Many insects, like caterpillars and leaf beetles, feed on plants. We call these insects phytophagous. Some phytophagous insects eat a variety of plant species, while others specialize in eating only one, or just a few. If the larvae or nymphs feed on plants, the insect mother usually lays her eggs on a host plant. So how do insects find the right plant?
We don't have all the answers to this question yet, but here's what we do know. Scientists believe that insects use chemical smell and taste cues to help them recognize host plants. Insects differentiate plants based on their odors and tastes. The chemistry of the plant determines its appeal to an insect.
Plants in the mustard family, for example, contain mustard oil, which has a unique smell and taste to a foraging insect. An insect that munches on cabbage will probably also munch on broccoli, since both plants belong to the mustard family and broadcast the mustard oil cue. That same insect would probably not, however, feed on squash. The squash tastes and smells completely foreign to a mustard-loving insect.
Here's where it gets a little tricky. Do insects just fly around, sniffing the air and following odors to find the right host plant? That might be part of the answer, but some scientists think there's more to it.
One theory suggests that insects first use visual cues to find plants. Studies of insect behavior demonstrate that phytophagous insects will land on green things – plants – but not brown things – soil. Only after landing on a plant will the insect use those chemical cues to confirm whether or not it has located its host plant. The smells and tastes don't actually help the insect find the plant, but they do keep the insect on the plant if it happens to land on the right one.
This theory, if proved correct, would have implications for agriculture. Plants in the wild tend to be surrounded by a diversity of other plants. An insect looking for a host plant in its native habitat will invest a good deal of time landing on the wrong plants. On the other hand, our monoculture farms offer pest insects a nearly error-free landing strip. Once a pest insect finds a field of its host plant, it will be rewarded with the right chemical cue almost every time it lands on something green. That insect is going to lay eggs and feed until the crop is overrun with pests.
Insect learning may also play a role in how insects find and choose food plants. Some evidence suggests that an insect develops a preference for its first food plant – the one where its mother laid the egg from which it hatched. Once the larva or nymph consumes the original host plant, it must go in search of a new food source. If it happens to be in a field of the same plant, it will quickly encounter another meal. More time spent eating, and less time spent wandering around looking for food, yields healthier, stronger insects. Could the adult insect learn to lay her eggs on plants that grow in abundance, and thus give her offspring a higher chance to thrive? Yes, according to some researchers.
The bottom line? Insects probably use all of these strategies – chemical cues, visual cues, and learning – in combination to find their food plants.
- "Host selection in phytophagous insects : a new explanation for learning in adults." J. P. Cunningham, S. A. West, and M. P. Zalucki.
- "Host-Plant Selection by Insects." Rosemary H. Collier and Stan Finch.
- Insects and Plants. Pierre Jolivet.
- The Handy Bug Answer Book. Gilbert Waldbauer.