Pests Have Enemies Too!

by Robert Wiedenmann

The title of this article is taken from a new INHS publication about biological control of pests.

Several years ago, while hacking my way through the steamy sugarcane fields of Central America, intently searching for stemborer pests and nervously scanning my surroundings for the venomous far-de-lance, I thought of my colleagues in air-conditioned comfort back home who had wished me well as I departed on what some regarded as an "exotic vacation." More recently, on another of these "vacations," I would have welcomed the company of a few of these well-wishers as lookouts, as I searched for parasitized stemborers in a dry stream bed in South Africa. My host had just pointed out a spot, not 200 meters away, where a leopard attack had occurred the week before!

Exploring for insects in such exotic locations makes for some good stories for the folks back home, but the hunt, while challenging, is neither glamorous nor easy. Nevertheless, for the past century, entomologists, undaunted, have been involved in adventurous, worldwide searches for beneficial insects to recruit as mercenaries in the bug-eat-bug war of biological control.

Bugs -more properly referred to as "insects"- are only one type of several potential biological control agents provided by nature. Biological control refers to the practice of using living organisms, such as parasites, predators, or diseases, to suppress other unwanted organisms (pests). Actually, humans have relied on biological control for centuries, but it is underused today due to our increasing reliance on artificial (usually chemical) means of controlling pests. Unfortunately, artificial controls often have undesirable side effects that can damage the environment as well as attack the pests for which they are intended. Scientists are now systematically attempting to apply Mother Nature's remedies to control pests in ways that are more earth-friendly.

The Illinois Natural History Survey's (INHS) Center for Economic Entomology is part of the vanguard, spearheading the development of biological control for the state and nation. Our biological control specialists are concentrating their efforts on two groups of pests: insects and weeds.

THE MAKING OF A PEST
But how do the sugarcane fields of Mexico and Honduras or the dry stream beds of South Africa fit into the Illinois picture? Why would entomologists, hoping to control a pest insect feeding on Illinois corn or soybeans, head to these locales?

To answer these questions, it is important to understand exactly what makes a particular insect a "pest" in the first place. Currently Illinois has roughly 17,000 known insect species, but only a small number of them are ever a nuisance, much less a crop- or ornamental-devouring pest. Every one of these potential pests has many enemies, including predators, parasites, or pathogens (disease-inducing viruses or micro-organisms) that keep its population in check. When populations of native insects erupt into pest outbreaks, at least part, if not all, of the blame can be traced to human actions that disrupt natural controls. For example, converting a prairie into a cornfield may not only provide a potential pest virtually unlimited food, but also create a habitat that does not provide shelter for its native natural enemies. Left unchecked by an overabundant food supply and no natural enemies, a "pest" is born. Another human-induced activity, unwise or poorly timed applications of pesticides, can decimate natural enemy populations, allowing pest populations to explode.

Not all of our pests, however, are of the homegrown variety. The most insidious pests (both animal and plant) are exotics, those that were accidentally (or even purposefully) imported from distant lands. About 75% of pests in the United States are exotics. Some of these may be familiar to Illinois residents -the European corn borer, gypsy moth, and Japanese beetle. Ironically, these invaders are rarely pests in their native lands; it is not until they enter the United States (or another country) that they reach their full pest potential. Their new-found "pest status" is due to the fact that, when they arrive here, they are almost never accompanied by their own natural enemies -those species that kept them in check at home.

THE ENEMY OF OUR ENEMY CAN BE OUR FRIEND
The strongest weapons in our arsenal for fighting exotic pests are their natural enemies. These enemies help us recreate a balance by keeping pest populations in check. We must, however, use the utmost caution to ensure that a species imported as a natural enemy to control an exotic pest doesn't become a pest itself.

There are two basic approaches to importing enemies of exotic pests. The first is to "reunite" an exotic pest with its traditional enemy. To accomplish this reunion, we explore a pest's place of origin to try to locate the natural enemies that have historically kept it in check. (Unfortunately, these exotic pests often come from equally exotic places, and that's why we sometimes find ourselves in snake- or leopard-infested locales in our search for a pest's natural enemy.) In most cases we are looking for parasitic wasps that lay their eggs on or inside another insect; the wasp's offspring then prey on the insect from the inside out.

The second approach is to create a novel union for a pest and an enemy. In this case, we determine which of the pest's close relatives exploit similar habitats or exhibit similar behaviors. Then we seek out the natural enemies of these close relatives and bring them back to our laboratories to see if they might also keep our target pest in check.

EXTREME CAUTION ADVISED!
Once the exotic pests or their close relatives are collected, they are sealed in secure containers and shipped back to the United States. When they arrive, they are opened in a secure, quarantined facility. A period of quarantine (often many months) safeguards against any "hitchhiker" pests that could escape into an environment that has no natural enemies to keep them in check. If we are lucky, some of the pests we have collected will have been infected or parasitized by an enemy -for instance, by a pathogen or a parasite wasp that laid eggs in the pest. Once the pathogen or parasite emerges, we can begin our experiments to determine the safety and usefulness of the parasite for biological control.

Before we seek permission from the U.S. Department of Agriculture or the U.S. Environmental Protection Agency to release the natural enemy, it must be tested to ensure that it will attack the target pest and, perhaps even more important, to determine which nontarget species it will attack. The total number of species (both target and nontarget) an organism attacks is called its host range. We strive to find a natural enemy that has an extremely narrow host range- ideally, only the target species. It is crucial that we know the host range of a natural enemy, including nontarget species. We especially want to know if the host range includes endangered or threatened species that could be decimated by the natural enemy.

Scientists at INHS are using biological control against a variety of pests, including the gypsy moth, a threat to forests; the diamondback moth and imported cabbageworm, both threats to vegetables; and the Asian tiger mosquito, a pest in enclosed conservatories. In our lab we are trying to develop biological-control-agents for the European corn borer, which is a widespread pest in Illinois and across the entire cornbelt. Although biological control of the European corn borer has been tested in Illinois since early in this century, there has been little success here using traditional, introduced enemies. We are, therefore, searching for new parasites to form novel unions (the second biological control approach discussed above) with the corn borer.

We've investigated three wasps of the genus Cotesia. These tiny wasps, about 3mm long, come from far-off places. One originated in Southeast Asia and has been used successfully for 20 years to control the sugarcane borer in the Rio Grande Valley in Texas. Another comes from Japan, and the third from Kenya; each controls stemborers in its own place of origin. We found that all three species readily attack corn borer larvae by laying eggs in them, but for some reason these eggs never hatch and therefore leave the corn borer unaffected. One of my students is now investigating why the eggs do not hatch.

On the other hand, wasps of another Asian genus, Xanthopimpla, do produce eggs that hatch in corn borers. This surprised us, because Xanthopimpla was first imported into the United States more than 70 years ago but was never released as a biological control agent. It ultimately died in quarantine. Apparently the scientists who originally studied this parasite failed to recognize its unique attack strategy and mistakenly concluded that it wouldn't attack European corn borers. But indeed it does. Later importations of this wasp in the 80s, for use against the sugarcane borer, led to the discovery that it could be used against the corn borer as well.

This wasp attacks corn borer pupae by drilling with its ovipositor (a stingerlike tubeon its posterior) through a plant stem and into a pest pupa enclosed within the stem. Unbelievably, this wasp does not recognize -and will not attack- a corn borer pupa that is not inside a plant stem.

Now that we know how Xanthopimpla attacks its target pest, we are investigating its nontarget pests. So far, we know that it will also attack beet armyworm pupae that have been placed in corn stems. However, in the real world, beet armyworms do not pupate in corn stems, they do so in the soil. So, most likely, Xanthopimpla will not attack beet armyworms outside the laboratory under natural conditions.

This raises some challenging questions. Before we can test this parasite in the field, we must be certain about its host range. Although Xanthopimpla can attack several potential nontarget species in the laboratory, it almost certainly will not attack them in the field. So, to ensure that the information we gather in the laboratory will apply to the real world -and will help us make informed decisions- we must be careful how we design and interpret the host-range experiments.

FINDING THE RIGHT ENEMIES
Insect pests are not the only ones we are dealing with in our lab. We are currently closing in on biological control agents for the invasive, exotic wetland weed, purple loosestrife. This weed grows in the northern part of the state and, in late summer, covers acres and acres with its colorful blossoms. Unfortunately, it is causing major problems in Illinois' few remaining wetlands. A single loosestrife plant can produce as many as 2.7 million seeds, which can be carried by water and may germinate years later.

Purple loosestrife is a perfect candidate for biological control because it is perennial, is restricted to wet and stable habitats, and is not closely related to an agricultural crop or an endangered plant species. Furthermore, conventional means of eradication do not appear to work against it. We are currently working with several species of exotic beetles to control this weed. Two species of leaf beetles feed on the foliage and growing tips of the plant, one weevil (weevils are a type of beetle) feeds inside the roots, and two weevils feed on the flowers.

Again, because we are working with exotic natural enemies, we have to be sure we are not releasing a species that will create a worse problem than the pest itself. Before importation, scientists with the International Institute for Biological Control, the U.S. Fish and Wildlife Service, and U.S. Department of Agriculture tested these beetles for host specificity. All five species passed the specificity tests with flying colors. In fact, when the leaf-feeding beetles were caged (in a no choice test) with winged loosestrife, the closestrelated plant, the beetles fed very little, laid only a fraction of their potential number of eggs, and lived only a short time. This means that when the weed is controlled, the beetles will not switch to feeding on another plant species. It was only after each species had passed the specificity tests that they were imported into quarantine and permission for release was obtained.

INHS, with funding from the U.S. Army Corps of Engineers, is now rearing the leaf beetles and root weevils for release in a large field trial. Because purple loosestrife is a problem from the Atlantic to the Pacific, this is part of a nationwide effort.

Realistically, even with biological control we will never be able to call a truce in the war on insect and weed pests. Each pest is unique and requires a creative approach and, furthermore, there will always be new pests to challenge us. That is one of the joys of working with biological control. We are trying to recreate nature on a small scale. But, unlike nature, we don't have the luxury of letting species sort themselves out over long periods of time. We have to get it right the first time. Whether we're looking at reuniting an exotic weed with its herbivorous natural enemies, or using parasites against insect pests, we still have to find the "enemies" and go through the careful testing procedures, not just to ensure winning individual battles, but also to ensure the continued success and acceptance of the earthfriendly strategy of using biological control.