There are more than 350,000 identified species of beetles in the order Coleoptera. These insects constitute one-half of all the known animals on the planet. That’s a lot of beetles! There are also predatory beetles that consume other insects as part of their diet. But the beetle species that have become most infamous in the garden are herbivores, consuming plant foliage, roots or woody tissue as a food source. While species that munch on plant roots as larvae (including white grubs, iris borers, wireworms and the like) are certainly problematic, those species that feed on foliage are more frequently encountered in the vegetable patch.
The beetle family is host to an enormous range of life cycles and feeding habits. Some beetle species are decomposers, feeding on animal and plant wastes, while others feed on fungus, pollen or nectar. All members of the beetle order have two pairs of wings. The outer wings constitute a pair of hardened elytra that create a shell-like covering over the membranous wings used for flight. All beetle species go through complete metamorphosis, passing through life first as an egg, then a larva, a pupa and finally an adult. Here’s the lowdown on some of the most common pest garden beetles and what you can do to keep their population in check.
1. Colorado Potato Beetles (Leptinotarsa decemlineata)
Adult Colorado potato beetles are 1/3 inch long with hard, rounded wing covers that are black-and-tan striped. The fat, reddish-pink larvae are 1/2 inch long, have rows of black dots on their sides and a small black head. Colorado potato beetles are very common across the U.S., except in the Pacific Northwest and the Deep South. They feed on all members of the tomato family, though potatoes are by far their favorite.
Adult Colorado potato beetles overwinter in the soil, emerging in spring to feed and breed, producing up to three generations each year. Both adult and larval Colorado potato beetles skeletonize the leaves of host plants very quickly. To manage them, cover newly planted seed potatoes with floating row cover and leave it in place until the potatoes are ready for harvest—pollination doesn’t need to occur for potatoes to be produced. Handpicking both adults and larvae is also very effective. Because Colorado potato beetles have developed resistance to many synthetic pesticides, use biological pesticides based on Bacillus thuringiensis var. San Diego or var. tenebrionis (commonly called Bt—just be sure to select the right variety). Other effective biopesticides include those based on spinosad.
2. Mexican Bean Beetles (Epilachna varivestis)
Adult Mexican bean beetles look a lot like ladybugs on steroids, though the absence of white markings between the head and body easily distinguish them from their friendly cousins. Their wing covers are copper-colored with 16 black spots. Mexican bean beetle larvae measure about 1/3 inch long, are light yellow and are covered in bristly spines. They’re found in almost every state east of the Rocky Mountains.
Mexican bean beetles spend the winter as adults nestled under garden debris. Eggs are laid in late spring on the undersides of leaves of host plants, including nearly every species of bean, with each female laying hundreds of eggs. The spikey larvae and adults feed on leaf backs, leaving only the leaf veins intact. They’ll also feed on the beans themselves. Damage is most severe in July and August.
To prevent an onslaught of these beetles, choose early bean varieties that mature before the pest becomes problematic. Handpick adults and squash larvae—their spines are very soft. Cover susceptible plants with floating row cover immediately after planting, but remove the covers when the plants begin to flower to allow pollination. Mexican bean beetles fall prey to numerous species of beneficial insects, including parasitic wasps, pirate bugs, assassin bugs, ladybugs and many others. Plant a lot of flowers in the veggie patch to provide nectar for these predators. Effective product controls include Bt var. San Diego or var tenebrionsis, and anything with the active ingredients of spinosad, neem and citrus oil—all three can be effective.
3. Cucumber Beetles (striped: Acalymma vittata; spotted: Diabrotica undecimpunctata howardi)
Both common species of cucumber beetle, striped and spotted, measure about 1/4 inch long. The adult striped beetles are bright yellow and bear three broad, black stripes running the length of their wing covers. Spotted beetles are greenish-yellow with 11 (Eastern species) or 12 (Western species) black spots on the wing covers. Both species overwinter as adults in weedy areas. Beetles emerge in spring to mate and lay eggs in the soil or on plants. Larvae burrow into the ground and feed on roots for several weeks.
Adults chew ragged holes in plant leaves and can consume entire blossoms of favorite plants, including all members of the Cucurbitaceae family. Newly planted seedlings can be consumed quickly. Most importantly, cucumber beetles can transmit deadly bacterial wilt and cucumber mosaic virus, so controlling the beetles is key to preventing the spread of these pathogens. You should only plant varieties with a known resistance to these pathogens.
To control cucumber beetles, trap adults on yellow sticky cards placed just above plant tops. To attract more beetles, attach cotton balls soaked in allspice, clove or bay oil to the cards. These oils contain eugenol, a pheromone that attracts female cucumber beetles. For added protection, delay planting cucumbers by a few weeks to help break the beetle’s feeding cycle and prevent major damage from the initial early spring feeding period.
A species of beneficial nematode (Heterorhabditis bacteriophora) is particularly effective at attacking the soil-bound larvae and killing them before they reach adulthood. These nematodes can be mixed with water and sprayed throughout the planting area any time during the growing season, as long as the soil temperature is above 65 degrees F. Spinosad-based products are also effective against adults.
4. Japanese Beetles (Popillia japonica)
Introduced to North America from Asia in the early 1990s, Japanese beetles have become a notorious pest. Adults are metallic green with copper-colored wing covers. They measure about 1/2 inch in length and are half as wide. Their ground-dwelling larvae are C-shaped, grayish-white grubs with light-brown heads. The larvae grow up to 1 inch long and spend the winter several inches beneath the soil.
As adults, Japanese beetles consume more than 300 different ornamentals, beginning in midsummer. They release aggregation pheromones as they feed, resulting in large numbers of adults coming together to “feed and breed” on the same host plant. As larvae, Japanese beetle grubs attack the roots of turf grass and many ornamentals. When serious infestations are present—10 or more grubs per square foot of soil—the turf might peel back in a carpet-like fashion. Grub damage is most evident in spring and fall when the grubs are actively feeding in the upper layer of soil.
Hand-pick the adults as early as possible. Product controls for adults include spinosad- and neem-based products. Their larvae tend to cause the biggest problems in lawns that are fed excessive amounts of chemical fertilzer and are frequently, but shallowly, irrigated. Stop watering and allow your lawn to go naturally dormant in summer’s heat. Effective, chemical-free grub control comes from Heterorhabditis bacteriophora, a species of beneficial nematode, applied to the soil each spring. The nematodes are mixed with water and sprayed over the lawn.
5. Blister Beetles (Meloidae family)
North America hosts some 300 species of blister beetles, but only a handful are harmful to gardens. Common food sources include legumes, Japanese anemones, potatoes, phlox, members of the Asteraceae family, amaranth, zinnias, and many other garden vegetables and ornamentals. Adults of pest species consume plant tissue while their larvae are seldom seen. All blister beetle larvae are predators, often using only one species of wild bees or grasshoppers as hosts. As small, newly hatched larvae, some blister beetle species piggyback on adult bees, who carry them back to the nest where the beetle larvae consume the larval bees.
Blister beetles acquired their common name because of their ability to produce a defensive compound that can cause the skin to blister when exposed (largely through accidental crushing). Adults are black, gray, orange or bronze, with various patterns of stripes and solids, and can measure 3/4 to 1 inch long. Their elytra are leathery, rather than rigid. Blister beetles can poison cattle and horses if they ingest infested alfalfa or hay.
Blister beetle adults can be controlled with careful hand-picking (do not squash them!), or with spinosad-based organic pesticides.
Pest beetles can become problematic in even the most well-maintained organic garden. Regular trips between crop rows to scout for them enables effective management and keeps their numbers at a tolerable level. Arm yourself with a little beetle know-how and have your best garden ever.