Caterpillars Foiled When Tomato Plants Summon Parasitic Wasps

Scientists have known for the past two decades that many wild and agricultural plants launch an immune-like chemical defense when attacked by insects. That chemical resistance response can make the plant a poorer food for the insect and it may send out an aromatic SOS that hails the insect's natural enemies.

In a recent study of tomato plants, beet armyworms and parasitic wasps, Jennifer Thaler, a UC Davis postdoctoral fellow, found that wasps on tomato plants whose defense systems had been artificially stimulated killed twice as many caterpillars as did wasps on untreated plants. Thaler, who conducted her study with UC Davis entomology professor Richard Karban, will report her findings in the June 17 issue of the journal Nature.

The finding provides important information for developing environmentally friendly agricultural pest control methods.

"This confirms researchers' assumption that a plant's resistance mechanism provides a net benefit to the plant by increasing the effectiveness of natural enemies against plant-eating insects," Thaler said. "Next we'll want to see how we can make use of this mechanism for large-scale pest management in production agriculture."

"Because these responses can be manipulated using elicitors -- chemicals that can be sprayed onto fields of plants to stimulate natural defenses -- we may be able to reduce the use of traditional pesticides," she said.

Thaler chose to study the resistance response in the tomato plant, an important California agricultural crop that has been chemically well-characterized in previous research.

One of the tomato's common pests is the beet armyworm, a greenish 1-inch-long caterpillar that feeds on tomato leaves and fruit. Thaler was curious how effective the tomato plant's resistance response was in defending the plant by calling in the beet armyworm's natural enemy, a tiny parasitic wasp.

The chemical resistance response in a plant is technically known as the "octadecanoid pathway," a complex chemical chain-reaction that is triggered when an insect feeds on the plant. A wound from the insect signals the plant to produce a chemical known as "jasmonic acid," which in turn causes increased production of chemicals responsible for the leafy green odors of plants.

"Wasps can smell those compounds through their antennae and can more easily find the caterpillars when the caterpillars are less than 1 centimeter long," Thaler said. "The plants are essentially sending up a chemical 'smoke-signal' to attract the wasps."

With its ovipositor, the wasp pierces the caterpillar's flesh and places a single egg inside. A wasp larva hatches from the egg and feeds on the caterpillar's internal organs, efficiently saving the brain for the very last. The wasp larva then emerges from the caterpillar's decimated body and builds a cocoon. A week later, the adult wasp emerges from the cocoon and the cycle begins again.

To study this interaction, Thaler set up 103 field plots, each containing 4-6 tomato plants. Half of the plots were sprayed with the resistance-triggering chemical jasmonic acid and the others were sprayed only with water. She chemically tested the plants and found that the resistance response was nearly three times as great in the plants sprayed with jasmonic acid as in the control plants.

Three weeks later, Thaler found there were twice as many parasitized caterpillars on the chemically treated plants as there were on the control plants.

To further pinpoint the cause, Thaler placed "sentinel caterpillars" in cups beneath the leaves of both the treated and control plants. The caged sentinel caterpillars were fed an artificial diet and, while they could not feed on the plants, they were accessible to the parasitic wasps attracted to the plants. A day later, Thaler found a 37-percent greater rate of parasitism among the sentinel caterpillars beneath the treated plants, evidence that the plant, rather than the caterpillar's diet, was attracting the wasp.

"This further suggested that the plant's resistance response increased the plant's ability to attract or retain the parasitic wasps and increased the rate of mortality among the caterpillars," Thaler said.

She did find that there was some negative effect on the wasps from the plant's resistance response. In a laboratory experiment, Thaler observed that the wasps developed more slowly from egg to pupa if they parasitized caterpillars from the plants that had been chemically treated to stimulate the resistance response rather than caterpillars from untreated plants. Despite this negative impact on the wasp, the net effect for the plant was beneficial in that the number of naturally occurring parasitized caterpillars was doubled on the treated plants.

"Confirmation of this net benefit to the plant is potentially very important for pest control," Thaler said. "Future management of agricultural pests will rely on a combination of strategies. The results from this study suggest that artificially induced plant resistance and use of biological controls in the form of naturally occurring enemies are two tools that can be combined synergistically to control crop pests.

"Naturally occurring chemical elicitors in plants, such as jasmonic acid, that can be sprayed onto the plant to stimulate natural defenses provide a safe and effective means of pest control," she said. Further research is needed to determine how these mechanisms might be manipulated in large agricultural plantings to provide effective pest control, she added.

This study was funded by the U.S. Department of Agriculture.