Nibbling Pests Trigger Scent Alarm

Chewing on a corn leaf spells doom for the beet armyworm, because a bite makes the plant produce odors that attract armyworm enemies. U.S. Department of Agriculture scientists reported that the female Cotesia marginiventris wasp tracks the plant-produced scents to find an armyworm in which she can lay her eggs. Eggs develop into larvae that feed inside the armyworm.

Ironically, the scents are probably manufactured in the plant to be insect repellents targeted at future armyworm nibblers. But Ted C. J. Turlings, James H. Tumlinson III and W. Joe Lewis, with USDA’s Agricultural Research Service, found that these repellents actually attract the wasp directly to the armyworm. This is the first evidence that beneficial wasps use a plant’s defensive scents to hunt caterpillars.

“We believe the same system works with other crops and with other parasitic wasps,” said Turlings, a research associate from the Netherlands. The research team hopes to make better biological control agents of the wasps by exploiting their proficiency in detecting and responding to chemical scents produced by plants. The work could give farmers biological controls for this and other pests that gobble up billions of dollars worth of crops a year, said Tumlinson, a chemist and leading expert on insect chemical communication.

Lewis, said that some of the scents could potentially be used to “simulate damage” and help a crop recruit wasps to an infected field. Most of the scent chemicals are commercially available, and those that aren’t have already been reproduced in the laboratory. Another potential is for insect rearing companies to “train” parasitic wasps to recognize the scents before they get to the field. Earlier studies by this research team showed that such training sharpens the wasps’ ability to find caterpillars. “These wasps are much better at homing in on scents if they have had previous experience associating them with a host or its feces,” Lewis said.

The training could even get as specific as manipulating wasps to focus on a specific caterpillar on a specific crop. The scientists have preliminary evidence that leaves chewed upon by different kinds of caterpillars produce different scents.

“We think the wasps can differentiate between the scents,” Turlings said. For example, C. marginiventris will attack both corn earworms and beet armyworms, but prefers an armyworm. If a corn field has both, the wasp would tend to home in on the chemicals from plants nibbled by the armyworm to locate that specific insect. But if farmers wanted them to focus on corn earworms, the “pre-release training” could be with chemicals produced when earworms feed on corn.

Tests conducted with corn plants, wasps and beet armyworms revealed that a leaf damaged by a pair of scissors or razor blade did not produce the scents. But when armyworm saliva was applied to the damaged sites, the chemicals appeared in about 10 hours, similar to when a caterpillar chews on a leaf.

This research is a continuation of work in which the scientists found that another parasitic wasp, Microplitis croceipes, locates corn earworms by tracking chemicals released from earworm feces. Now the team will explore in more detail how these two and other beneficial wasps respond to feces odors and plant defense chemicals.

For more information:

Ted C. J. Turlings or James H. Tumlinson
Insect Attractants
Behavior and Basic Biology Research Laboratory
Agricultural Research Service
USDA, Gainesville, Fla. 32604 USA