The Chemical Conversations of Plants and Bugs
By Eric R. Eaton:
We do not give plants enough credit for being alert, smart, talkative, and resilient in the face of insect attacks. No, seriously. The flowers, vegetables, trees, and shrubs in your yard and garden are very capable of defending themselves, recruiting “good bugs” to the rescue, and alerting nearby plants of impending attack, thanks to a language of chemicals.
Welcome to the world of “semiochemicals,” after the Greek word for “signal.” Most of us are familiar with one class of semiochemicals: pheromones. Insects in particular make use of pheromones to draw the opposite sex, create scent trails to a food source, send an alarm, attract others to congregate in a localized area, or otherwise influence the behavior of individuals of the same species. Less familiar are three other classes of semiochemicals, collectively known as allelochemicals: allomones, kairomones, and synomones. In the case of plants, allelochemicals play no part in growth, development, or reproduction, but are manufactured to give the plant advantages over other plant species in its habitat and to defend against herbivores. Allomones influence the behavior of another species to the benefit of the organism producing the allomone. Plants often use allomones to resist insect attacks, such as recruiting beneficial predatory and parasitic insects and mites to feast on the herbivorous “bugs” plaguing the plant. Kairomones are mediators of interspecific relationships that benefit the receiver of the chemical cue, but not the originating organism that emits the kairomone. Synomones also involve two different species, but benefit both the sender and receiver.
Because insects are largely chemotactile animals that perceive the world chiefly through smell and touch, they are tuned in to the allelochemicals produced by plants. Insects that feed specifically on roses, for example, recognize a rose plant by the airborne and plant-surface chemicals unique to the rose. The fragrance of a rose flower attracts welcome pollinators, but the “smell” of the leaf, stem, or root draws unwanted herbivores. Butterflies are known to have taste receptors on their feet; the female insect scratches the leaf surface to release volatile chemicals (kairomones in this case) that will tell her if this is the appropriate plant on which to lay her eggs. An incorrect choice means her caterpillar offspring will starve or be poisoned.
An example of the complexity of plant communication was revealed by a study of lima beans and their spider mite pests. When attacked by spider mites, lima bean plants emit a distress signal that is received by other bean plants, and by predatory mites that then hurry over to eat the spider mites. Remember that plants defend themselves against attack by producing chemicals toxic or repellent to their herbivore attackers. Alerted by the distress allomones of its nearby kin, a plant can then ramp up production of its chemical defenses in preparation for the arrival of herbivorous enemies. In this case, the semiochemicals generated by the lima bean work both as allomones that affect other bean plants, and as kairomones that recruit helpful predatory mites.
There is another cool thing that plants can do in their own defense. A tree becoming infested with aphids, for example, may sacrifice a particular twig or branch to the attacking insects. The tree directs its chemical arsenal everywhere else, thereby making that one chemical-free patch more attractive to the aphids. This results in “clumping” of the aphids, making them more conspicuous to predators and parasites. Birds notice aphid clumps, as do lady beetles, lacewings, and every other aphid predator and parasite. Or you might observe a patch of milkweed and notice that just a few individual plants have milkweed bugs or beetles eating them. The same principle applies: the insects are seeking the least-toxic plants.
What does this mean for pest control in your garden? First, it points to the fact that chemical pesticides play into the hands of herbivorous insects that are already built to deal with plant toxins. Is resistance to insecticides any surprise? Pesticides also kill the beneficial predatory and parasitic insects that distressed plants are recruiting with their own semiochemicals. Native plants, and climate- and soil-appropriate cultivars will be better able to marshal their own defenses and/or endure the aftermath of an insect onslaught. Noxious weeds and other successful invasive plants make use of allelochemicals to suppress competition from our native plants, and crops. Always avoid purchasing plants that have the potential to become invasive. (Ask if you’re uncertain.)
It is in our best interest to reinforce the existing capacity of plants to defend themselves, detect each other’s signals, and access the natural, native defenders (predatory and parasitic insects) as well as pollinators. The closer your garden approaches the native ecosystem in which it lies, or at least mimics the natural landscape, the more likely your plants will be to flourish.
Eric R. Eaton is principal author of the Kaufman Field Guide to Insects of North America and writes the blog “Bug Eric.” He lives in Colorado Springs with his wife, Heidi.