Botanists have long opined that a weed is just a plant out of place. To farmers, gardeners and lawn owners, a weed could be classified as public enemy No. 1. To herbicide manufacturers and distributors, a weed is a source of income. To a number of insect species, a weed is merely something good to eat.
About half of the insects in the world feed on plants, and these six-legged creatures don't shun a food source just because it's called a weed by humans. What insects are plant feeders? Almost all of the butterflies, moths, grasshoppers and walking sticks chow down on plants. So do most of the true bugs, about 30 percent of the beetles and a few bees, wasps and flies.
Individual plant species vary as to the number of insect species that make a meal of them. That is because plants defend themselves against insect feeding with physical barriers, such as spines and thorns or chemicals toxic to the insect.
Milkweed plants are a good example. Milkweeds have hairs on the leaves, a milky sap and a chemical called cardiac glycoside, all of which cause many insects to avoid feeding on the plant. But in spite of those defenses, more than 400 insect species use the milkweed as a primary or a partial food source.
The most widely recognized of the milkweed feeding insects is the monarch butterfly; however, but most people who look at milkweed will also notice milkweed beetles, a black-and-gold tussock moth caterpillar and a number of aphids. These insects have overcome plant defenses.
Another plant that, like the milkweed, can sometimes be considered a weed is the sunflower. When I was growing up in Kansas, this plant was considered a major weed along roadsides and in crop fields. Now we use it as an oilseed crop and a garden ornamental.
Because sunflowers are a crop, insects that feed on it are called pests. Sunflower-feeding insects are numerous and include these pests: banded sunflower moth, cutworms, headclipper weevil, palestriped flea beetle, thistle caterpillar and the sunflower beetle.
The thistle caterpillar is so-named because it feeds on thistles. Most of us would consider such an insect beneficial because thistle plants are weeds of the worst sort. But when that thistle caterpillar, the larvae of the painted lady butterfly, starts consuming a plant that we want, it becomes a pest. This is the case when it feeds on sunflower or soybean crops.
Another example is the kudzu bug. As the name suggests, this bug feeds on kudzu, the vine plant that is prevalent throughout the southeastern part of the US. Some would consider the bug beneficial because it feeds on a plant they dub a weed. Now, however, the insect is showing up on soybeans, and that is not a good thing.
Some scientists have suggested that using insects to control unwanted plants is a good idea. For example, purple loosestrife, an invasive perennial weed that was accidentally introduced to the US from Europe. It has been suggested that insects might help control this unwanted weed. So 120 insects have been evaluated for that purpose. Two species of leaf beetle and three species of weevil have been introduced to feed on purple loosestrife. The feeding by these insects sometimes results in total defoliation of the plants, but whether or not it is effective in eliminating the plants is unknown at this time.
Hemp, more popularly known today as marijuana, is another plant attractive to insects. In fact, a lot of insects - 272 species in all - have been reported to use hemp as a food source. Hemp-feeding insects include stem borers, leaf miners, root feeders, sap suckers and cutworms.
One of those stem borers is the insect called the European corn borer. This well-known pest of corn is, as its name suggests, a native of Europe. Before corn was introduced to Europe from America, the European corn borer fed on hemp and hops. It quickly adapted to feeding on corn in its native habitat.
The European corn borer was accidentally introduced to America and was first reported in 1917. Since then it has moved westward and has became one of the major insect pests of corn. It still feeds on hemp, making it a true "weed" eater!