Larviciding is widely considered the prime means of reducing mosquito populations. Killing larvae is much easier than killing adults, because the larvae are in limited areas of water, whereas adults can be flying around anywhere in the air up to a height of something like 20 feet or sheltering in or under plants, in tree hollows, under porches and basement entryways, and in other areas where airborne spray may not penetrate well.
Furthermore, larvicides are not chemical toxins and affect only mosquitoes and some other species of insect pests, whereas airborne pesticides have collateral damage, killing many non-targeted insects and possibly harming other species.
The larvicide you can buy in the hardware store is the bacteria-based Bti “dunks.” According to the PA DEP, “Bti produces toxins that specifically affect the larvae of only mosquitoes, black flies and fungus gnats. These toxins do not affect other types of insects including honey bees.”
Another widely-used larvicide, methoprene is a growth regulating “juvenile hormone” that prevents insect larvae from maturing into adults.
PA West Nile website home page extols the benefits of larviciding. Indeed, larval control is an important part of the Chester County Health Department’s 2018 application for a grant from PA DEP (download here: DEP DH contract for 2018:
So wouldn’t we expect that the County would be regularly larviciding in West Chester Borough, which the Health Department has been regularly identifying as having high “Vector Indexes” (likely from breeding in storm drains)? Unfortunately not. In fact, the County has conducted only ten larvicide events in the Borough in the last 4 years! See the locationsn black on this map constructed by the 2018 Don’t Spray Me! / Sierra Club intern Kyle Erisman on the basis of geographical coordinates obtained by a Right To Know request to the PA DEP:
We hope that the Borough will, as planned, successfully take larviciding into its own hands beginning in 2019.