plant defense

Plant Defenses Against Insects

By Pamm Cooper for UConn Extension

caterpillarHistorically, insects have been the most important bane of the plant kingdom. The fatal attraction that exists between plants and insects has woven an intricate balance between good and evil, survival and devastation, and benefits versus harm. While insects play a significant role in pollination, and while over 90% of insects are not a problem, the few that are plant pests can wreak destruction.

Some insects are vectors of disease, especially those that feed by piercing plant tissue. Aphids, plant hoppers and the familiar cucumber beetle may pass along viruses even though feeding damage is not significant. Introduced insects seem to have a field day and may prove to be more damaging than native insects in the long run.

But in all the dramas that occurs in nature, the ones that may be overlooked are the strategies plants can use to defend themselves against insects. Whether it is simply structural impediments such as thorns, prickles, thick bark, waxy cuticles and objectionable chemical compounds, plants are not helpless against attacks. While some defenses are always present, like the above physical qualities, there are other means by which plants can release substances as needed that either repel the feeding insects, or attract predators of the same.

One plant of interest is the geranium that produces a chemical in its petals that can temporarily paralyze the Japanese beetle while it is feeding. This may provide a window for any predator that happens by. Native wild tobacco plants change the time of day that flower buds open in response to caterpillar feeding. This discourages certain sphinx moths that pollinate by night as they are attracted to the scent and color of the flowers, and lay eggs on the plant, which doubles as both an adult and larval food plant. Pollinators such as butterflies and hummingbirds will visit by day and the plant loses no ground in reproduction and survival. Other caterpillars like the tobacco budworm have no problem feeding on geraniums, petunias, snapdragons and other tobacco relatives. Their saliva counteracts the production of induced defenses in the plants.

Some plants release hormones or other substances upon feeding injury that attract predatory insects or even birds. It is like a silent alarm calling in the troops. Many caterpillars may be parasitized because their feeding releases chemicals in the plants that attract predatory insects such as brachonid wasps. Grass releases a strong aroma when cut, either when cut when insects are feeding on it. Perhaps this is why starlings and other birds flock into a yard or pasture that is under attack from cutworms or other grass pests.

Milkweeds contain strong chemical defenses that are passed along to insects that feed on leaves but are unaffected by them themselves. The monarch caterpillar and others avoid much predation because of the absorption of these chemicals, which make them bitter to the taste of hungry birds. The latex released by milkweed as insects begin chewing hardens quickly when exposed to the air and may cause mouthparts to stick together so the insect starves. Monarch cats avoid this by clipping off the base of the midrib first, which reduces sap flow to the leaf.=

Some plants have high lignin or tannin content that makes them unattractive to insects later in the season. Window feeding is a way some caterpillars avoid higher concentrations of toxins in leaves or high lignin content that is difficult to ingest, such as leaf veins and midribs. Many beetles avoid ingesting high concentrations of toxins by feeding in large groups, thereby “sharing in the load”.

Pyrethrins are ester compounds produced by chrysanthemum plants, which act as insect neurotoxins. Some commercially available insecticides are actually synthetic copies of pyrethrins, called pyrethroids. Tansy is a non-native escapee that has toxins repelling many insects. It has been used with some success as a companion plant with cucurbits, squash, roses and other plants to repel cucumber beetles, ants, Japanese beetles and other insect pests. Sprigs were used at windowsills to repel flies.

Trillium actually reproduces effectively by myrmecochory- using ants to carry away its seeds and thus protecting them from becoming consumed by various animals. Ants are attracted to eliaosomes attached to the seeds and bring them back to their nests. After consuming the eliaosomes, the ants discard the seeds and they are still viable. The little ant helps survival of the species along.

While we may be blissfully ignorant of all the events taking place among the plants surrounding us, at least where fending off insects is involved, there is at least as much drama as any to be found in the entertainment industry.