Effects of prairie and agricultural plant neighborhoods on insect herbivory of a perennial legume

Background/Question/Methods

Perenniality and diversity are two common attributes of ecological communities that are not common in many agricultural systems of today, but could be key attributes of a post-contemporary agriculture that does not rely on inexpensive fuel and fertilizer, is productive, is resilient, and maintains biodiversity.  Local plant neighborhood heterogeneity can influence the frequency and intensity of insect herbivore feeding and abundance.  Therefore, a shift in cropping system from annual monoculture to perennial polyculture could have implications for insect herbivores (pests).  We will examine, using two complementary studies, the influence of three components of plant neighborhood heterogeneity: density, diversity, and configuration, on insect herbivore impact of a perennial legume, Desmanthus illinoensis (Illinois Bundleflower), which is in the early stages of domestication as a grain crop.  The first study evaluates the influence of local plant neighborhood on insect herbivore impact of D. illinoensis in three native prairies.  The second study examines herbivore impact on D. illinoensis in biculture with Thinopyrum intermedium (Wheatgrass) with manipulations of density, diversity, and configuration. 

Results/Conclusions

In the three native prairies, on average, 80% of the D. illinoensis leaves were damaged by insect herbivores.  Despite significantly reducing leaf damage across all three sites (P< 0.001) by at least half using herbivore exclosure, there was no significant increase in inflorescence maturation (P=0.417) or height (P= 0.859) with herbivore exclosure in the prairies   On average, 65% of inflorescences matured.  During the first experimental year of the biculture, on average, 86% of D. illinoensis leaves were damaged by insect herbivores, which is comparable to leaf damage in the prairies.  Leaf feeding by insect herbivores was decreased by seeding configuration only at the lowest density of D. illinoensis; leaf damage was 93% in row seeding and 50% in solid seeding.  In the biculture 68% of D. illinoensis inflorescences matured across all density and configuration treatments; this is comparable to inflorescence maturation in the prairies.  Inflorescence maturation by D. illinoensis was reduced by seeding configuration, especially at lesser densities of D. illinoensis.  Our results may have implications for how much and what kinds of plant neighborhood heterogeneity could be incorporated into perennial grain polycultures to maximize yield and to minimize reduction in yield due to insect herbivores (pests).