Associational resistance, in which one species gains protection from its consumers by association with a competitor, is common among plants but has seldom been documented among insects. Here we show that parasitism of an aboveground herbivore, European corn borer, by its specialist parasitoid Macrocentrus grandii, is reduced 98% in the presence of a belowground herbivore, corn rootworm. We tested the hypothesis that this positive indirect interaction between corn rootworm and corn borer was mediated by corn rootworm's influence on the habitat: plant height was diminished by 33% and plant density by 20%, resulting in a more open habitat. We found that M. grandii showed a two- to five-fold preference for dense versus open habitats, and that experimental reduction of plant density in the absence of corn rootworm reduced M. grandii parasitism of corn borer by 13%. This result supports the presence of a habitat modification effect as a contributing factor to associational resistance for corn borer. We argue that associational resistance may be more common among phytophagous insects than previously appreciated.
Bibliographical noteFunding Information:
Acknowledgments This research was funded by the National Center for Environmental Research (NCER) STAR Program, EPA grant # U-91578501-2, USDA award 99-34103-7632, NSF award DMS-0083468, the Center for Community Genetics, and fellowships from the University of Minnesota graduate school. We thank J. Har-mon, G. Heimpel, and C. Neuhauser for reviews of earlier drafts of this paper, K. Karelis and J. Rowe at the University of Minnesota Outreach, Research and Education Park for field and logistical assistance, and D. Cook for statistical advice.
- Belowground/aboveground interactions
- Enemy-mediated indirect interactions
- Positive effects
- Spatial proximity effects