Roanoke Times
Copyright (c) 1995, Landmark Communications, Inc.
DATE: SUNDAY, July 23, 1995 TAG: 9507240123
SECTION: HOMES PAGE: D-1 EDITION: METRO
SOURCE: PAUL WILLS KNIGHT-RIDDER NEWSPAPERS
DATELINE: LENGTH: Medium
Several kinds of insects are targets of BT, but the most helpful to vegetable gardeners is the kind that kills the worms that develop into butterflies or moths.
This is a big target. It includes cabbage loopers of two kinds, which riddle not only cabbage but also broccoli, collards and cauliflower; tomato fruit worms that can wipe out tomatoes and damage sweet corn; the ever-present squash borers and pickle worms and the big hornworms that eat tomato plants.
Worldwide, BT is used by the hundreds of tons against army worms, gypsy moths, coddling moths, bean leaf rollers and various other caterpillars in forest and field. Because it does little or no damage to the environment, its use still is expanding.
The letters BT stand for ``bacillus thuringiensis,'' a race of bacteria that already is easily the most successful biological pest control ever discovered by man.
Spores of the bacteria may be bought under several brand names in either dry or liquid form, mixed with water and sprayed on plants as soon as the worms appear. After digesting the bacteria, the worms stop feeding and die in a few days, their insides destroyed.
BT can be sprayed on the crops right up to the day of harvest, for it will not harm anybody or anything that is going to eat the produce.
This is a pretty impressive record for athat came to man's attention because it caused a fatal disease in silkworms - a big economic problem in the days when silk was a high-priced luxury item throughout the world. In 1901, researchers discovered the guilty party was BT bacteria.
Slowly, the scientists discovered the nemesis of the silkworm was the guardian of garden, field and forest, because BT killed the larvae of other plant pests as efficiently as it killed the silkworms.
It was so attractive as a pesticide that microbiologists began searching for strains that would attack other classes of insects. And they found them.
First, a strain of BT was found in an Israeli pond that could kill the larvae of mosquitoes and black flies without harming the fish, birds or water plants.
In 1985 came a major breakthrough. A variety of BT was found that killed the larvae of the Colorado potato beetle, a major pest that is from an entirely different insect group from moths and butterflies. Known as BT/San Diego, this strain went on the market in 1989 after tests in many parts of the world proved it was effective if consumed by potato beetle larvae while they are still small.
As any gardener knows, the flesh-colored grubs of the Colorado potato beetle are voracious, capable of eating all the foliage off Irish potatoes in short order. Furthermore, they quickly develop resistance to chemical sprays and within three or four generations become almost immune, but apparently cannot become immune to BT.
In recent years, BT research led to one strain that kills fungus gnats attacking potted plants and isolated another that controls a type of moth that infests beehives.
After a targeted insect eats the bacteria along with the foliage, the BT organisms begin multiplying rapidly and releasing a protein crystal that attacks the digestive system of the worm. The larvae may cling to the plant for a few days, but they eat no more and soon die. If a bird were to eat the insect in the meantime, it would not be harmed by the BT.
Since the insecticide consists of living microorganisms, BT should be applied late in the day or on an overcast but not rainy day. Hot sunshine can quickly kill the bacteria before the insects ingest them.
Because BT microorganisms stay alive on the leaves only a few days at best, they are not very effective as a preventive spray.
They work best and fastest when the larvae are very small, so it's important to keep close watch for the beginnings of damage, such as a cluster of worms eating the underside of a tomato leaf without damaging the other side.
BT to control the larvae of moths and butterflies is widely sold in garden centers under such names as Dipel, Thuricide, Biotrol or Biological Worm Spray.
Scientists are searching for ways to take the toxic protein crystals out of the bacteria and use them as a pesticide, or, better yet, to manipulate the plants into producing the crystals and poisoning the insects that attack them.
Gardeners and other environmental protectors can look forward to a successful search, for BT homes in on its targeted enemy without doing any damage to innocent bystanders such as the environment, a claim that mighty few insecticides can make.
by CNB