Bt, An Organic Insecticide
Copyright 2011 McGroarty Enterprises Inc.

Most gardeners are anxious to get out into their gardens as soon as the weather warms up in the spring and the first green sprouts appear in their gardens. Unfortunately, plant-eating insects are just as anxious as we are to get into the garden. They seem to think that our carefully-tended plants and vegetables were planted for their benefit alone. Bt, or Bacillus thuringiensis, is an organic insecticide that is a useful addition in the gardeners' arsenal against many of these plant-eating insects.

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Bt is a naturally occurring bacteria that is common all across the world, and early in the twentieth century it was discovered to be an excellent natural insectide. Bt became available to home gardeners in the 1950s, but in recent years new strains of this organic insecticide have been developed that attack an even wider range of hungry insects.

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Bt is an organic insecticide that works by affecting the insects' digestive system. When an insect munches on a plant that has been treated with Bt, the Bt bacteria paralyzes the insect's digestive system and the insect stops feeding and expires from starvation within days.


Bt kurstaki is the most common strain of Bt, and it kills only caterpillars that feed on leaves and needles. As an organic insecticide, Bt kurstaki is especially useful for eliminating corn earworms and those little green cabbage worms that like to infest broccoli. If tomato hornworms are destroying your tomato plants, a dose of Bt kurstaki will quickly dispatch those big green caterpillars. Bt also acts as a natural insecticide for tent caterpillars, bagworms and webworms that damage trees and shrubs. 

Because this organic insecticide will kill most any type of caterpillar, it should be used sparingly if you have a butterfly garden or seek to attract more butterflies to your garden. Bt does not discriminate between nasty, damaging caterpillars and the caterpillars that will spin cocoons and emerge as beautiful butterflies. 


Each strain of Bt is specific to certain pests or families of insects. The san diego strain of Bt is an excellent organic insecticide for controlling the Colorado potato beetle. Potato beetles can quickly reduce potato plants to shreds, but Bt san diego stops the potato beetle larvae in their tracks before they can do significant damage.

Another type of Bt, the israelensis strain, will kill the larvae of mosquitoes, black flies and fungus gnats. This is good news for those who suffer through black fly season each summer. To control mosquitoes, Bt is placed into the standing water where mosquitoes breed. For this purpose, the natural insecticide is formulated into granules or solid rings that slowly release the Bt throughout the season. When exposed to sunlight, Bt typically degrades and becomes ineffective after about 24 hours, but in the water it remains viable for much longer. If wet conditions persist for several weeks, more Bt may need to be applied to keep the mosquito population under control. For fungus gnats, Bt is applied as a soil drench when the population becomes evident.


Bt is sold by various manufacturers as either a liquid or a powder. In either form, this organic insecticide must be eaten by the insect to be effective. Bt has no effect on adult insects, it is harmful only in when the target insect is in its immature, or larval stage. So Bt will not kill the pretty butterflies or moths in your garden, but if those butterflies or moths lay eggs in your garden and the larvae munch on plants that have been treated with Bt, they will not survive.

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Bt is however, safe to use around birds, bees and pets. Bt breaks down rapidly when exposed to sunlight and it does not persist in the environment. Even though it is safe to use, if the Bt you purchase is in powder form, you should wear a face mask to avoid breathing it in, as you would when working with any other powder.


The various forms of this natural insecticide can be purchased at garden centers, gardening catalogs and many other places where plants and gardening supplies are sold. Bt should be stored in a cool, dry and dark place. It will lose some effectiveness after six months, so buy only as much as you'll use in one gardening season.

by Michael J. McGroarty
Copyright 2011