Illinois Natural History Survey - University of Illinois

Using Natural Enemies for Pest Control

Biological control means intentionally using natural enemies--predators, pathogens, or parasites (or parasitoids)--to control pest populations. Biological control has been successful against many pests, including stemborers, the species on which we focus our laboratory work. Biological control approaches include reuniting accidentally introduced pests with their exotic natural enemies, as well as creating novel associations by using a natural enemy that attacks a host that is closely related to the target species. We are trying to understand how novel-association natural enemies operate, how to use them effectively, and how to use these natural enemies safely so they do not harm nontarget species. Our goal is to develop tactics to use against stemborers, including the European corn borer, a serious corn pest in Illinois.

Stemborers attack grass crops, such as corn, sorghum, rice, and sugarcane. In our lab we have three species of Old World wasps (Braconidae) in the genus Cotesia that attack stemborers in those various grass habitats. One of these, Cotesia flavipes, originally from Asia, has controlled sugarcane borers in Texas for over 20 years. The other species are from Japan and Kenya. The adult wasps are about 3 mm long, live 1-2 days, and insert, in a matter of seconds, 30-80 eggs into a host larva, inside which the parasite larvae develop. After 12 days, the parasite larvae chew their way out of a dying host and spin cocoons. A week later, adult wasps emerge from the cocoons and search for a new host.

Our lab is trying to determine the kinds of cues parasitic wasps use to find habitats and hosts, and to understand the physiological interactions between parasites and their hosts.

Parasites use cues that allow them to find a habitat in which their hosts reside. By responding only to certain cues, parasites do not search aimlessly, but limit searching to appropriate habitats. Our braconids respond to volatile chemicals emitted by plants used by stemborers, whether stemborer hosts are present or not. One Ph.D. student, Claire Rutledge, exposes wasps to cues from different crop and noncrop grasses, and to nongrass plants, to see if the wasps prefer certain plants. She has found that each wasp species has its own likes and dislikes. Each parasite finds certain grass crop plants more attractive than others, is equivocal about some native grasses, and definitely finds some nongrass plants unattractive. This allows us to hypothesize that each species will use only habitats containing grasses, but that each species will prefer a different habitat. Next, we will add stemborer larvae to the test plants to see if less-attractive plants become more attractive when stemborers feed in them. Knowing plant preferences and the degree of habitat fidelity the parasites show allow us to predict whether these natural enemies can negatively impact nontarget species.

wasp.gif
Braconid wasp, a stem borer parasite, searching near entrance of tunnel inhabited by stem borer larva.

A second Ph.D. student, Marianne Alleyne, is testing what happens after the parasite oviposits into a host. Several outcomes of parasitization can occur: the parasite uses the host successfully (parasite wins), the host kills the parasite progeny (host wins), or intermediate outcomes in which neither host nor parasite wins. We have seen all of these outcomes, indicating differences in physiological compatibility between the wasps and stemborer hosts. For parasitization to succeed, attacked hosts must provide parasite progeny with sufficient nutrition for development and a safe place to develop. On the other hand, some hosts can kill the parasite progeny by a process called encapsulation. Hosts have circulating blood cells (hemocytes) that initiate an immune response when the host is invaded by parasite eggs. Encapsulation is one immune response that hosts use to defend themselves against parasites. In turn, some parasites can defuse the immune response by interfering with the ability of the hemocytes to recognize and encapsulate parasite progeny. It is this interplay of response and counter-response that Marianne will tease apart to find mechanisms that determine the different outcomes of parasitization and test whether different hemocyte types are responsible for differences in the immune response.

Both avenues of research may seem basic and possibly esoteric, whether determining the roles of hemocytes or the kinds of plant odors parasites like. Yet these kinds of basic research are what we must pursue to develop applied solutions. Following both lines of research will allow us to predict both the utility and safety of using certain parasites against specific stemborer hosts. Parasites have a range of hosts they can and will attack, whether that is determined ecologically--by choosing only certain habitats, or physiologically--by developing only in certain hosts. Our task is to determine the physiological and ecological host ranges and to develop the means to deploy parasites against those hosts. Only with those kinds of information can we make informed decisions and predict the safety and utility of using natural enemies for biological pest control.

Robert Wiedenmann, Center for Economic Entomology



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Last Modified 7/03/96



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