Bacterial Insecticide

What is Btk?
Btk (Bacillus thuringiensis kurstaki) is a naturally occurring bacterial pathogen of the gypsy moth. It commonly occurs in soils and on plants throughout the world. It is employed in a biological insecticide used to suppress the gypsy moth and many other insect pests. Widespread use of Btk to control the gypsy moth began in the 1970’s and has been applied to millions of acres of forest in the east and mid-west since.  

Is the Btk used in Roscommon County Organic?
Yes, the Organic Materials Review Institute (OMRI) has determined that the formulation of Btk “Foray48B®” is organic. OMRI is a national nonprofit organization that approves products that are used on operations that are certified organic under the USDA National Organic Program.

How Btk Works

The gypsy moth caterpillar must eat the Btk for it to be effective. Within minutes, the toxin binds to specific receptors in the gut wall, and the caterpillar stops feeding.  Within hours, the alkaline gut and enzymes that only caterpillars have activate the toxin that kills the cells lining, stopping the caterpillar from feeding and dies within 1 to 2 days. This alkaline condition is not present in the stomachs of humans, birds, or fish and other animals because they have acidic stomach environments.

How Btk Is Made

Btk is cultured using water and nutrients such as sugars and starches, in a fermenting process similar to brewing beer. The final product contains almost all water, the leftover growth medium, carbohydrates, inert ingredients that are approved as food additives, and the active ingredient. 

The biological insecticide is a very effective suppresent. It has been used successfully on gypsy moth infestations in Michigan and other states since the 1980s; has a proven safety record with people, pets, birds, fish, livestock, and other insects such as bees; 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.

What Btk Affects

Different strains of Btk affect different insects. Btk has no effect on humans, pets birds, fish, livestock, honey bees or other wildlife. Btk is so specific to only certain moth and butterflies that it is an approved control product for use by certified organic gardeners.
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. Public health officials state Btk is not a public health risk.   
Common sense precautions as with the application of any control product, people, pets or domestic animals with known respiratory ailments or other health concerns my wish to avoid exposure by staying indoors during the spray operation and until the Btk product is dry (usually 30 minutes). The Btk applied in a spray does not multiply or accumulate in the environment.


Btk is applied at a rate of 0.5 gallons per acre. It is applied after all caterpillars have hatched. It is most effective in mid- to late May or early June when caterpillars are young and have a gut lining thin enough for the Btk toxin to punch holes in it. Once the larvae have gotten larger than 5/8-inch long, they are rather difficult to kill.

Why We Use Btk

  • Breaks down in sunlight within 7 days
  • Highly effective: nearly 80% mortality of gypsy moth caterpillars in treated areas
  • Highly effective with high gypsy moth caterpillar populations
  • Readily available
  • Known to be toxic to feeding caterpillars of moths and butterflies and is safe to humans, birds, fish, pets, beneficial insects and other non-target organisms
Do-It-Yourself Application
For landowners whose property is not included in a spray block, or for treatment to address later hatch caterpillars on smaller trees and lower foliage:
The landowner may purchase pesticides containing Btk from retailers and garden stores. (Whatever product you use, confirm that the label explicitly states it is effective on gypsy moth or Lymantria dispar. Avoid using broad spectrum pesticides which may not work on gypsy moth/LD, but are very harmful to a wide variety of native and non-pest species.) Treatment is most effective on small caterpillars in their first and second instar stages (less than 1/2" long). Btk application must be administered to foliage in a very fine mist and must be ingested by the caterpillar to be effective.

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