Methoprene and vernal pools

Thursday’s Rutland Herald article by Will Mathis, Director of Operations of the BLSG Insect Control District, focused on the larvicide program. Although the article was titled “Mosquito spraying program explained,” no mention was made of malathion or permethrin which are sprayed along town roads to kill adult mosquitoes. Instead, the article focused on the program to treat standing water to kill mosquito larvae before they hatch.

The major effort of the larvicide program is spreading bacteria from a helicopter over the Otter Creek floodplain swamps and fields. The two bacteria used (Bacillus thuringiensis israelensis, and Lysinibacillus sphaericus, until recently known as Bacillus sphaericus) contain toxic crystals which are activated by conditions in a mosquito larva’s gut and kill the larvae. These bacteria have shown no toxicity to people or animals other than mosquitoes and a couple of other types of small flies. Well-timed application of these bacteria can dramatically reduce the number of certain species of mosquitoes.

Not all mosquito species breed in floodplain swamps and fields, and helicopter application cannot efficiently target many dispersed small pools. So another type of larvicide, methoprene, is applied by hand to some water bodies. Methoprene is not a bacteria, it is an insect growth regulator. Insect larvae exposed to it never metamorphose into adults. Methoprene is not specific to mosquitoes and interrupts the development of many aquatic insects and other invertebrates. Mosquitoes are very sensitive to methoprene, so if its concentration in water is low, the primary toxicity will be on mosquitoes and some other small invertebrates.

A vernal pool in the Salisbury Town Forest. April 29, 2009

The loss of some harmless non-target creatures might be an acceptable price to pay for controlling mosquitoes, unless those creatures are part of a critical ecosystem. Vernal pools are small temporary ponds that are wet just long enough in the spring to support ephemeral populations including amphibians, fingernail clams, and fairy shrimp. Amphibians hop or crawl away before the pools dry up, but the clams, fairy shrimp, and other invertebrates persist for months until the pools fill up again. There is not much data on how these invertebrates tolerate methoprene, but vernal pool communities are sufficiently valued that it might be wise to avoid the risk of serious disturbance. Some authorities have banned the use methoprene in vernal pools, for example in a California mosquito control district “Because of the effects of methoprene on fairy shrimp and a lack of information on how long the agent remains in the soil, use of the larvicide methoprene within vernal pools or swales at any time, in either wet or dry conditions, is prohibited.”

It will be good to learn more about where methoprene is used in the BLSG district, and how the concentration of methoprene is kept low enough to affect only mosquitoes and other small invertebrates.


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