Crops vs. caterpillars

Insect spit a key weapon in ongoing war

Penn State graduate student Loren Rivera-Vega removes the salivary glands from a cabbage looper caterpillar. She studies how proteins in caterpillar spit combat a plant's chemical defenses. Credit: ©Brittany DodsonAll Rights Reserved.

Next time you chew a stick of mint gum or pop a peppermint candy, think of insects.

That distinctive flavor comes from essential oils the mint plant makes to defend itself against hungry insects. Strong flavors and smells of other plants, such as basil and cabbage, are also plant defense compounds. These weapons halt insect feeding in many ways. Plant compounds can taste or smell bad, fortify cell walls so insects can’t penetrate a leaf to feed, or affect digestion, eventually killing the attackers. But insects aren’t helpless against these plant defenses. They find ways to fight back — and one of their best weapons is their spit.

“It’s all chemistry,” says Loren Rivera-Vega, a doctoral candidate in entomology at Penn State. Plants may recognize physical damage caused by insect feeding and chemicals in insect feces or spit. “But some proteins in insect spit can suppress plant defenses instead of triggering them,” she says. “In a way, the plant is fooled.”

Chemical warfareInsects capable of this trickery are most often specialists that eat one type of plant. Generalists – insects that have broader tastes and eat many different plants – are usually more sensitive to plant defenses than specialists. “It’s difficult for an insect to eat multiple plant species because different plants have different defenses,” Rivera-Vega says. Yet some of our worst agricultural pests are generalists. Rivera-Vega studies one of them, the caterpillar of the cabbage looper moth, Trichoplusia ni. Despite their name, these caterpillars can eat and damage more than forty vegetable crops and ornamental plants. Rivera-Vega wants to find a new way to control them. “If you understand how the plant responds to the insect and how the insect avoids defenses, you can try to manipulate that to your advantage,” she says. “My question is, how can the cabbage looper eat so many different plants and still survive?”

She thinks the secret may be in their spit. She’s studying how the spit changes when cabbage loopers feed on different plants. To set up an experiment, Rivera-Vega fills a growth chamber with trays of tiny plastic cups containing two or three newly hatched cabbage looper caterpillars each. She makes baby caterpillar food for them by blending pinto beans, brewer’s yeast, and other nutrients. Her caterpillars eat this artificial diet for 10 to 12 days, until they’re big enough for the experiment. So far, Rivera-Vega has tested them on cabbage and tomato plants. These crops are easy to grow in the lab, and they contain different defense chemicals that affect cabbage loopers in different ways. The caterpillars are healthiest eating cabbage, their preferred food source. They survive eating tomato, but grow more slowly. She explains, “It’s a nice comparison between a host where they do really well versus a host where they don’t do as well.”

Once the caterpillars are big enough, Rivera-Vega takes them to the lab greenhouse where her cabbage and tomato plants are growing. She can’t haphazardly release the caterpillars -– if she did, they’d all head for the cabbages. To prevent runaways and keep each caterpillar eating its prescribed diet, she clips small homemade cages onto the leaves and places a single caterpillar in each cage. She lets the insects feed for two to three days and then harvests both the leaves and the caterpillars.