(Beyond Pesticides, October 4, 2017) Beetles in the family Cantharidae are known as soldier beetles, a name that is based on the resemblance of the elyra (wing cover) to certain military uniforms. They superficially resemble fireflies (family Lampyridae), but lack light-emitting organs and the covering obscuring the head of fireflies. Like fireflies, soldier beetles are distasteful to most predators.
There are 16 genera containing 455 species of soldier beetles native to North America, including Chauliognathus marginatus, which is commonly seen on goldenrod in late summer and early fall. Worldwide, there are about 5,100 species in 160 genera, widely distributed in all but polar regions. Most frequently active in summer and early fall, adults can be found on various flowers including sunflowers, tansy, zinnia, marigold, goldenrod, and coneflower. Females lay eggs in clusters in the soil. Larvae are mostly carnivorous, feeding on soil insects. They live through the winter under loose soil, and become active during spring. Larvae normally pupate in early summer and adults first emerge in mid-summer.
The soldier beetle’s body is around ½ to ¾ inch long. Adults are black or brown, usually with red to yellow markings, an “aposematic” signal to predators, warning of an unpleasant taste. The elytra are soft wing covers, hence the nickname “leatherwings.”
Adults and mature larvae have chewing mouthparts. After hatching, the larvae are tiny and white. However, 24 hours after their first molt, larvae begin to move and darken. To quote University of Minnesota Extension Entomologist Jeff Hahn, the dark larvae come to “resemble miniature alligators.”
Soldier beetle adults and larvae, when attacked, can emit a spray of dihydromatricaria acid from glands along their bodies, causing the majority to be either rejected or avoided outright by potential predators such as birds, mice, and jumping spiders.
Ecological Role and Threats to Existence
After they hatch in the summer, larval activity increases with each successive molt. Soldier beetle larvae are carnivorous, foraging for aphid eggs, worms, slugs, and snails among assorted plant debris. As they feed, soldier beetle larvae reduce the number of eggs and larvae of other soft-bodied insects, such as aphids, thereby limiting the ability of those insects to damage crops.
Soldier beetle adults feed on the pollen and nectar of flowers, as well as other insects, pollinating the flowers as they move about. Beetle-pollinated flowers are generally open and fragrant, allowing beetles to pollinate the flowers as they scramble across them. The same flowers serve as mating sites for the soldier beetles.
Although harmless to humans, the soldier beetle is among a long litany of “non-target” species who may be poisoned, or whose source of both food and habitat may be poisoned by the use of pesticides.
How to Protect the Species
Swaths of wild flowers, native shrubs and trees, as well as urban green spaces, provide good habitat for adult soldier beetles and other pollinators. Similarly, since the beetles deposit eggs into soil, or loose leaf litter, it is critical to eliminate the use of synthetic fertilizers and toxic pesticides that threaten soil life. Adopting organic land management practices such as planting pollinator habitat conservation strips and cover crops, using mulch for weed control, and adding compost to gardens, lawns and farm fields, helps to build and protect biodiversity. The resulting rise of soil organic matter provides healthy hunting grounds for voracious soldier beetle larvae.
University of Minnesota Extension: https://www.extension.umn.edu/garden/insects/find/soldier-beetles/
University of Kentucky Entomology: http://www.uky.edu/Ag/CritterFiles/casefile/insects/beetles/soldier/soldier.htm
Virginia Cooperative Extension: https://pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/ENTO/ENTO-53/ENTO-53-pdf.pdf
Encyclopedia Britannica: https://www.britannica.com/animal/soldier-beetle
USDA Forest Service: https://www.fs.fed.us/wildflowers/pollinators/animals/beetles.shtml
Mother Earth News: https://www.motherearthnews.com/organic-gardening/pest-control/soldier-beetle-facts-zw0z1302zkin
Iowa State University Extension and Outreach: https://hortnews.extension.iastate.edu/2009/7-1/soldierbeetle.html
Soldier Beetles – Family Cantharidae http://www.cirrusimage.com/beetles_soldier.htm
Bug Guide: Family Cantharidae – Soldier Beetles http://bugguide.net/node/view/118
R.E. White, 1983. A Field Guide to the Beetles of North America. Peterson Field Guide Series, Houghton-Mifflin Co., Boston, MA.