A larvicide (or larvacide) kills larvae.
In a rare too-good-to-be-true moment, the biological agent Bti prevents mosquito larvae from maturing. It’s made from bacteria called Bacillus thuringiensis israelensis, which affects no other forms of life except two other nuisance insects (black flies and midges). It is organic and totally non-toxic.
You can use mosquito dunks in any standing water that you can’t dump out regularly: a small pond (such as the artificial one pictured here; these larvae leave shadows on the bottom) or stagnant stretch of stream in your yard, or a built-in plant soaking area that no longer drains, etc.
For a small surface area, just break up the dunks in pieces. In any case, reapply every month or so to be sure the Bti is still actively working.
Dunks usually come in packs of 6. If you buy more than one at a time at ACE hardware store in East Bradford (720 W Strasburg Rd., just west of West Chester borough, ask for the Don’t Spray Me! discount.
Just to remind you, we oppose the use of pesticide sprays and SPRAYS HAVE NO EFFECT ON LARVAE (or on eggs, or on pupae, the cocoon-like form that larvae go through before emerging as adults).
For detailed information on Bti, see “Best Natural Mosquito Control: Bti” at mosquitoreviews. Use Bti only on your own property or, with permission, on someone else’s property, but not on public property.
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.
If you have anything like this on your property, it’s time for immediate action! This photo shows about 100 mosquito larvae. The corresponding video shows them happily snapping their way (that’s how they move) around the shallow water rich in organic organic matter, their ideal habitat.
The good news is that an application of several tablets of the non-toxic larvicide Bti in a surface area of about 100 square feet virtually wiped out the larvae within 2 days.
With a bit of practice, it becomes easy to detect larvae in standing water. Sunlight helps show them, or a good flashlight. They are easily visible, about 3/8″ long, and a slight disruption of the water encourages them to zip around looking for shelter.