“Natural pesticide” is a confusing term that people often encounter in their search for safer alternatives for their gardens. In this article, we use the term to refer to material found in nature that is minimally altered by humans and not likely to be harmful to you or your garden. Most natural pesticides are the desirable choice because they are nontoxic to humans, other mammals, and nontarget organisms, and are specific to the few insect pests for which you might want to use them. They can be derived from microbes or from plants and should be handled with the same safety precautions you would use with any other pesticide. Bacillus thuringiensis (Bt) is a microbial pesticide that illustrates how natural pesticides can be safe and effective for the gardener.

Microbial diseases kill insects much the way they do humans. The pathogens that cause these diseases often affect only a specific host, or group of insects, and don’t attack others. The Bt products sold in stores are commercial formulations of the naturally occurring microbial organism. The organism has been taken directly from the wild and is not genetically altered by humans. Most of the formulations you will find for retail sale indicate they contain Bacillus thuringiensis var. kurstaki. This strain is limited to controlling certain caterpillars.

Other available strains of Bt are effective treatments against larval beetles and some flies. For example, incorporating B.t. var. israelensis in the soil can kill fungus gnats in greenhouse soil or in potted indoor plants. One strain, B.t. var. san diego, can be used to kill the Colorado potato beetle. The package label will say for which insects the product is effective.

How Bt works

The Bt microbe actually contains two parts: an active spore and a thick-walled storage spore that includes a toxic protein crystal. Once the Bt is ingested by a larval insect, the walls of the storage spore dissolve, releasing the toxic crystal. The toxin destroys the gut wall of the insect, permitting the active spore to pass into the blood stream, where it multiplies. The insect then dies from blood poisoning. The pH of the insect’s gut (how alkaline or acid it is) is critical to dissolving the walls of the storage spore. Not all insects have the same acidity in their gut, and this is why some insects are susceptible to Bt poisoning and others are not.

A number of common vegetable and fruit pests are difficult or unpleasant to suppress by handpicking, washing off with water, or treating with soap sprays. These include about 150 species of pest moths and butterflies that are susceptible to Bt in their larval stage, including tomato hornworm, corn earworm, cabbage looper, imported cabbageworm, and the diamondback moth.

A perfect example of a pest for which Bt is effective is the tomato hornworm. In spite of their large size, the larvae are difficult to see. They are perfectly colored to blend with the tomato plant, and you can look right at one and not see it. Instead, you can locate it by the characteristic barrel-shaped fecal pellets on the leaves and then look directly upward to find the caterpillar. It can be unpleasant to handpick tomato hornworms because of their size and the stubborn way they resist being pulled off the plant. Fortunately, using a Bt spray is a safe alternative if hunting and hand picking is unappealing or too time consuming.

In addition to being a convenient way of controlling pests, Bt is also a good way to maintain the natural equilibrium of the ecosystem in your garden. Biological enemies, from predators to parasites, are vital to the long-term balance of pest control in the garden. They are not always apparent to the gardener because they are under good natural control, but they play an important role in the proper functioning of the ecosystem. If you can control the pest and still leave its most important natural enemies alive, you have a superior pest control tool because these enemies will help keep pest levels below a reasonable threshold.

Most insecticides on the market, including insecticidal soaps, kill a wide range of insects, including many predators and parasites. Once these natural enemies are killed, the pest can resurge and reestablish itself before its enemy can do the same. You can also trigger a new, or secondary, pest problem if the product you use kills off natural enemies of other pests. The initial pest problem may be resolved but there could be a new one in its place. Bt is a superior least-toxic pest control product because it targets the pests only and doesn’t kill the natural enemies. It works with the existing ecosystem and won’t cause secondary pests.

Monitor to determine application time

Proper timing of the application of Bt is important, so to get it right you need to monitor daily. Since Bt is a stomach poison, the insect must eat it for it to be effective. This means you must treat caterpillar pests when they are actively feeding. Once the caterpillars are fully grown, they usually move off the surface of the plant and stop feeding, in preparation for making a cocoon. The Bt spray must be deposited on the leaf surfaces after the eggs have hatched but before the larger larvae move on to pupation sites.

You will usually find egg clusters on the undersides of leaves. If you find a group of small caterpillars that has recently hatched, look for the eggs nearby and learn to recognize them. Then look around and see if you can spot more egg masses. Flag those you find by tying a ribbon or something bright around the plant at that point. Check those masses each day for hatching. This will give you a sense of what is happening to all the other egg masses you don’t see. Wait until the last flagged mass has hatched before treating the caterpillars.

Many caterpillar species feed on the undersides of the leaves in the first stages of their life, when they are most susceptible, so position the spray nozzle so the undersides of the leaves can be treated. A gentle spray is all that is needed. Larger (older) caterpillars eat through both sides of the leaf, so with them, spray placement is not so critical. Larger larvae will require a higher dose to kill them, so an extra pass with the nozzle or a reapplication in a few days may be necessary to get most of them. However, don’t worry about killing all the pest larvae; it’s usually impossible. It’s also inadvisable, since leaving a few pests is one way to keep some natural enemies alive.

A living organism as a wettable powder

Bt products available in garden shops are commonly sold as wettable powders that can be mixed with water and sprayed on plants with conventional garden spray equipment. A wettable powder goes into solution easily and has a long shelf life. Once the powder is mixed with water, however, it will last for only a few days, so prepare only what you think you will use and don’t try to save it for another time.

Because Bt is actually a living organism, the pH of the water used to make the solution is important. Many failures with Bt happen because it is mixed with hard water (which is alkaline and high in calcium). If your water is very hard, you are likely to know this already because of the calcium deposits left behind when it evaporates or boils away. If you are not certain about how hard your water is, put some in a jar and add ordinary soap (not detergent). If it is soft enough to use for spraying Bt, lots of bubbles will form when you shake the jar. If your water is hard, buffer it with citric acid before using it for a solution. Some mail-order supply houses will carry buffering or acidifying materials, as may garden shops, especially if the local water supply is known to be hard.

Other additives for use with Bt products include nontoxic feeding stimulants, which encourage caterpillars to feed on plant leaves. This addition can be useful with some harder-to-kill species or later-stage larvae, since they need to ingest more Bt for effective control.

Although Bt has a reasonable shelf life in the dry form, keep in mind that the dry powder is a composed of living spores. Therefore, it is advisable to buy from a shop that seems to be selling the product regularly, so you know it is unlikely to have been sitting around for a long time. Store the product according to the recommendations on the package.

Bt, like most natural pesticides, is generally regarded as safe, but you should still follow all the safety directions on the label regarding mixing, using, and cleaning up afterward. The unmixed portion of the package should be placed in an airtight container and stored in a cool, dry place.

This article originally appeared in Kitchen Gardener #28 (August 2000).

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