Bark beetles engage in one of the most pronounced examples of group procurement of defended plants. Their aggregation pheromones attract both sexes and are essential to overcome constitutive and rapidly inducible lethal defenses. The relative benefits to senders versus receivers of these signals are only partly understood. Because the initial stage of host entry can be hazardous, there may be benefit to a cheating strategy, whose practitioners respond to pheromones but do not engage in host searching. Several disadvantages to cheating have been proposed, but the role of predators has not been considered. Predators exploit bark beetle pheromones to locate prey, accumulate at the breeding site, and consume adult bark beetles before they enter the tree. Preliminary experiments quantified arrival patterns in the field. We used a laboratory assay to investigate relative predation on pioneers (those that initially select and enter hosts) and responders (those that arrive at a host in response to pheromones) during host colonization. Our model system utilized the pine engraver, Ips pini, which exhibits male harem polygamy. We allowed male I. pini to colonize host tissue and added females 1 day later. Also 1 day later, we variably added additional males and predacious checkered beetles, Thanasimus dubius. These treatments included two densities of males and three densities of predators that were selected to emulate field conditions. Responding males experienced higher predation than pioneers. T. dubius ate more males than females, independent of the presence or absence of responding males. T. dubius affected the distribution of females per male, although the number of females that survived to construct ovipositional galleries was constant. We discuss the viability of cheating, implications for biological control, and predator-prey coevolution in this cooperative, group-colonizing herbivore.
Bibliographical noteFunding Information:
Acknowledgements S. LaFontaine, L. Kuiper, N. Erbilgin, E. Hladilek, J. Gruber, B. Burwitz, A. Boyd, and K. Gardner assisted in these assays. M. Clayton provided statistical advice. We thank the Wisconsin Department of Natural Resources for providing the trees and trapping sites. This study was supported by USDA NRI AMD 96 04317, the University of Wisconsin-Madison College of Agricultural and Life Sciences, S.C. Johnson and Son, Inc., and an Elsa and Louis Thomsen Wisconsin Distinguished Fellowship awarded to B.H.A. We thank D. Mahr, D. Hogg, A. Ives, R. Lindroth, J. Handelsman, D. Wood, J. Cronin, and two anonymous reviewers for their helpful discussions and suggestions.
- Ips pini
- Male pioneering
- Reproductive success
- Thanasimus dubius