From IPM in the South
By Rosemary Hallberg
Research by a doctoral student at Auburn University into how certain insect parasitoids “sniff out” their prey will help farmers better utilize them to control insect pests.
Tolulope Morawo, a Ph.D. student at Auburn University, won a Friends of Southern IPM Graduate Student award for his research on two parasitoid species (parasitic wasps) that attack caterpillar pests. His goal is to discover how these parasitoids find their hosts through olfactory responses.
“The first thing that chemical ecologists want to do is to make sure that there is a behavioral response of an insect to whatever cues we are testing,” Morawo says. “It might be odors from a plant or odors from the insect host. .”
The parasitoid species, Microplitis croceipes and Cotesia marginiventris both attack tobacco budworm, Heliothis virescens, on several crops including cotton and soybean. Microplitis croceipes specifically attacks Heliothis and Helicoverpa species, while C. marginiventris is more generalist and attacks most noctuid caterpillars that are pests of cotton, tobacco, soybean, corn, and vegetable crops.
Morawo uses various cotton volatiles (scent chemicals) to try to attract the parasitoids. Using a branched test tube setup that looks like a large Y, he attaches one tube to a jar with a caterpillar-damaged plant and the other tube to a jar with no plant in it. Then he places the parasitoid in the main branch and records which tube the parasitoid chooses.
“What we’re trying to test is the attraction of the parasitoid to the odor emitted by the plant,” he says. “In some cases the insect will choose the treatment; in other cases the insect will choose the control. We just have to record it.”
When only a mechanically damaged leaf is in the treatment tube, as he showed me during a visit, the parasitoid may not choose the tube with the damaged leaf scent. Most of the time, Morawo said, the parasitoid is really looking for the pest insect.
Morawo also tested whether parasitoid species were attracted by scents of freshly damaged leaves versus leaves that were being eaten over a longer period of time. His results showed that the freshly damaged plants seemed more attractive to the generalist parasitoid while the old damaged plants seemed more attractive to the specialist parasitoid.
The next step is to see whether or not the insect reacts to some of the compounds emitted by the plant. Using an electroantennogram, he finds out whether or not the insect has a reaction to any of the compounds.
“The EAG doesn’t tell us whether or not the insect is attracted or repelled; it just tells us if the insect has a reaction to that particular compound,” he says. Any results from that test must be further tested in behavioral experiments with an olfactometer.
He then uses a gas chromatography- mass spectrometer (GC-MS) to identify different chemicals emitted by a plant.
“The GC-MS will identify the spectra from different compounds,” he says. “It will tell us not only the identity of each compound; it will tell us also how much of each compound is emitted by the plant.”
In a recent publication in Journal of Chemical Ecology, he reported that caterpillar pests become more attractive to parasitoids after feeding on plants. The caterpillars emit chemicals that help parasitoids locate them at short distance. As a result of his research into olfactory responses, Morawo has identified two major plant-associated compounds that attract M. croceipes to H. virescens (tobacco budworm) after it has fed on cotton. The interesting thing about this finding is that the same compounds have been reported to help some blood-feeding insects to locate their mammalian hosts. He noted that other not-so attractive components of the chemical blend may provide context for the parasitoid in identifying its hosts.
His research on how these two parasitoids respond to odors will aid other researchers in their quest for synthetic lures to attract natural enemies of pests or lead pests away from the cash crop. Findings such as Morawo’s will also be crucial in helping sustainable and organic growers minimize pest infestation in their crops.