Biological control of annual crop pests provided by mixed prairie of varying diversity

Background/Question/Methods

Biofuel crops composed of mixed native prairie plants can potentially offer ecosystem services such as biological control that benefit nearby annual crops. Such ecological interactions between different crop types can help create a landscape optimized for both food and fuel production.

Communities of native plants are thought to increase the diversity and abundance of predatory insects that feed on crop pests. However, it is not clear if it is diversity of the native plants per sethat increases biological control or whether particular plant species within the mixture provide most or all of the benefit.

We manipulated the diversity of thirty-three 9m x 9m plots of native prairie species at the Cedar Creek Ecosystem Science Reserve, Minnesota, and planted soybean plants adjacent to these plots. Soybean is a major U.S. agricultural commodity and is attacked by the soybean aphid, Aphis glycines, which can cause yield reductions up to 40%.

We inoculated each of 198 soybean plants with soybean aphids and weekly counted the number of aphids and their enemies on each plant. At the end of the season we measured soybean plant biomass and yield. We hypothesized that soybean plants adjacent to higher diversity prairie plots would show reduced aphid numbers, increased enemy numbers, larger soybean plant size, and higher yield.

We also excluded enemies from half of the soybean plants with mesh for two weeks to confirm that enemies are responsible for suppressing aphid numbers within our experiment.

Results/Conclusions

The mesh sub-experiment showed a significant increase in aphid numbers (p<0.001) when enemies were excluded, confirming the occurrence of biological control. As expected, soybean yield was reduced when aphid density was high (p<0.001).

Surprisingly though, there was no significant effect of prairie plant diversity on aphid numbers, soybean plant size, or soybean yield. Plant diversity did  predict abundance of aphid enemies, but with more enemies in lower diversity plots (p=0.006) – the opposite of our hypothesis! This relationship was driven by parasitoid wasps, the second-most abundant enemy group (26% of observations). These parasitoid wasps specialize on aphids and may prefer lower diversity plots where aphids are easier to find. Or they may be subject to higher rates of intraguild predation in plots with higher plant diversity.

These results show that different crop pest enemies have differing responses to plant diversity, implying that the diversity of biofuel crops cannot be used alone to predict biological control strength.