In his detailed and fascinating 1964 book, Beneficial Insects, Lester Swan comments on several beneficial species of soldier beetles and their associated prey, then notes that, unfortunately, they have not been studied extensively. Not much has changed in the ensuing four decades. “Oh, those! I have them in my garden, but I didn’t know they were beneficial,” is the now-familiar response when I point them out to garden visitors. Yet soldier beetles surely warrant the same recognition given to lady beetles and lacewings. In suitable habitat, they are a reliable and valuable ally. It is far easier to supply ideal living conditions for soldier beetles in gardens than in agricultural fields. This lack of potential for commercial use may help explain why soldier beetles have been so little studied, despite a voracious appetite for aphids, caterpillars, grasshopper eggs, mites, and other small pests. They are even reputed to attack cucumber beetles—reason enough for gardeners to agree that soldier beetles deserve further study!
The soldier beetle adult is a narrow, parallel-sided beetle, with long thread-like antennae. Most species have red or orange heads, and dusty-looking grey, brown, or sometimes bluish elytra (wing covers), or some variation on this basic theme. Unlike most beetles, the elytra are relatively soft, resulting in the second common moniker, leatherwings. The family of soldier beetles is closely allied with several families of more familiar beetles, such as blister and click beetles, fireflies and glowworms. Blister beetles are capable of raising painful blisters if picked up imprudently. Click beetles are well known to children, who enjoy turning the beetles on their backs to hear the satisfying “click” they make as they turn themselves right-side-up.
Sadly, the fireflies, or lightning bugs, are not found in the West, but their relatives, the glowworms, are sometimes seen in wooded areas of the Pacific Northwest in late winter. On finding out that glowworms prey on snails, the gardener’s attention perks up; but the glowworm is a gourmet, generally preferring native snails to the introduced species that is the bane of the ornamental garden. The glow is emitted by the female, which, curiously, is able to reproduce while remaining in the juvenile form, a phenomenon known as paedogenesis. The adult male glowworm demonstrates its close relationship to the cantharids by looking a lot like an adult soldier beetle, but with long, feathery antennae. The glowworm male is undoubtedly using those elaborate appendages as locators to find the females, who emit pheromones (a powerful insect “perfume”) to advertise their presence.
The female soldier beetle sometimes attracts hordes of males with the pheromones she emits, but generally only one male is successful. Most beetles don’t engage in elaborate courtship behaviors, but some soldier beetle males may ‘nibble’ females. Considering that soldier beetles usually only mate once, when there are a lot of these beetles in the garden, there seems to be a lot of ‘nibbling’ taking place! Since each female has a huge supply of eggs, building up good garden populations need not take a long time. The nocturnal larvae hatch in spring and are found in damp areas beneath rocks, in leaf litter, or under bark, where they prey on insects and other small organisms. A year or more after hatching, the larvae pupate and emerge as adults.
Soldier beetles have a varied diet, feeding on aphids and other homopterans, grasshopper eggs, caterpillars, root maggots, and other soft-bodied insects. Many genera of soldier beetles, such as Cantharis, Podabrus, and Pacificanthia, are primarily carnivorous in both the larval and adult stage, but a few are minor pests in the larval stage, feeding on roots. Larvae primarily eat eggs and larvae of beetles, moths, grasshoppers, and other insects. Adults are frequently found on a variety of flowers, where they feed on pollen and nectar in addition to insect prey such as aphids and mealybugs. Because they are generalist predators, soldier beetles may also eat beneficial insects, such as lacewing larvae and aphids that have been parasitized by wasps.
Among the earliest insects evolutionarily, beetles were here long before flowering plants (angiosperms), and were among the first pollinators. They were initially associated with plants such as cycads, then, later, the early angiosperms, such as magnolias and our own native spice bush (Calycanthus occidentalis). Soldier beetles favor flowers in the sunflower (Asteraceae) and umbel (Apiaceae) families, and other species with clusters of small flowers, such as milkweeds. Beetle pollination is known as cantharophily, appropriately named after the flower-loving Cantharidae, which are most often noticed when visiting blossoms of flowers such as yarrow, cosmos, fennel, and goldenrod.
Encouraging a resident population of soldier beetles is easy in gardens. Choose suitable flowers to bloom over a long season. Any habitat garden must include a water source; soldier beetles are particularly known to frequent moist habitats. It is important to the life cycle of soldier beetles (and many other beneficial organisms) that they have undisturbed, mulched soil in which to pupate, so include permanent perennial plantings in gardens. A fragile and important community thrives at the interface between soil and organic matter. In permanent plantings, avoid raking and add organic material to the surface of the beds as needed to keep the soil in good fertility.
Each spring, I look forward to the day I first spot soldier beetles, knowing that soon, there will be an abundance of these helpful allies patrolling my garden.
In a Nutshell
Soldier beetle, leatherwings.
Order: Coleoptera. Superfamily: Elateroidea (Glowworms, Fireflies, Click, Net-winged, and Soldier Beetles).
Family: Cantharidae (Soldier Beetles).
Cantharis, Podabrus, Pacificanthia.
Many species of soldier beetles are cosmopolitan, and widely distributed throughout the West. In California, alone, there are 159 species in ten genera.
Holometabolous (complete metamorphosis from egg to larva to pupa to adult).
Eggs: microscopic, soft, smooth-surfaced in clusters. Larvae: velvety appearance due to covering of fine hairs. Pupae: Exarate, where the appendages are free from the body and recognizable even in the pupal stage. Adults: Usually a half-inch or less, narrow, parallel-sided. Often with reddish head and velvety bluish, brown, or grey elytra (wing covers).
Larvae, one to three years; adults, less than one year.
Larvae are generally carnivorous and feed on small soil organisms; a few feed on roots of grasses, potatoes, and celery. Adults eat nectar and pollen, and often other insects such as aphids, mites, mealybugs, small caterpillars and other soft-bodied arthropods. Adults of many species are important predators of aphids.
Nectar and pollen plants, such as goldenrod, milkweeds, yarrow, and umbellifers.
May offer effective biological pest control for home gardens; populations build up when suitable environmental conditions are provided.
Some species are minor pests in the larval stage, when they feed on plant roots.
Larvae, pupae, and adults all produce defensive secretions that discourage predators. Several insect species mimic the appearance of soldier beetles, taking advantage of the bad taste of soldier beetles to protect themselves. Some species of Cantharidae are attacked by a fungus (Eryniopsis) that causes a lethal infection; dead adults are found in a “death grip,” with their mandibles firmly imbedded in a plant, wings extended, and body bend upwards.
No commercial sources. Populations can be increased by gardening with good pollen and nectar plants. Establish beetle habitat by providing areas of permanent mulch, a compost pile, and readily available water.
A useful first stop when trying to identify an insect is www.whatsthatbug.com. A good resource for identifying local soldier beetles is the nearest university with an entomology collection. Field Guide to Beetles of California (2006) and Introduction to California Beetles (2004), both by Arthur Evans and James Hogue (University of California Press), are excellent books on California beetle fauna; both include many beetles from outside the area. Beneficial Insects, by Lester Swan (Harper and Row, 1964), is an old treasure well worth seeking out. It includes information not readily available elsewhere, and is a well-thumbed reference for this author.