Taking advantage of nature’s weapons

Photo by Stephen Ausmus, USDA Web site.
In a strawberry patch next to an apple orchard, Technicians David Roys, left, and Catherine Peters, along with Entomologist Tom Unruh, inspect plants for strawberry leaf rollers, insects they hope will provide a winter home for Colpoclypeus florus, a beneficial parasitic wasp that attacks apple leaf rollers in spring.

In the 1860s, cottony cushion scale was found on some acacia trees near San Francisco, which spread rapidly to the citrus groves in Southern California. “It became incredibly devastating,” says Tony Shelton, professor of entomology with Cornell University College of Agriculture and Life Sciences NYS Agricultural Experiment Station in New York. C.V. Riley, chief of the USDA’s Division of Entomology at the time, discovered that the vedalia beetle was keeping the population in check in its native Australia, so he brought back 514 of the beetles in 1888-1889 and released them in Southern California. A year or two later, the scale was under control.

The beetles are still an important control factor, Shelton says. “Riley only introduced 514 vedalia beetles, and we’re still reaping the benefits.”

Since then, whenever a new pest species is discovered in a new environment, researchers trace it back to its center of origin to find its natural enemies. They’ve developed a system for identifying and maintaining the safest and most effective ones.

In the importation or introduction phase, according to the North Carolina State University Department of Entomology (NCSU) Web site, researchers look for natural enemies that are present when the pests first appear, can find pests when they’re scarce, consume them rapidly, and survive when the pest population is very low. They also lay many eggs quickly.

They’re tested to ensure that they’re specific to the pest species and compatible with the new habitat so they don’t become pests themselves, Shelton says. Then, they’re registered for use, and raised and released in the hopes that they’ll become established and suppress the pest.

In the augmentation phase, if a beneficial is doing well in controlling the pest, but is unable to survive or persist on its own, it is raised in large numbers, often in a tent-like structure called an augmentorium, and released into the area.

PHOTO BY KEITH WELLER, USDA WEB SITE.
A researcher examines roots of a leafy spurge from Russia for pathogens that could be used to control leafy spurge in the United States.

In the conservation phase, already present beneficials are protected. In the early 1980s, there were outbreaks of the diamondback moth, whose larval stage is very damaging to cruciferous vegetables. The moths can be held in check by a parasite, but a new class of insecticide was very toxic to the parasite while at the same time becoming less effective on the diamondback moth. It was a worst-case scenario.

“We can develop tactics to conserve beneficial insects most of the time,” Shelton says, including reducing or eliminating insecticide applications, or using more selective ones, to avoid killing beneficials. Cultural methods should be used to keep the pest population down, like plowing down crops at the end of the season. If a crop does have insects or diseases, get rid of it as soon as the harvest is finished so pests don’t increase. Shelter and alternative food sources should be provided for natural enemies during the off-season.

There’s no shortage of beneficials. A wide variety of insects, mites and pathogens, including bacteria, viruses and fungi, kill invasive plants and insect pests. Some kinds of vegetation, especially brassicas, are believed to control fungi and nematodes.

Insects that control invasive plants

Vegetation-eating insects weaken or injure plants. According to the Oregon Department of Agriculture, the cinnabar moth and the flea beetle control tansy ragwort, an invasive plant that’s toxic to livestock, the golden loosestrife beetle controls purple loosestrife, and seedhead weevils control knapweeds.

PHOTO BY SCOTT BAUER, USDA WEB SITE.
A site coordinator baits traps that catch fruit flies in this augmentorium. The flies that emerge from the infested fruit are trapped inside. Beneficial parasitoids that emerge from the fruit fly pupae escape through the yellow screen at the top of the tent and parasitize a new generation of fruit flies.

Researchers at NCSU found that Chrysolina quadrigemina, a chrysomelid beetle, has largely eliminated klamath weed, also known as St. John’s wort and goatweed, which is poisonous to livestock, in the western United States.

According to the Texas A&M University’s Department of Entomology Web site, a flea beetle, Agasicles hygrophila, is controlling alligatorweed in Texas waterways. Also, puncturevine weevils have reduced puncturevine in west Texas, although it’s still a pest in a number of other states.

Animals and insects that control insects and mites

Many animals, including birds, amphibians, reptiles, fish and mammals, feed on insects. Beneficial insects, such as lady beetles, ground beetles, rove beetles, flower bugs, lacewings and hover flies, as well as spiders and some families of mites, consume both insects and mites.

Some insects, such as parasitic wasps, lay their eggs on the inside or the outside of a host. The young feed on the host, eventually killing it. They emerge as adults and continue the cycle. Hosts include aphids, beetles, caterpillars, flies, sawflies, scale insects and true bugs. Trichogramma wasps primarily parasitize caterpillar eggs, according to Texas A&M.

Pathogens that control insects and invasive plants

Bacteria, fungi and viruses cause diseases that reduce the rate of which pests feed and grow. They slow are prevent their reproduction or they kill them outright, according to the University of Maine Cooperative Extension Bulletin #7144.

  • Bacteria—“Of the total insecticide market in the world, only 1 percent is microbial,” Shelton says. The leader is Bt (Bacillus thuringiensis), which contains a protein that causes caterpillars to starve to death. They have to feed on the leaf exactly where the droplet is, though, and it’s hard to get even coverage on all the leaves.
  • Fungi—According to the Sustainable Agriculture Research and Education (SARE) Web site, most insect-pathogenic fungi invade their hosts through openings in their cuticle, so they don’t need to be consumed. Compared to other pathogens, they have an extensive range of hosts.

Beauveria bassiana causes white muscadine disease, which covers its victims with a layer of white mold. It controls pests such as whiteflies, aphids, Japanese beetles, Colorado potato beetles, grasshoppers and European corn borers, as well as beneficials like lady beetles.

Another fungus, discovered by researchers at the University of Florida Department of Plant Pathology, is an isolate of the pathogen Dactylaria higginsii, which controls a number of sedges, including purple nutsedge, yellow nutsedge, annual sedge, globe sedge and green kyllinga. The infected leaves die back, spores form on them and a secondary infection begins within 20 days.

  • Viruses—Most viruses attack young larvae of butterflies and moths, according to SARE. They usually need to be eaten by the insect, but may spread in other ways. They interfere with feeding, egg laying and movement.

The ones most commonly used for pest control are nuclear polyhedrosis viruses. According to the Cornell site, they affect the gypsy moth, beet armyworm, corn earworm, imported cabbageworm and almond moth.

  • Nematodes—According to SARE, nematodes transport disease-causing bacteria inside their host, and then the bacteria kill the host. The nematodes use both the bacteria and the host’s body for food and shelter, where they mature, mate and reproduce. When young nematodes emerge, they search for another host. Because they’re highly mobile and can locate and destroy victims within a few days, nematodes make excellent candidates for biocontrol.

Steinernema carpocapsae is especially effective against webworms, cutworms, armyworms, girdlers and wood-borers, according to Cornell University’s Web site.

Vegetation that may control fungi and nematodes

Brassicas include members of the broccoli and cabbage families, as well as the mustards. They all contain glucosinolates, which are believed to be effective against fungi and nematodes. Plants with higher levels, such as Oriental mustard, are believed to be the most effective.

According to Michigan State University’s Department of Crop and Soil Sciences, brassica cover crops may reduce or suppress Verticillium in potato; Pythium, Fusarium and Rhizoctonia root rots in beans; Pythium in lettuce; Aphanomyces, Pythium, Rhizoctonia and Fusarium root rot in peas; and cavity spot and Fusarium in carrots.

Considerations

“There are a lot of limitations to biocontrol,” Shelton says. “Still, everything’s got a plus and minus side to it, and where there is potential, the benefits can be enormous.”

The environment can determine whether or not beneficials succeed. They work best in stable environments like greenhouses, but people may not want to see any insects at all on high-value crops and flowers, even if they are beneficial. Sunlight tends to break down viruses and bacteria, so if they’re sprayed in a field when it’s sunny, they may be effective for only a few days. Fungi need a cool, moist environment to germinate.

Time is also a factor. Biocontrols work better in fields with perennial crops than annual ones because often they take more time to become established and suppress pests than other kinds of pesticides. Pathogens can take three or four days or more before they become effective. Parasites can attack an insect long before it eventually kills it. On the other hand, although natural enemies usually take longer to become effective, they’re often self-perpetuating.

Even their specificity can become a limitation. When there are multiple pests, biocontrol often can take care of one pest, but not the others.

“Other kinds of pesticides will always be useful,” Shelton says. “What we try to do from an environmental standpoint is try to use as little as possible to allow other tactics like biological control to come into their full potential.”

The author is a freelance writer based in Altadena, Calif.