Predation risk and diet expansion in a native arthropod predator

Background/Question/Methods

A generalist predator should expand its diet to include nutritionally suitable, novel prey.  However, the predator may avoid novel prey associated with habitats in which it faces an increased risk of being preyed upon.  The native, predatory coccinellid Coleomegilla maculata, an extreme generalist, seldom colonizes soybeans despite the introduction of an abundant and suitable novel prey, the soybean aphid.  Because soybean habitats are associated with an invasive, intra-guild predator, Harmonia axyridis, we hypothesized that use of the novel prey by the native predator is limited by mortality due to predation.  

We compared native predator mortality in soybean to mortality in maize, where it is abundant, using four blocked treatments of maize and soybean in central Minnesota.  Throughout the 2008 and 2009 growing seasons, we conducted weekly visual counts of coccinellids in fifteen randomly selected locations per treatment/block.  We calculated mortality using the KNM method and determined p-values by randomizing the data.  In 2010 we placed C. maculata egg masses on ten plants per treatment/block at peak egg laying and a week later and measured predation after one day.  Percent predation by treatment was compared using ANOVA.

Results/Conclusions

Our data do not support the hypothesis of higher predation in soybean.  Although egg mortality was high in both habitats, it was lower in soybean compared to the preferred habitat, maize, in both 2008 and 2009 (Fisher’s combined probability test, p=0.012).  Predation on sentinel egg masses was also lower in soybean than in maize (p<0.05). 

Our results are not consistent with the explanation that predation risk prevents the native predator from exploiting the novel prey.  However, very low abundances of the native predator in soybean suggest that some other factor limits the native’s use of the novel prey.  Potential factors may include limited ability to detect damage by soybean aphids on soybean plants or lack of preference for soybean aphid compared to other potential food sources.