Habitat modification contributes to associational resistance between herbivores.

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.