The MDA determines which product to use to treat spongy moth by looking at the results of annual spongy moth survey data, alternate life survey results, and size of the treatment area. Depending on the product, treatments are conducted to target either very young caterpillars or adult moths.

In areas that have many male moths captured and where egg masses are found during the alternate life stage survey, we use Bacillus thuringiensis kurstaki (Btk) formulated in the product Foray. It is used where there is evidence of a reproducing population, and is used against small caterpillars.

Product Questions

Btk as a naturally, occurring bacterium that is found in the soil. You may come into contact with it on a daily basis. The product is applied to the treetops where spongy moth caterpillars are feeding. When ingested, the bacterium is toxic to certain susceptible caterpillars like the spongy moth. Caterpillars stop feeding and die within a couple days.

Foray is the commercial product containing Btk. The inert, or inactive, ingredients used in Foray are certified organic food-grade ingredients and contain no petroleum solvents. The product is certified organic by the Organic Materials Review Institute (OMRI). The Foray Btk is not genetically engineered and the product is gluten free.

The product is applied to the treetops where spongy moth caterpillars are feeding. This is done in May or June when the caterpillars are very small. We use aircraft or spray trucks. The aerial treatments are at low altitudes (approximately 50 feet above the treetops).

When the caterpillars feed on the leaves, they ingest the products. The bacterium in Foray is toxic to the caterpillars, and they stop feeding and die within a couple days.

Treatments occur twice, about a week apart, to catch late-hatching caterpillars. Spraying normally takes place early in the day when low winds and high humidity allow the spray droplets to land on tree tops where they will be the most effective, and also because fewer people are active outside. A treatment site will typically be finished before children are walking to school or people are heading to work.

Human Health Questions

Btk has a proven safety record with people, pets, birds, fish, livestock, and other insects such as bees; and has been registered and re-registered many times by the Environmental Protection Agency (EPA) for use in the U.S., to use on more than 200 food and fiber crops.

People are exposed to Btk through contact with soil in the natural environment or through their diet, at very low levels. If you eat fresh fruits or vegetables, you probably have already ingested this bacterium. It is commonly used on commercial and organic food crops.

Some people have reported mild skin reactions or mild eye, ear, and nose irritations after Btk treatments. Others have reported mild hay fever reactions. Health officials have studied these reports extensively and have not been able to determine if the reactions were caused by Btk or by pollens, molds, or dust generated during the treatments, or were unrelated to Btk treatments. The following are two cases often cited from the Internet.

A study of an entire community in New Zealand was done for a 2002 application of Foray. Surveys of 292 study participants about their own symptoms and their self-perceptions of their health were conducted before and after the sprays; there were multiple applications of Foray. Among the findings: there were significant increases in the rate of reported throat and gastrointestinal symptoms, and for sleep problems, following the sprays. Yet symptoms of exposure are quite general, and are often the same as those from other causes such as flu or food poisoning. The study authors cautioned that these self-reported symptoms were not necessarily confirmed by healthcare providers. Also, most participants in the New Zealand study reported their health (and that of their children) were not affected by the spray program, nor was there any reported increase in visits to either medical practitioners or alternative healthcare providers. 

There have also been concerns that aerial sprays of Btk-based pesticides, such as Foray, could impact vulnerable populations in the spray zone. Canadian researchers explored this issue during a 1999 application of Foray over Vancouver Island. The lung symptoms and breathing capacity (lung function) of 29 children with asthma living in spray zone were compared to those of 29 children with asthma living outside the spray zone. Measurements, as conducted by parents, were done before and after the spray period. There were no differences in asthma symptom scores between either groups, neither before nor after the spray. There were there no significant changes in measured lung function, as reported by parents of study participants, after the sprays.

Public health officials state Btk is not a public health risk.

Although we don't have evidence that Btk will affect any given group of people, individuals with a weakened immune systems or serious food allergies may choose to avoid any potential for exposure by staying indoors during and at least 30 minutes after the treatment, or leaving the area during the application. 

Given the long history of use and the knowledge that regulatory and public health officials have about Btk and Foray, there is no reason to suspect that either Foray or Btk is carcinogenic.

You can read more about Bt and human health from the National Pesticide Information Center.

Environmental Questions

No, Btk is not toxic to humans, mammals, fish, birds, or honeybees. Btk only works under the alkaline conditions present in the stomach of insects in the caterpillar stage of development. This alkaline condition is not present in the stomachs of humans, mammals, fish, birds, or honeybees, which is why the Btk does not affect them.

To have any effects, caterpillars must be actively feeding within approximately two weeks after treatment, they must eat the Foray, and they must be susceptible to the bacteria. The MDA works with the Minnesota Department of Natural Resources and the U.S. Fish and Wildlife Service to identify and evaluate impacts to any threatened or endangered species, and ensure that no sites of sensitive, threatened, or endangered species of moths and butterflies overlap with this year’s proposed Foray treatments.

Without treatment, spongy moth caterpillars can quickly ravage the vegetation that these native moths and butterflies depend on.

Minnesota first used Btk for spongy moth in 1984. Since then, approximately 50 sites in the state have been treated for the insect. Only one area has needed to be retreated. Why was a retreatment needed? It is likely the original treatment area was too small to eradicate all the spongy moths. The MDA strives to find the right balance when drawing treatment areas.

We do our very best to design our management blocks as small as possible yet large enough to avoid missing the spongy moth population and having to conduct a second year of application. In addition, we want to treat the population as quickly as possible to avoid the population from expanding and thus resulting a much larger treatment area in following years.

The spray residue is not harmful to humans, animals or outdoor structures. The spray can be easily washed off. It will show more readily on dark colored vehicles and it can be washed off. If left in the sun for a few days without washing, the spray deposit on vehicles may have to be rehydrated in order to remove it. Take an old bath towel and soak it in sudsy water and drape it over the vehicle for a few minutes. This will help soften the spray deposit and it can be removed. More ‘elbow grease’ may be required if the spray deposit is left on the vehicle for several days.