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.


Blackberry Leaves Decompose to Thwart Mosquito Breeding

Beyond Pesticides, December 21, 2019)

A study at the University of Maine (UMaine) finds that adding blackberry leaf litter in stormwater catch basins creates an “ecological trap,” enticing mosquito females to lay eggs in sites unsuitable for larvae survival. Employing this new and incredibly viable “attract-and-kill’ tool for mosquito control shows potential for preventing the breeding of mosquitoes that may carry insect-borne diseases, especially in urban environments. Stormwater catch basins regularly accumulate leaf litter, which serve as habitat for the mosquito species Culex pipiens (Cx. Pipiens) that may carry West Nile virus.

Previous University of Maine research discovered decomposing leaf litter from Amur honeysuckle (Lonicera maackii) and common blackberry (Rubus allegheniensis) produces chemical compounds that attracts and stimulates Cx. Pipiens female to oviposit, or lay eggs.

Investigating the attractiveness and lethality of varying catch basin conditions to mosquitoes, researchers hypothesized that blackberry leaf litter could be shown to be lethal to developing mosquito larvae, and, therefore, act as a natural ecological trap for Cx. Pipiens….

read more at Beyond Pesticides

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.

Who says storm drains are a big mosquito problem?

The PA Department of Environmental Protection, that’s who!

We usually call them “storm drains” but others use the term “catch basins” or “inlets.” In any case, that’s where runoff from streets goes through a grill and disappears from our sight. But then were does it go? It should drain by gravity though a pipe system and eventually flow into a stream. That’s a problem for the stream, because street runoff can be polluted, e.g, by car and animal wastes.

But when water is able to remain standing in the storm drain, it provides an ideal habitat for mosquitoes to breed! Then those storm drains need to be “treated” (with a larvicide like Bti) to keep larvae from maturing there into adult mosquitoes. Here’s what PA DEP says (you have to keep refreshing the URL in your browser to find this photo and commentary):

What is supposed to happen? Has it been happening?

Some questions derived from CDC mosquito control guidelines:

1. Where has the County engaged in source reduction, as recommended?

The only source reduction we know of has been undertaken by West Chester Borough to prevent water from standing in storm drains. Does anyone know of other examples?

2. Where has the County engaged in larval mosquito control, as recommended?

They have told us that they do so, but so far have said they do not have records for 2015-17 and do not have time to tell us where for 2018; our Right to Know request on this with the PA Department of Environmental Protection is pending.

3. Has the County maintained a database of aquatic habitats to identify the sources of vector mosquitoes and a record of larval control measures applied to each (last paragraph below)?

From point 2 above, it would seem doubtful; but the public has a right to know, and we will.

Source material: Centers for Disease Control and Prevention, Division of Vector-Borne Diseases, “West Nile Virus in the United States: Guidelines for Surveillance, Prevention, and Control,” 2016, p. 33. (See the points we have put in boldface below. Download the full publication here).

Integrated Vector Management

Mosquito abatement programs successfully employ integrated pest management (IPM) principles to reduce mosquito abundance, providing important community services to protect quality of life and public health (Rose 2001). Prevention and control of WNV and other zoonotic arboviral diseases is accomplished most effectively through a comprehensive, integrated vector management (IVM) program applying the principles of IPM. IVM is based on an understanding of the underlying biology of the arbovirus transmission system, and utilizes regular monitoring of vector mosquito populations and WNV activity levels to determine if, when, and where interventions are needed to keep mosquito numbers below levels which produce risk of human disease, and to respond appropriately to reduce risk when it exceeds acceptable levels.

Operationally, IVM is anchored by a monitoring program providing data that describe:
• Conditions and habitats that produce vector mosquitoes.
• Abundance of those mosquitoes over the course of a season.
• WNV transmission activity levels expressed as WNV infection rate in mosquito vectors.
• Parameters that influence local mosquito populations and WNV transmission.

These data inform decisions about implementing mosquito control activities appropriate to the situation, such as:
Source reduction through habitat modification.
• Larval mosquito control using the appropriate methods for the habitat.

• Adult mosquito control using pesticides applied from trucks or aircraft when established thresholds have been exceeded.
Community education efforts related to WNV risk levels and intervention activities.

Monitoring also provides quality control for the program, allowing evaluation of:
• Effectiveness of larval control efforts.
• Effectiveness of adult control efforts.
• Causes of control failures (e.g., undetected larval sources, pesticide resistance, equipment failure)….

and p. 34:

Larval Mosquito Surveillance

“Larval surveillance involves identifying and sampling a wide range of aquatic habitats to identify the sources of vector mosquitoes, maintaining a database of these locations, and a record of larval control measures applied to each. This requires trained inspectors to identify larval production sites, collect larval specimens on a regular basis from known larval habitats, and to perform systematic surveillance for new sources. This information is used to determine where and when source reduction or larval control efforts should be implemented….

The Truth About Mosquitoes, Pesticides and West Nile virus

This is a very thorough indictment of the expansive tendency in mosquito spraying. We give just a few excerpts here. Please see the full article for all details.

A Beyond Pesticides Factsheet

While communities have good intentions, many existing policies and programs may be dangerous to children, adults and wildlife and inadequate by relying too heavily on spraying pesticides to kill adult mosquitoes….

Less than one percent of those infected with WNv will develop severe illness, according to the Centers for Disease Control. The vast majority of people (about 80%) who become infected with WNv will show no symptoms and never become sick. Some 20% may experience mild flu-like symptoms within 3 – 15 days….

A person who has been infected with WNv may have life-long immunity even if they show no symptoms….


According to the Centers of Disease Control and Prevention (CDC), spraying adulticides, pesticides intended to kill adult mosquitoes, is usually the least efficient mosquito control technique.

Adulticiding programs spray pesticides indiscriminately and do not get at the mosquitoes until they have matured. They also do not restrict, control, or prevent mosquitoes from carrying WNv or continuing to breed.

Close to 99.9 % of sprayed chemicals go off into the environment where they can have detrimental effects on public health and ecosystems, leaving 0.10% to actually hit the target pest.

Mosquitoes develop resistance to pesticides over time, rendering the chemicals ineffective. A 2003 study finds that mosquitoes carrying West Nile virus and malaria developed resistance to organophosphate and carbamate insecticides as a result of a single genetic mutation.

After Hurricane Floyd caused a surge in mosquito populations in Florida, state officials took bite counts before and after widespread aerial spraying and found that mosquito populations surged back to pre-spray levels within three days of the treatment….

In 2003 the city of Boulder, CO did not adulticide and showed an 80% reduction in mosquito populations and lower rates of serious illness per population than surrounding cities where adulticiding took place….

see lots more info, including non-toxic methods of mosquito control, at Beyond Pesticides

A serious case

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.

DSM’s secret weed-killing formula

by Jim Hudgings

Here is the sidewalk spray recipe used by the Sierra Club Youth Corps program in summer 2017, as a good alternative to toxic herbicides:

1 gallon distilled white vinegar
1/2 cup table salt
1/2 teaspoon liquid dish soap

I pour a small amount of vinegar from the jug into my sprayer in order to make room in the jug for salt, then pour the salt into the jug of vinegar and shake it vigorously to dissolve the salt so that it won’t clog my sprayer nozzle. Pour the salted vinegar into the sprayer and add the liquid dish soap. Briefly swirl the sprayer to mix in the soap, but not enough to generate suds. Spray it on the sidewalk, ideally in the morning of a very sunny day.

When finished, I spray plain water for a few seconds to clean the wand and nozzle in order to avoid re-crystallized salt from clogging the nozzle.