Of Earth’s 350,000-plus species of beetles, many serve as the planet’s cleanup crew by getting rid of waste matter and decaying wood. Others operate as ever-important pollinators. Some beetles are plant-eating pests and still others are carnivores—predators that feast on other members of the insect clan. Beetles also serve as prey, being readily consumed by everything from birds to snakes. Their sheer numbers, not to mention their myriad roles in nature’s food web, make them important insects.
Beetles and weevils are in the insect order Coleoptera, meaning “sheath wing”—an appropriate name due to their thick exoskeleton and hardened, protective forewings. Those that are destructive in gardens tend to be the herbivorous, or plant-eating, species. Pest beetles can be controlled by covering susceptible crops with protective row covers, hand-picking both adults and larvae, timing crops to avoid peak insect activity, and, when necessary, applying spinosad-based organic pesticides according to label instructions.
Learning to recognize pest beetle species as adults, and sometimes larvae, encourages both the use of appropriate control measures and an appreciation of their intrinsic, yet occasionally detrimental, significance to the landscape. Here are some of the beetles you’re apt to come across in your garden.
North America hosts some 300 species of blister beetles, but only a handful are harmful to gardens. These beetles acquired their common name because of their ability to produce a defensive compound that can cause human skin to blister when exposed (largely through accidental crushing). Blister beetles can poison cattle and horses if infested alfalfa or hay is ingested. Adult beetles are 3/4- to 1-inch, elongated insects that consume a range of plants, including legumes, Japanese anemones, potatoes, phlox, zinnias, and many other garden vegetables and ornamentals.
Both common species of cucumber beetle, striped and spotted, measure 1/4 inch long. Adults are yellowish green. Striped species have three broad black stripes on the wing covers, while spotted species have either 11 or 12 black spots. Cucumber beetles damage members of the cucurbit family, including cucumbers, melons, pumpkins, and squash, as well as other vegetable crops. They chew ragged holes in flowers and foliage and spread diseases, including bacterial wilt and cucumber mosaic virus. Their larvae reside below ground, where they feed on plant roots until pupation occurs.
Colorado Potato Beetle
Both the tan-and-black-striped adult beetles and their black-spotted, fat red larvae feed on the foliage of potatoes, tomatoes, eggplants, and other vegetables. Severe infestations can be devastating, as the beetles can quickly defoliate entire plants. The larvae are often found feeding in groups before they drop to the ground to pupate. They can go from egg to adult in 21 days.
Though this beetle has but one host, asparagus, its damage is extensive. Adults chew ragged depressions in developing spears, while the army-green larvae chew foliage, limiting photosynthesis and decreasing yields. Adults are 1/4 inch long. They are black with several cream-colored spots and red wing borders. Their black, elongated eggs are often found attached to spear tips.
Black Vine Weevil
Adult weevils are black and 1/2 inch long with a short, broad snout. Black vine weevils, like aphids, are parthenogenetic, meaning females can lay viable eggs without the help of males. This pest feeds on some 100 species of plants, including many broad-leaved evergreens and perennials. The nocturnal feeding of adult weevils produces distinctive crescent-shaped notches in the leaf margins. The ground-dwelling larvae cause significant damage to plant roots in spring and early summer. The adult beetles do not fly, as their wing covers are fused.
Originally published in Organic Gardening Magazine, April/May 2012