Nature Notes: Stopping Mosquitoes
It hit zero degrees Fahrenheit in Noyac early Sunday morning. An email from someone in northeastern Montauk told me it hit 5 degrees there. I haven’t recorded a zero temperature since 1979 when I moved here. I’ve had an outside thermometer up for almost all of those 36-plus years, so this may have been the coldest temperature the Noyac-Sag Harbor area has witnessed since that time. But alas, I cannot verify my claim.
I can no longer call up my favorite weatherman of the century, Richard Hendrickson, who always had the correct answer to my queries. That wonderful man passed from us two weeks ago.
A few days before that cold one, it was 60 degrees and I could see insects in flight from my window. (I hope they were overwintering mosquitoes, which came out of their torpor for a few hours only to freeze to death prematurely.) Active mosquitoes cannot survive zero temperatures. However, if secreted and “hibernating” in a safe, semi-warm spot, they can survive many winters.
With global warming in the wings, the distribution of all of nature’s living things, including mosquitoes, is in flux. Mosquitoes, of all of the disease-carrying vector organisms, are by far the worst, much worse than ticks, sand flies, rats, white-footed mice, and all the rest of them. It is particularly important that we watch what happens to foreign mosquito immigrants as the upper latitudes warm up. Already, perhaps, the worst of the disease-bearing mosquitoes, Aedes aegypti, that carries a potpourri of disease microbes and viruses including dengue, malaria, and, now, Zika, has established from a tropical base in Florida, Texas, Hawaii, and, within the last four years, in Washington, D.C.
In the first quarter of the past century there were a few cases of malaria in western Long Island, which prompted the creation of mosquito control districts with offices, staff, and equipment to protect us all. Before we were able to determine which mosquito species — ones that bred only in freshwater, or those which bred in both freshwater and saltwater — were responsible, we took steps to eradicate them all. These districts treated both fresh and salt waters. Ponds were covered over with oil, ditches were dug throughout Long Island’s saltmarshes, miles and miles of them. In East Hampton alone, more than 50 miles of such ditches, two to three feet wide and one to two feet deep, were dug, with special shovels distributed by Franklin Roosevelt’s Works Progress Administration. Multiply those 50 miles, principally in Northwest Creek, Accabonac Harbor, and Napeague Harbor marshes, by the rest of the towns on Long Island, many with more salt marshes than East Hampton, and you end up with more than 1,000 miles of such interconnected, crisscrossing, tidal inroads across the island.
The oil on the surface of freshwaters was to prevent the mosquito larvae from getting oxygen. The ditches were to allow the tidal waters to move in and out, drain the tops of the marshes more quickly, and let the killifish and other predators get to the larvae before they metamorphosed into adult mosquitoes and flew out of the marsh.
Lots of ecological damage was caused by all of these ditches. Ditch reed, Eurasian phragmites, moved in to occupy the mounds of dirt left beside each ditch, the water drained off the marsh too quickly, and the peaty banks of the ditches eroded more quickly by ice and storms. Thus the area of each ditch marsh covered with spartina grasses and other native saltmarsh species receded.
Suffolk County Vector Control no longer creates new ditches or clears out older ones, and no longer spreads oil on the surface of mosquito-breeding ponds. While the department used to treat marshes with DDT and other poisons, that hasn’t been done for more than 50 years now. The poisons have been replaced by bacteria which are pathenogenic to mosquito larvae, such as Bacillus thuringiensis israelensis (Bti) and methoprene, a synthetic hormone, which prevents the larvae from metamorphosing into adults. Of these, Bti is the least harmful to non-target species, while methoprene can also inhibit molting and achieving adulthood in a wide array of marine organisms, including insects such as saltmarsh dragonflies and crustaceans such as crabs and lobsters.
Both of these agents are used by county vector control in Accabonac Harbor and Napeague. The latter area just may be the most mosquito-prone area on the South Fork.
On the other hand, there are at least two mosquito-borne diseases on Long Island, equine encephalitis and West Nile disease. We’ve experienced both in Suffolk County, the former in Montauk as well, and each year “mosquito pools” with West Nile viruses are found here and there across Long Island and, for that matter, throughout most of the United States.
But what is truly scary is dengue fever and the Zika virus running rampant in Brazil, Colombia, and other Latin American countries. It’s only a matter of time before these diseases spread deep into the United States as it continues to warm up during the millennium, principally as a result of the redistribution northward of the number-one carrier, Aedes aegypti.
And what about the other introductions? The Asian tiger mosquito is rampant on Long Island. It is not yet a serious vector for any disease, but mosquito species tend to evolve almost as quickly as they spread.
There are some very efficient natural controls, including the saltmarsh dragonfly and killifish and the becoming-naturalized mosquitofish, Gambusia affinis. Killifish live in fresh, brackish, and saltwater. Some birds such as swallows and flycatchers, our wood pewees, phoebes, and great-crested flycatchers, as well as some bats, also can feed on flying mosquitoes.
One biocontrol method that has been tried locally, and which is working, is also good for the saltmarsh. East Hampton, with the help of Emerson Hasbrouck and Cornell Cooperative Extension, has applied it to about 20 miles of marsh in Accabonac Harbor and Northwest Creek. The vector control ditches are dammed at their outfalls with sandbags or piles of sand, to the level that only lets the top of flood tides in and flood tides out during the tidal cycle. Thus there is almost always enough water behind the dams to support killifish and other organisms, including dragonfly larvae that feed on mosquito larvae.
The dams also prevent the water from leaving the top of the marsh too quickly during flood tides, which is good for the native marsh plants but hard on phragmites, and tend to keep the E. coli bacteria from defecation by wildlife species and waterfowl in the ditches and out of the adjacent shellfish waters. As the ditches mature they tend to repopulate with spartina grasses as well, thus increasing the amount of good vegetation coverage.
Another advantage of these dams not too long ago discovered by Dr. Christopher Gobler and his graduate student Stephanie Talmage Forsberg, now a Ph.D. graduate from SUNY Stony Brook and a former East Hampton Town Trustee, was that the water coming out of the dammed ditches on the falling tide had less nitrates and nitrogent products in it than the water behind the dams. In Accabonac Harbor, for example, many of the ditches are down-gradient from homes with septics, a major source of nitrogenous materials.
We can’t stop global warming in its tracks, but in the long run biological controls such as those practiced in East Hampton since the mid-1990s, and others being developed around the globe such as genetically modified adult male mosquitoes — first developed in England and now being loosed against the Zika and dengue viruses crisis in Brazil — give the larvae stemming from fertilization a condition which prevents them from maturing into adults. Such non-toxic controls, once perfected and applied, will be the best way to keep salt marsh mosquito populations down, and good for the salt marshes and the coastal waters they border on and help cleanse.
A bit ironical, isn’t it, that the synthetic hormone which prevents the mosquito larvae from attaining adulthood and flight, is the same one that keeps the saltmarsh dragonflies that feed on mosquitoes from becoming dragonflies?
Larry Penny can be reached via email at [email protected].