Effects of olfactory cues on the foraging behavior of a carabid beetle, Calosoma scrutator, a predator of a forest defoliator

Background/Question/Methods

Arthropod predators often use environmental cues such as kairomones and host plant volatiles to locate herbivore prey. Carabids, also known as ground beetles, are an abundant and diverse family of predatory beetle that feed voraciously on invertebrates. Calosoma scrutator is a Carabid that specializes on Lepidopteran larvae and, despite the common name, often climbs into forest canopies at dusk to feed.  It is unknown whether C. scrutator uses olfactory cues to select an infested tree in which to forage.  In this study we conducted olfactometer experiments to determine if C. scrutator can detect larvae, frass, and damaged oak leaves in a laboratory setting. We used the fall cankerworm, a forest pest in central Virginia, and leaves of its preferred host, oak. A Y-tube olfactometer was oriented at an angle to mimic a tree trunk, and each treatment offered a choice between one of the three cues and a control (odorless air).  All beetles were sexed, and choice and time to choose were recorded.  The Pearson’s chi-square test was used to determine significance in preference for treatment against control.  ANOVAs were used to test for the effects of sex and treatment on response variables. 

Results/Conclusions

We found significant differences between the damaged leaf treatment and control experiment. We found no significant difference between the two remaining treatment and control experiments.  Approximately half of the beetles did not advance beyond the Y-joint in the larvae and frass experiments, thus, those trials were excluded from the analyses. Those that did advance beyond the Y-joint chose the treatment or control in proportions that were not significantly different from random. Time to choose was not affected by any of the treatments. There was no effect of sex on treatment choice. The results of the damaged leaves experiment support the hypothesis that C. scrutator uses olfactory cues to locate infested trees. Future experiments will test interactive effects of olfactory cues, such as frass and leaf volatiles, and caterpillars and leaf volatiles, and for a tactile response to frass on the forest floor. Understanding how predators locate prey has implications for understanding predator-prey curves and trophic effects on population dynamics.