Becoming biorational

If gardening and farming involved nothing more than sticking plants in the ground, watering them now and again, and pulling some weeds, no one would ever spend a dime on what might be called Stupid Chemical Tricks.

Farmers didn’t adopt DDT in hopes of thinning raptor eggshells and nearly decimating the American bald eagle, the peregrine falcon and other hawk relatives; they used the magic bullet because it knocked the stuffing out of pests that were chewing their way through the season’s profits.

Dieldrin is another sneaky pete that proved to be nearly as good at killing people as insects, but it enjoyed wide popularity among bug-battered growers for upward of 20 years.

While organic growers with small gardens or abundant uncommitted hours may manage insect invasions by hand-picking bugs, those who tend larger parcels not unreasonably seek weapons of mass disruption to fend off the voracious hordes.

Enter biorational pest control.

The appellation is self-explanatory, suggesting a thoughtful approach to plant protection that’s rooted in the logic of life. Just as soft-energy theorist Amory Lovins has argued that using nuclear fission to generate electricity is like cutting butter with a chain saw, biorationalists view Stupid Chemical Tricks as roughly akin to destroying a village in order to save it.

Biorational controls include bacteria, insect-growth inhibitors, insecticidal soap, horticultural oil and botanicals.

The most commonly used bacterial bug killers are those containing bacillus thuringiensis, which have a short residual activity and are specifically toxic to a handful of species. The toxin produced by this bacterium disrupts caterpillar metabolism. Susceptible larvae that consume material containing Bt more or less die of starvation. Bt breaks down quickly in sunlight, so repeated applications may be necessary to combat an ongoing outbreak of hornworms, corn earworms, cabbage worms or other targeted munchers.

Even the most biorational idea can be carried to an irrational extreme, as demonstrated by genetic engineers who have created crop species that produce their own Bt. Introducing the toxin on a widespread basis has had two unforeseen effects. Target species have rapidly developed immunity to the toxin due to constant exposure combined with the relentless power of natural selection and evolution. And nontarget species like monarch butterflies have been poisoned by pollen drift from Bt corn crops, threatening a beautiful insect whose presence boosts tourism on the Blue Ridge Parkway as well as in Mexico (where they winter).

Growth regulators are used to control aphids, whiteflies, mealybugs and scales. These substances either prevent maturation of larval forms or trigger premature molting, which disrupts the insect life cycle and causes early death. These substances are not poisons; they have no toxic effect on adult insects, though some types cause adult females to lay infertile eggs.

Insecticidal soaps and horticultural oils are contact controls that must be sprayed on the target animal to kill it. Soaps erode the waterproofing on an insect’s exterior and cause desiccation — the bug’s internal moisture evaporates, and then it dries to death. Oils clog an insect’s breathing apertures (spiracles), and the invader suffocates. Both substances are broad-spectrum, killing most insect species that they touch, whether adult or immature and during any season. The stronger “dormant oils” are often applied in winter to kill overwintering insects and eggs.

Horticultural oils should not be applied too frequently and never at temperatures above 80 degrees Fahrenheit. (When in doubt, follow label directions.)

Diatomaceous earth is another nontoxic insecticide that kills through desiccation. Diatoms are tiny, ocean-dwelling creatures whose skeletal structures are made of silica. Dead diatoms pile up on the ocean floor, which eventually becomes dry land when the earth’s crust heaves and rises, doing its tectonic thing. The dust in a diatomaceous boneyard is microscopically sharp. While it feels like flour running through one’s fingers (and is entirely edible, if unpalatable), it scratches the waxy surface of insect carapaces and terminally dehumidifies a wide variety of bugs, including ticks, fleas, roaches and carpet beetles, to name just a few.

There’s even a solar-powered solution to some pest problems, particularly in fields beset by pathogens or infestations of insects that spend part of their life cycles in the soil. Solarization — most effective when the daytime high temperature is at least 85 degrees Fahrenheit — involves clearing surface plant material and tilling the soil to level the treatment area. The entire area is then covered with a clear plastic sheet and the perimeter secured with soil.

When the cover is left in place for at least four weeks, the localized greenhouse effect will drive soil temperatures up to at least 140 degrees Fahrenheit. Most weed seeds, pathogens and insect eggs in the top inch of soil will be killed. Subsequent planting should be done without tilling to avoid bringing up pests beneath the one-inch kill zone — some of which will continue to die off as their normal annual cycles expire without the customary seasonal wake-up call.

Several varieties of beneficial nematodes have been identified and are now available commercially. Nematodes have no connection to toads (or, for that matter, to Nemo, the lost Disney clown fish). Rather, they are microscopic worms that infest and feed on more than 250 different kinds of pest larvae. They consume their prey from the inside out. While beneficial nematodes occur naturally in every landscape, introducing extras will help beat back Japanese beetles, June bugs, black-vine weevils, cabbage-root maggots, corn-root worms and the insidious-sounding filth-fly maggot.

Does anyone love filth flies? (Other than, presumably, others of their ilk — and, perhaps, Howard Stern?)

Equally biorational is the practice of introducing or encouraging predator species. Ladybugs, big-eyed bugs, damsel bugs, green lacewings, rove beetles and soldier beetles view aphid infestations as all-you-can-eat smorgasbords, while braconid, tachnid, ichneumon and trichogramma wasps regard caterpillars the way dancing-pig billboards portray barbecue pits. (For more on pest predators, see “Bug Buddies,” July 5 Xpress.) Experts say that a profusion of flowers — both annual and perennial — is one of the best ways to encourage beneficial insect species.

Crop rotation is yet another biorational approach to pest management. It’s particularly useful in connection with a few families of plants that have common enemies. Cole crops — including broccoli, brussels sprouts, cabbage, collards and kale — should be rotated annually. Similarly, the solanaceous crops (such as eggplant, pepper, tomato and potato) will do better if not replanted in the same soil year after year. And with cucurbits — including cantaloupe, cucumber, gourd, watermelon and squash — the best approach is to skip a year between plantings.

Squash bugs, in particular, are very species-specific. The adults overwinter under debris, emerging the following spring to follow the plants’ scent to a food source. By alternating years, you can force re-emerging adults to exit your garden in search of a meal. Vine borers — which infest all of the cucurbits — lay eggs in the soil, and the alternate-year program helps ensure that emerging larvae won’t find a suitable host plant come spring.

A rational approach to gardening is all well and good, but getting biorational is better still. And remember that any sort of pesticide works better if used sporadically, since a steady diet of any one toxin is apt to engender resistance among the survivors.