Bacillus thuringiensis (BT) is a naturally occurring bacterium found in the soil. There are many different types, and some can be used to kill a specific insect or class of insects. When a target insect takes a bite of a plant sprayed with the type of BT the insect is sensitive to, the insect gets infected and stops feeding. Inside the insect, the bacterium releases a protein that causes the pest to die within a few days.
Each type of BT is effective only on one specific insect (or group) and only on insects that actually eat it. However, that doesn’t mean you can spray it indiscriminately. For example, the type that kills cabbage loopers can also kill the caterpillars of the beautiful butterflies you’re trying to attract to your garden. Only spray it when you know you have a pest problem, and only spray the pest-infested plants.
Most formulations of this bacterium are sold as a liquid or wettable powder that you dilute with water and then spray on the plants you want to protect. Some products are sold in the form of dusts or granules that you dust directly on plants.
Because BT usually is effective only against the nonadult stage of pest insects, you must time applications carefully. As soon as you spot the pest larvae, thoroughly coat the affected plants with the spray or dust. (For corn pests, deposit a little of the granular product into the whorl or on the corn silk.) Avoid spraying during the heat of day. BT breaks down a day or two after spraying, so you may need to reapply it if you’re up against a severe infestation. As with all sprays or dusts, always wear goggles and a mask to prevent contact with the bacterium when you apply it to your plants (there have been a few reports of allergic reactions in those who have inhaled it).
Pests controlled: The most common strain of the bacterium—BT var. kurstaki (sometimes called BT var. berliner)—kills hundreds of different kinds of caterpillars, including cabbage loopers, tomato hornworms, cabbageworms, corn earworms, European corn borers, and squash vine borers. BT var. tenebrionis (a new name—until recently this one was called BT var. san diego) kills Colorado potato beetles.
Don’t confuse these beneficial nematodes with destructive root-knot nematodes. Once inside a pest, parasitic nematodes release bacteria that kills the insect host within a day or two. Although these good nematodes occur naturally in the soil, there usually aren’t enough of them in one place to control pests that have gotten out of hand in your garden. But you can buy them by the billions for use as a living—and organic, safe, and nontoxic—form of pest control.
The dormant nematodes are shipped in a moist medium, which you mix with water when you’re ready to apply. When you receive a shipment, put the sealed container in your refrigerator until you are ready to use it (the nematodes will keep there for about 4 months). Try to use them as soon as possible, though; their effectiveness declines the longer you store them. Once the nematodes are mixed with water for application, they are only viable for a very short time. Use all of the mix within a few hours—don’t try to save any of it.
Apply the nematodes to moist soil that has reached a temperature of at least 60°F, either in the evening or when it’s overcast, at a rate of about 23 million nematodes per 1,000 square feet. Thoroughly cover the area with the nematodes, then water them in. Exception: If your pest is in the plant (the squash vine borer or corn earworm), mix up a small batch of nematodes and use a garden syringe or eye-dropper to apply them just inside the tip of the ear of corn, or into the squash vine entrance holes.
Pests controlled: Nematodes attack and invade armyworms, corn earworms, squash vine borers, soil-dwelling grubs (including Japanese beetle larvae), weevils, root maggots, and cutworms (in their soil-dwelling stages).
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