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?”