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Bug of the Week is written by "The Bug Guy," Michael J. Raupp, Professor of Entomology at the University of Maryland.

Ground beetles up in the air: Calosoma wilcoxi and Calosoma scrutator


Calosoma wilcoxi (right) is about one third the size of its cousin, the fiery searcher Calosoma scrutator (left).


In last week’s episode we met cankerworms pillaging forests in western and southern Maryland. While ogling fall cankerworms as they shredded leaves of maples and oaks I was surprised to see many carabid ground beetles foraging in the canopies of trees. Now, as the name implies ground beetles usually have their six tiny feet firmly planted on the ground where they provide great service hunting and eating a wide variety of soft-bodied pests, including rootworms, cutworms, and slugs, that plague our agricultural crops and backyard gardens. However, part of the clan, members of the genus Calosoma, are famous for their arboreal skills, ascending trees to deliver murder and mayhem on caterpillars.


Down the hatch in less than five minutes, an arboreal ground beetle makes short work of fall cankerworm.

 One of the largest members of Calosoma is a drop-dead gorgeous beetle that goes by the name the fiery searcher or caterpillar hunter. For an insect, it is big, often attaining a length of an inch and a half. My first encounter with the fiery searcher was disconcerting. Back in the days of Maryland’s gypsy moth plague, I lifted a burlap band on a tree to count gypsy moth caterpillars and out popped a fiery searcher that landed on my shirt. When I captured the assailant, it unleashed a potent chemical defense that lingered on my fingers for several hours. Calosoma beetles are notorious for producing potent defenses including methacrylic acid and salicylaldehyde.

A guardian of the maple leaf searches for the next victim.

 Although I did not see fiery searchers during my visit to cankerworm central, I was delighted to see scads of its smaller cousin Calosoma wilcoxi devouring cankerworms in the treetops and feasting on unlucky inchworms that had fallen on the ground. These beautiful hunters live two to three years or possibly longer. Romance happens in the month of June when mating pairs are active. Eggs are deposited in the soil in early summer and they hatch in a roughly a week. According to published accounts, larvae are ground dwellers where they consume a wide variety of prey including gypsy moths and eastern tent caterpillars. Adult beetles hunt both day and night and in addition to fall cankerworms, on the menu are spring cankerworms, gypsy moth larvae, and eastern tent caterpillars.

 Several literature accounts associate the presence of Calosoma wilcoxi with the demise of cankerworms and the end of their devastating outbreaks in natural forest settings. One has to wonder why some locations in Maryland, Virginia, and North Carolina have eruptions of cankerworms lasting years or even decades. One answer to this mystery may lie in the degradation of habitat that often accompanies the process of urbanization. In a recent review of responses of ground beetle communities to urbanization around the world, we discovered large forest dwelling ground beetles like our friend Calosoma wilcoxi to be the most severely impacted when forests become cities. Despite the presence of seemingly adequate prey for adults in some urban areas, other factors such as overwintering sites, food for larvae, cover for larvae and adults, and favorable thermal regimes may be inadequate or missing in developed areas. For our beleaguered oaks, maples, birches, and other hardwoods, let’s hope that Mother Nature’s hit squad of predators and parasites can bring the cankerworm catastrophe to a quick conclusion.      



These powerful jaws are made for munching caterpillars.


We thank Dr. Shrewsbury and Rebecca Waterworth for providing the inspiration for this episode. Interesting accounts including “The genus Calosoma including studies of seasonal histories, habits, and economic importance of American species north of Mexico of several introduced species” by A. F. Burgess and C. W. Collins,   “A meta-analysis of the effects of urbanization on ground beetle communities” by H. Martinson and M. Raupp, and “Calosoma wilcoxi, A ground beetle” by T. Erwin were used to prepare this episode.