We all know that many insects are considered pests to certain plant species. However, we don’t often hear about the methods some plants use to attack insect pests. Obviously, insects have the advantage of being mobile, but there is a lot that plants can do when it comes to altering an insect’s behavior. Plant and insect relations are sometimes mutually antagonistic because they have evolved together. Some plants can produce defensive chemicals that have negative effects only on the insect pests that feed on them. On the other hand, some insects produce chemicals that damage only one type of plant. For example gall-inducing fly larvae release chemicals that only damage tall goldenrod plants. However, the tall goldenrod plant can release chemicals that negatively affect gall-inducing flies.
Tall goldenrod plants can cause damage to gall-inducing flies, or Eurosta solidaginis, as they are officially known, by releasing a chemical called jasmonic acid. However, before this defensive chemical is released by the plant, the male flies must first emit a particular odor. It happens that these odors are used by male flies to attract female flies.
The male fly spends nearly all of its time near tall goldenrod plants. Once the female fly senses the male’s attractive odor, the female meets the male near a goldenrod plant. Once the females eggs are fertilized the eggs are deposited into the stem of a goldenrod plant. Eventually larvae develops within the stem. The larvae immediately feeds on the internal plant stem. While the larvae feeds, it releases a chemical from its saliva that causes the goldenrod plant to grow abnormally. The plant will respond to this damage by regrowing tissue near the site of the damage. This regrown tissue results in a gall, or a protrusion from the plant’s stem.
The flies damage cause damage to goldenrod plants by decreasing the number of seeds that they are able to produce, and the seeds that are produced are undersized. The plant cannot dedicate as much energy to seed production because its energy is consumed with repairing the damage caused by the larvae. The plant does, however, possess one weapon to use against the flies.
Researchers exposed goldenrod plants to a chemical compound called E,S-conophthorin. This is the most abundant compound emitted by gall-inducing flies. After the goldenrods were exposed, they secreted more defensive chemicals. In other words, the plants “smelled” the flies. This insect-plant relationship suggests close evolutionary bonds.
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