Bti for Mosquito Control

Good advice on larviciding from the US EPA. It’s so much easier to get rid of larvae that can’t escape than winged adults that fly where they wish! And right now is the time to dispatch all possible larvae, before numbers start to multiply.

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1. What is Bti?

Bti is a biological or a naturally occurring bacterium found in soils. (Bti is short for Bacillus thuringiensis subspecies israelensis.) It contains spores that produce toxins that specifically target and only affect the larvae of the mosquito, blackfly and fungus gnat. EPA has registered five different strains of Bti found in 48 pesticide products that are approved for use in residential, commercial and agricultural settings primarily for control of mosquito larvae.

2. Does Bti pose health risks to humans?

No. Bti has no toxicity to people and is approved for use for pest control in organic farming operations. It has been well tested by many studies on acute toxicity and pathogenicity (ability to cause disease) for Bacillus thuringiensis including studies specifically on Bti. Based on these studies, EPA has concluded that Bti does not pose a risk to humans.

3. Where has Bti been used for mosquito control?

Bti is used across the United States for mosquito control. Bti is approved for aerial spraying, which has taken place in Massachusetts, Pennsylvania, Maryland and Michigan, among other states. Bti can be sprayed over waterbodies such as ponds, lakes, rivers and streams. Bti is used to kill developing mosquito larvae by being applied to standing water where those larvae are found. Bti can be used around homes in areas and containers where water can collect, such as flower pots, tires, and bird baths. Bti can also be used to treat larger bodies of water like ponds, lakes and irrigation ditches.

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4. Will Bti work to control mosquito larvae?

Yes, Bti has been shown to be effective in reducing mosquito larval populations and could be effective in controlling mosquitos carrying Zika, dengue and chikungunya in places like Puerto Rico and other areas where these diseases have been identified.

5. Are insects becoming resistant to Bti?

No. There is no documented resistance to Bti as a larvicide. A recent study (Tetreau et al. 2013) confirmed previous research showing a lack of Bti resistance in mosquito populations that had been treated for decades with Bti.

6. Are there special precautions to be taken during Bti spraying?

No special precautions are needed for applying Bti. A number of Bti products are sold as “homeowner” products and are easy and safe to use. People do not need to leave areas being treated. However, as is the case with many microbial pesticides, some commercial use Bti products may require applicators to wear a dust/mist filtering mask.

7. How will I know if aerial spraying is going to take place?

Decisions about where and when to spray will be made by local officials. Listen for announcements in your community with the dates, times and locations of upcoming sprayings on social media sites, newspapers or radios.

8. Does Bti pose risk to crops or water supplies?

No. Bti has no toxicity to people, so it can be applied safely to mosquito habitat without a detrimental impact on food crops or water supplies. In fact, Bti can be used for pest control in organic farming operations. It is important to follow the label for any Bti product to ensure that the product is being used correctly. There are multiple Bti products and some are allowed to be used on certain drinking water (e.g., cisterns) while others are not intended for that use.

9. Is Bti harmful to wildlife including honey bees?

Studies indicate Bti has minimal toxicity to honey bees. 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.

10. Is there a medical test to show whether I’ve been exposed to Bti?

Since Bti has no toxicity to humans, a medical test to show exposure to the active ingredient has not been developed.

11. What other measures should be taken to control mosquitoes besides aerial spraying?

Eliminate any standing water (even tiny amounts) to prevent infected mosquitoes from laying their eggs (breeding) in standing water.

Use window and door screens to block infected mosquitoes from entering your home, workplace or children’s schools.

Use EPA-registered insect repellents to prevent getting bitten. EPA-registered means the product works and is safe when you follow the directions.

Dress in light-colored clothing, long pants, and long sleeves and try to avoid areas where mosquitoes are present.

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To larvicide or not to larvicide?

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.

The “contractual guidelines/agreement with the Pennsylvania Department of Environmental Protection”

That is the phrase used for the first time we can recall in the County release about the spraying of West Chester schedule for this 9/11 evening (unless rain intervenes). For the full document, see “Happy Patriots’ Day, West Chester, from the Chesco Health Department.”

Now we are in possession of the document that must be at the origin of that phrase. Download it here: DEP DH contract for 2018. It’s basically an application to the PA DEP for a maximum of $102,680.40 to aid in anti-mosquito spraying in 2018. (That’s just a portion of the total expense, of course.)

Pages 1-22 are standard bureaucratic stuff. The interesting part comes on pp. 23-25 of the pdf: “2018 Addenda – Scope of Work.” The underlined paragraphs are the actual County submission for the state money.

So the Health Dept wrote its own ticket, telling the State what it wanted to do, the State said OK, and now the County is saying it has to abide by its contract with the state… which it wrote! Circular reasoning, anyone? And still, nothing there says when the County has to spray.

We need to dig deeper, but at this point it is hard to find evidence that the County has lived up to its stated intentions regarding public education, outreach to municipalities, and larviciding. We need to go the Right To Know route, since the information flow to the public has been cut off.

The only positive in the document is that the County commits to 48 hours notice (previously 24). They do not commit to skipping homes of hypersensitive individuals but in the last couple of years they have provided a small buffer around registered hypersensitives and registered apiarists.

Is there another “contractual guidelines/agreement”? We’ll find out.