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Abstract
The physical and chemical aspects of Diabrotica virgifera virgifera larval hemolymph were quantitatively assessed against two predatory beetle species in the laboratory. Adult Poecilus cupreus and Harpalus pensylvanicus (Coleoptera: Carabidae) were fed pupae, second or third instar D. v. virgifera or a palatable surrogate prey, i.e., Calliphora vicina or Sarcophaga bullata larvae (Diptera: Calliphoridae, Sarcophagidae, respectively) of equivalent size. The ethanol-soluble fraction of third instar D. v. virgifera hemolymph was extracted and suspended in a 0.24 M sucrose solution and offered to H. pensylvanicus (using a sucrose only control for comparison). The mean duration until first consumption was recorded for each predator, as was the amount of time spent eating, cleaning, resting, or walking for 2 min post-attack (or 5 min for the sugar assay). Maggots and D. virgifera larvae and pupae were attacked equally by both predators. But upon attack, D. v. virgifera larval hemolymph coagulated onto the mouthparts of the predators, which they began vigorously cleaning. Predators ate the sucrose solution for significantly longer than hemolymph + sucrose solution, indicating the presence of deterrent chemicals in the hemolymph. This research suggests that D. v. virgifera larvae are defended from predation by sticky and repellent hemolymph. We hypothesize that this defense partially explains the widespread success of D. v. virgifera as an invasive pest.
Acknowledgements
We thank Carson Dinger (USDA-ARS), Janet Fergen (USDA-ARS), Mallory Johnson (USDA-ARS), Hongmei Li (CABI), Sam Magnus (CABI) and Mike Wogin (CABI) for assistance with behavioral observations, and Urs Schaffner (CAB) for his knowledge and input on hemolymph defenses. Chad Nielson (USDA-ARS) and Lars Andreassen (CABI) provided helpful advice on rearing of insects. John Tooker (Pennsylvania State University) and Wade French (USDA-ARS) reviewed an earlier draft of this manuscript. This research was funded in part by an OECD Research Fellowship. Mention of any proprietary product does not constitute endorsement by the USDA.
