Beetles are infamous creatures in the minds of most gardeners, mere mention of the moniker conjuring up memories of Japanese beetles devouring grape foliage, flea beetles shot-holing broccoli leaves, cucumber beetles, Colorado potato beetles, asparagus beetles, Mexican bean beetles, lily leaf beetles, corn rootworm beetles, viburnum leaf beetles, vine weevils, Asian longhorn beetles, and emerald ash borers, to name a few serious offenders. We rotate crops to elude them, use row covers to exclude them, and plant trap crops to divert them. We become their primary predator, hand picking them at dawn, when they too are sluggish, tossing them into soapy water. We search for and destroy their eggs. And when every non-chemical effort fails, we give up in despair, tossing a once-favorite garden plant on the compost heap.
Why not pull out the big guns, one of the pyrethroid insecticides, or carbaryl (Sevin), or the systemic insecticide acetamiprid? Or the plant-derived insecticides such as azadirachtin, a naturally-occurring compound found in extracts of the neem tree, or pyrethrin, another frequently recommended botanical insecticide? For those of us who garden for wildness, the answer is simple: any chemical used to control one species of beetle would also kill non-target insect species, including nearby beneficial beetles.
In North America there are approximately 25,000 species of beetles. The eastern North American component of this total, approximately 14,000 species, equals one-fifth of all plant and animal species in eastern North America. What are all these beetles doing in our gardens?
Some beetles are pollinating garden plants. By their shear numbers, beetles are the largest group of pollinating animals, responsible for pollinating 88% of the 240,000 plant species on Earth. In North American north of Mexico, 52 native plant species are known to be pollinated by beetles, including asters, sunflowers, roses, butterfly weed, goldenrods, and spireas. Known as “mess-and-soil” pollinators, beetles move pollen around as they eat exposed floral tissues such as petals and stamens, often copulating and defecating as they go about feeding. In plants that are pollinated by beetles, the all-important ovules are protected by sturdy tissues that resist invasion.
Indeed, beetles were the first pollinators. Long before bees came on the scene, beetles were munching on cycads and primitive flowering plants, collecting pollen on their bristly bodies as they fed. Present day beetles still show a preference for pollinating descendants of those ancient plants, including magnolias and water lilies. Many of these plants, like the sweetbay magnolia (Magnolia virginiana), attract beetles by producing flowers with a fruity or fetid fragrance.
The most abundant pollinating beetles in our garden are several species of flower longhorn beetles of the Cerambycidae, a family of some 20,000 species worldwide. The common name points to the long antennae, in some species longer than the beetle’s body. Most are slender creatures, their outer wing cases marked with distinctive spots or stripes.
Flower longhorn beetles prefer flat open clusters of blossoms such as the umbels of Queen Ann’s lace and other members of the carrot family. Occasionally we also see them on flowers of the wild rose as well as goldenrod and other members of the aster family. To watch these diurnal beetles gorge on the pollen, stamens, and nectar of a garden flower, one has to wonder how much pollination is actually happening. Will anything remain of the flower when they are done? But if you look closely, you discover that much of the beetles body is covered with pollen that can be brushed on a nearby pistil in passing.
Evodinus monticola, a flower longhorn
beetle found foraging in our garden on crabapple flowers, the globose flower clusters of globe thistle, and the pendulous blueberry-like blossoms of enkianthus, is a specialist pollinator of bunchberry (Cornus canadensis), a native groundcover dogwood. The tiny flowers that make up the bunchberry blossom release their pollen in an explosion triggered by treading of the heavy foraging beetle. Much of the pollen, embedded deep within hairs on the beetle’s body, is then transferred to pistils in nearby flower clusters.
Lady beetles are among the most familiar of predaceous beetles, their abundance in the garden recognized as a sign that all is well. Adults tend to focus on eating aphids while the larvae are more generalists, preying on insect eggs, beetle larvae, and the adults of aphids, scale insects, mealybugs, mites, and whiteflies. Each lady beetle larva can consume about 25 aphids per day, while an adult lady beetle can eat about 60.
Lady beetles are well-known predators because of their diurnal habit. Much of the good work that other types of beetles perform in our gardens happens during the night and thus goes unnoticed. Various species of nocturnal ground beetles, often called “carabids” in reference to their family, the Carabidae, are carnivores, scampering about the garden under the stars in search of caterpillars, grubs, other beetles (including those that eat garden plants), fly larvae and pupae, aphids, earthworms, slugs, snails, earwigs, and other garden herbivores. Other carabid species are granivores, feeding on weed seeds in the soil. Still others are omnivores, eating both seeds and insects. Whatever its food preference, a ground beetle can eat its weight in insect prey and/or seeds every day.
You can find nocturnal carabids during the day if you go looking for them, turning over stones and old logs, lifting a potted plant, or digging at the base of bunch grasses where they seek shelter. They love to spend the day tucked beneath the foliage of any low ground cover. You will also run into them while turning the compost heap, one of their favorite hunting grounds, or when raking aside the leaf litter mulch covering a garden bed.
There are over 2000 species of carabids in North America. As a group, they vary in shape and color, most being shiny and dark, others metallic or patterned, and they range in size from one-third to two-thirds of an inch long. Many can be recognized by unique color patterns or body shapes that speak to their role in the garden food web. The snail-eating beetle (Scaphinotus andrewsii), for example, is equipped with long slender mandibles that allow it to reach deep into the shell of a snail, while the six-spotted tiger beetle (Cicindela sexguttata), a diurnal species, has a brilliant metallic green body that allows it to blend in with its surroundings, avoiding its predators while stalking its prey. Most carabids are swift of foot, preferring to outrun their prey rather than flying, and some species have lost the ability to fly.
The most common garden carabids belong to a group called “common black ground beetles” (CBGB), of which there are over 1,100 species worldwide. They share a common life cycle, breeding in late summer, females laying eggs just below the soil surface. Larvae hatch and spend the winter in the soil, emerging in early spring to feed and then pupate. Adults emerge in summer. Some adults may also overwinter.
CBGBs are at the center of a complex segment of the garden food web. As mentioned earlier, they prey on a host of garden herbivores (including army worms, cutworms, corn rootworms, potato beetles, and cucumber beetles), earthworms, slugs, and snails, while they are preyed upon by an equally large and diverse group of predators that includes other beetles (such as the six-spotted tiger beetle), wolf spiders, garden snakes, toads, frogs, birds (including robins, turkeys, red-winged blackbirds, blue jays, and mourning doves), shrews, skunks, and red fox.
To those of us who garden for wildness, it is clear that beetles are important members of the garden food web, many species essential in limiting populations of garden herbivores, others functioning as pollinators. We can increase the population of beneficial beetles in our gardens by minimizing disturbance of the soil where beetle larvae live, and we can create beetle habitat by leaving a few stones and rotting logs along garden paths and mulching garden beds with leaf litter. We can spend a few summer nights watching ground beetles at work and thus come to a greater appreciation for their existence.