By: Island County Extension Agent, Donald B. Meehan
"BT", A BIOLOGICAL INSECTICIDE THAT WORKS WELL
For the past couple of years I have been writing about the tent caterpillar invasion we have been having. That invasion continues on, and we are now beginning to see signs that it will be a major problem again this summer.
Whenever we are besieged by something, we want to have a fast and reliable method to get the problem in hand again. This kind of thinking has often led us to use things which are not as safe.
One of the pesticides that I have been recommending for controlling tent caterpillars has been "Bt". This is a biological pesticide unlike products like Diazinon, Sevin or Malathion which all give rapid knock-down. I thought it might be worthwhile to spend some time talking about this special tool for the gardener and homeowner.
"Bt" is short for Bacillus thuringiensis. It is a bacteria and was first discovered in 1901 in Japan during an outbreak of the silkworm. The organism was rediscovered in 1911 and 1927 when its potential as a pesticide was considered. Nothing was done with it until 1953 when large-scale commercial work began to develop it. FDA cleared the product for use on crops indicating that the material was so safe that no specific dosage was judged toxic to humans.
Why is this product so wonderful?? There are a couple of reasons. First and foremost is the safety of this product to the user. It is not regulated as a hazardous material, unlike Malathion, Diazinon and other synthetic pesticides. It also poses no danger to bees.
The second most important advantage is that "Bt" is host specific. In other words, only caterpillars are injured. Those insects which are beneficial to us like lady bugs, parasitic flys, and lacewings are not hurt. Hence they keep on working for us keeping other pest insects under control.
How does "Bt" work? This particular bacteria has the ability to produce spores and a diamond-shaped protein crystal. The crystal is toxic to many insects depending on the alkalinity of their guts. Tent caterpillars and about 150 other types of caterpillars have very alkaline stomachs. When they ingest the crystals the alkaline environment allows the crystal to dissolve. When dissolving has begun the toxin becomes available. A stomach paralysis sets in and the caterpillar stops eating. Even though the caterpillar is still alive, it is not well. The damage will have stopped, and it will be a day or two before the caterpillar actually dies.
In order for "Bt" to work it must be consumed by the caterpillar. This obviously is a crucial step in getting control. Leaves must be present and the caterpillar must be present. Leaves are the food source, hence, must act as the carrier of the "Bt", and since "Bt" has a fairly short lifespan it is important to have it applied fresh when the caterpillars are present.
"Bt" works best on young caterpillars. When applying the material keep in mind that ultraviolet light degrades the "Bt" spore. The effect of this is that any of the "Bt" exposed to sunlight will lose its effectiveness in about a day. However, if the "Bt" is on the leaf underside then it will last as long as 25 days. As a general rule you would be wise to re-apply after one week if the caterpillars are still present.
"Bt" comes under a number of product names. The first was Thuricide. Others followed: Dipel, Biotrol and Attack. All of these brand names are available here on Whidbey Island with the exception of Biotrol.
Return to Articles Index
Return to Garden Page
Providing Educational Support to the Island County Community