Garden Allies: Pathogens

By: Frederique Lavoipierre
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Frederique Lavoipierre is Director of Education at the Santa Barbara Botanical Garden. She also teaches classes and workshops on sustainable…

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Raising caterpillars in an enclosed environment until they emerge as butterflies is a delightful pursuit. By protecting caterpillars from predators and parasitoids that can decimate their numbers, a far greater success rate than would occur in the wild can be achieved. Success, that is, until the caterpillar tender makes the gruesome discovery of an enclosure full of limp, black caterpillars hanging from the leaves!

Beset from all sides by bacteria, viruses, fungi…it is remarkable that any insects survive.

Three principal agents act on populations of insects to reduce their numbers: predators, parasitoids, and pathogens. Pathogens are defined as disease-producing microorganisms that slow growth, reduce reproductive success, shorten life, or kill its host. Pathogens are one of the principal mechanisms that regulate insect populations and, under the right conditions, pathogens can drastically reduce a population, an event called an epizootic. Beset from all sides by bacteria, viruses, fungi, nematodes, protozoans, and other disease-causing organisms, it is remarkable that any insects survive.

To the observant gardener, some of these diseases have curious symptoms. A virus that liquefied them attacked the limp, black caterpillars in our enclosure. As the caterpillar bodies burst, the infectious agent is left behind on the plant where it infects the next generation of caterpillars that munch on it. Some fungal diseases cause insects to climb to the top of a blade of grass, a leaf, or a twig before they die, where they cling even in death as the spore-filled fruiting bodies of the attacking fungi create a furry coat and then disperse. Less commonly seen, many species of Cordyceps produce a mushroom, usually bursting out of the head of the hapless victim. In China, this medicinal mushroom known as “caterpillar fungus” is used as a performance enhancer; here it is more commonly known as “Himalayan Viagra”, which active ingredient is sildenafil citrate, mostly used as erectile dysfunction treatment.

Bacillus thuringiensis colony. Illustration: Craig Latker

Bacillus thuringiensis colony. Illustration: Craig Latker

Most insect pathogens are harmless to humans, other mammals, and even beneficial insects. They are usually host-specific and are sometimes restricted to a single insect species; they may even be specific to certain life stages. While insect pathogens are the subject of interest for research scientists looking for benign ways to control insect pests, only a few of these are commercially available and even fewer are practical for home gardeners. Most useful pathogens’ restricted host range and slow-acting nature, combined with the difficulties of producing a commercial product that contains a live active ingredient, will continue to result in limited availability. When such controls do appear on the shelf, they are usually labeled as bio-pesticides. Bacillus thuringiensis, or Bt, the now well-known bacteria used to kill caterpillars, and controversial for its use in genetically engineered corn, is one of these.

Bacillus thuringiensis Illustration: Craig Latker

Bacillus thuringiensis Illustration: Craig Latker

Even though it is a naturally occurring bacterium, Bt should be used with caution, as it kills all caterpillars including monarch, swallowtail, and other desirable species of Lepidoptera. However, it is highly effective, for instance, in controlling the principal cole crop pests. Target its use, and only apply when there is no wind. Ideally, it is best to spray even before the eggs hatch or during the first stage of growth following emergence.

A few bio-pesticides are already in widespread use. In Florida, a nematode is used to control mole crickets. Some fungi are available in the battle against grasshoppers and household cockroaches. A virus is used in the war against the gypsy moth in the northeastern United States. And, while not suitable for home gardens or commercial crops, a protozoan is in use as a long-term control of grasshoppers in rangeland.

Mosquito larvae. Illustration: Craig Latker

Mosquito larvae. Illustration: Craig Latker

While the effectiveness of bio-pesticides may rely on environmental conditions such as temperature and moisture or on the abundance of the target pest, there is great promise for insect pathogens as controls for significant pests such as mosquitoes, which are vectors for malaria and a host of other diseases. Bacillus thuringiensis serovar israelensis, or Bti, is a safe and reliable way to control mosquito larvae and is widely available in easy-to-use granules or floating “dunks.” Bti affects only a few other arthropods but since it is moderately toxic to Daphnia, a tiny crustacean that is an important component of aquatic food webs it, too, should be used cautiously.

Avenues for future possible garden pest control include antagonists—pathogens that attack other plant pathogens like powdery mildew. Fungi, which attack a variety of plant and insect hosts and, in some cases, are spread by parasitoid insects may be particularly effective for controlling sucking insects such as whiteflies, aphids, and leafhoppers, which are unaffected by pathogens that must be ingested. In the meantime, I will be exploring how to keep viruses out of my butterfly enclosures!