June 19, 2003 at 3:46 p.m.
It is a naturally occurring bacteria that attacks the larval form of these insects but has no effect whatsoever on people or pets, or even on other beneficial insects, such as bees, lady beetles, spiders and lacewings.
There are several different strains of Bt that target different insects, so be sure to get the right strain for the insect you are trying to control. Also, be aware that Bt is effective only when the larvae are still small.
1) Bt Kurstaki - Products: Dipel (dust or wettable powder), Thuricide (liquid concentrate)
IMPORTED CABBAGE WORM, CABBAGE LOOPER, DIAMONDBACK MOTH
These three "worms" are moth and butterfly caterpillars and are common pests of cabbage, cauliflower, broccoli, kale, rutabaga, radishes, turnips, collards, horseradish and other crucifers. They all cause similar feeding, chewing damage and have several generations per season. Bt Kurstaki can be used up to the day of harvest.
Sod webworm is the most common blade-feeding lawn pest in Minnesota. It is the larva of the "miller" moth.
Spring and fall cankerworms, also called inchworms, are common shade tree defoliators, feeding on the leaves of elm, apple, hackberry, basswood, oak, boxelder, maple, and ash trees, as well as shrubs growing beneath them. Inchworms go through a natural cycle of 13 to 18 years, when they are very prevalent for 3 or 4 years, followed by long periods of lower populations, according to Jeffrey Hahn, U of M Extension Entomologist. Bt Kurstaki can achieve good control when sprayed while inchworms are still less than 1/2 inch.
2) Bt tenebrionis or Bt San Diego - Products: Novodor, M-Trak and Raven. Also, look for products for Potato Beetles with Bt as the active ingredient.
COLORADO POTATO BEETLE
According to Jeff Hahn, this beetle has developed resistance to virtually all of the synthetic organic insecticides available to home gardeners. Bt, however, does provide some control, when applied frequently and while the beetle larvae are young. Large larvae and adult beetles still must be picked off by hand. Spray Bt tenebrionis when eggs first begin to hatch.
Some interesting research, done by G. E. Brust, an entomologist at North Carolina State University, showed that potato plots with straw mulch and no insecticide use other than Bt supported significantly greater numbers of the beetles' natural enemies: ground beetles, lady beetles, lacewings, stink bugs. Remember, Bt has no effect on beneficial insects! Better potato yields resulted.
Several varieties of leaf beetles are common in Minnesota and attack elms, willows, cottonwood and other trees and shrubs. Robert Wawrzynski, U of M entomologist, states that they cause little damage, except when trees are already under stress from drought or disease or when a tree is young or recently transplanted. Then, the additional stress from leaf-eating beetles can cause the death of the tree. Spray Bt tenebrionis when larvae are still small.
3) Bt H-14 or Bt israelensis- Product: Knock-Out Gnats, Gnatrol
Fungus gnats around houseplants are commonly mistaken for fruit flies. They lay eggs in the soil of houseplants. Water once a week with Bt H-14 solution for three weeks to kill fungus gnat larvae.
Some of the above products can be purchased at local garden centers. If you can't find it, ask if it can be ordered for you. They can also be found in mail-order garden supply catalogs, such as Mellingers and Gardens Alive, as well as from their online stores, mellingers.com and gardensalive.com. Other internet sites that sell biological controls include biconet.com and planetnatural.com.
As always, read the labels carefully and follow instructions. Use only on the plants listed and only for control of the insects listed on the labels.
The Master Gardeners have a variety of native plants and heirloom tomato plants for sale at the Extension Office. They will also be having a plant sale at the office Saturday, June 21, in conjunction with North Branch Midsummer Day Festival.
For more information on the U of M Extension Service web page go to www.extension.umn.edu and search. Or, you may call our Yard and Garden Line anytime and leave a message at (612) 624-4771. A Master Gardener will return your call. The Master Gardeners also staff the Extension Office on Monday from 4:30 to 8 p.m. Call (651) 674-4417 or drop in with your samples and gardening questions.