That’s a good thing, because — even with an 8 ft. drop in lake levels — all of the water vessels docked here could still navigate its waters from the slips to the lake. Lake Conroe, itself stood at 67% of capacity as of 11 November (NOAA Drought Information Statement 11-11-2011). Meanwhile, rainfall records for the city of Conroe were 35 inches below normal, and though some rainfall was expected from isolated thunderstorm activity, the long-range forecast through January of 2012 was not promising. Drought conditions were expected to continue through mid-winter, as climate signals are indicating the commencement of a new La Niña episode, with warmer than normal temperatures and below normal rainfall in the coming months. In other words, Allison Harpold, along with every other marina manager on Lake Conroe, was busily studying her options in case worse comes to worse and lake levels drop below the threshold needed for maritime navigation here.
That kind of situation was very painful to contemplate…
Though enormously complicated in the past, the conditions at Lake Conroe are likely to present dramatically more acute challenges in the future. The level of this 21,000-acre lake has always been subject to wide fluctuations. The lake depends entirely on rainfall within the Lake Conroe watershed for all of the water that flows into it. It must respond immediately to demands placed by authorities in the Houston metropolitan area, a 10-county region that owns two-thirds of the water rights of Lake Conroe. Those demands are huge now, and destined to grow dramatically in the coming years. Though the worst of the drought ended in 2012, and a welcome increase in rainfall that year and the first half of 2013 managed to bring lake levels back to normal and keep them there, the future for reservoirs like Lake Conroe appears bleak, at best.
According to the 2010 census, the Houston–SugarLand–Baytown metropolitan area was the sixth-largest in the United States, containing at that time nearly 6 million people. Population here is projected to grow much faster than most of the nation through 2030, when estimates are that it will become the fifth-largest metropolitan area in the U.S. At this rate, before long even a minor drought in this locale will impose unsustainable demands on Lake Conroe until and unless major additions and improvements are made to regional water utilization rates, transport infrastructure (it is said that over 50% of the clean water that enters Houston’s water distribution system is lost due to leakage), and the size and number if regional water collection reservoirs.
These are daunting challenges, indeed. Epidemics of chironomid midges and tetragnathid spiders are serious issues, and must be dealt with to improve the quality of life for those living, boating, and working in areas where these organisms abound. In the process, we must also keep in mind the sensitive nature of the water reservoir and those who depend on it for a myriad of interrelated life processes. Emphasis on the use of Integrated Reduced Impact Methods, to achieve PestAvoidance through Habitat Modification (IRIM-PAHM™) will, as always, overshadow and govern everything we do.
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