Submitted to: Journal of Entomological Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/17/2012
Publication Date: 4/1/2012
Citation: Lundgren, J.G., Harwood, J.D. 2012. Functional responses to food diversity: the effect of seed availability on the feeding of facultative granivores. Journal of Entomological Science. 47(2):160-176. Interpretive Summary: Most beneficial natural enemies of insect pests are best classified as omnivores. Our ability to rely on these omnivores for insect pest management depends on how their consumption of prey is affected when non-prey foods become available. Here, we examined feeding behavior and population responses of crickets and carabid beetles (both groups containing important omnivorous predators) in the presence and absence of locally abundant foxtail seeds in perennial alfalfa. Using discriminant analysis, we first examined how the omnivore communities responded to local seed availability. Then we applied ELISA-based and microscopic gut content analysis to determine how crickets altered their feeding behavior in the presence of seeds. The insect communities were very diverse and abundant, but were structurally similar in the presence and absence of seeds. Interestingly, members of this community clearly partition their activity patterns such that each is particularly active during only a portion of each 24 h period (diel cycle). We found that the black field cricket, Gryllus pennsylvanicus, ate more overall food when seeds were available, and that they consumed more plant-based foods when seeds were available than when they were absent. However, their frequency of consuming insect prey was not affected by seed availability. The feeding behavior of a second cricket, Allonemobius sp., was unaffected by seed availability. This research shows that complex soil communities are able to coexist through specializations in their diet and in their activity patterns. Also important, biological control of insect pests by omnivores may not be disrupted when non-prey foods become abundant in a habitat.
Technical Abstract: The relative importance and availability of different foods to animals is critical in determining how they function within food webs. We examined how the diverse communities of carabid beetles and crickets in a perennial hayfield respond to seed availability numerically and in their feeding behavior. Treatments compared how these insect communities responded in their abundance and composition to local seed rain of Setaria viridis over the first week of seed availability. We determined how the diets of two prominent facultative granivores (Gryllus pennsylvanicus and Allonemobius sp.) were affected by the availability of seeds using monoclonal-antibody-based and microscopic gut content analyses of 1,270 total specimens. Although there were three distinct insect communities identified over the diel cycle, these communities were similar in the treatments with and without seeds. Gryllus pennsylvanicus consumed plant material more frequently, and ate more food in the fields with greater seed availability. However, they consumed prey with equal frequency in the two treatments. Allonemobius sp. consumed food less frequently than G. pennsylvanicus, and their diet was unaffected by seed availability. Availability of non-prey food resources does not necessarily affect soil insect communities in the short term, but some omnivores that reside in these systems are quick to alter their diet to exploit non-prey resources. Understanding the trade-off involved in consuming foods from different trophic tiers is crucial to understanding where omnivorous animals fit within complex food webs.