Direct and indirect trophic interactions of ants, spiders, and soybean pests

Biological control practices rely on natural enemies within cropping systems to suppress pest populations. These natural enemies are often generalist predators, allowing them to consume a wide range prey including other predators. We examined these interactions between two important generalist predators– the striped lynx spider (Oxyopes salticus) and the turfgrass ant (Lasius neoniger) – with green clover worm (Hypena scabra) larvae. Our prediction was that each predator would reduce H. scabra populations and leaf damage, but the predators would interfere with each other when both were present. To study these interactions, large-scale field cages containing potted soybeans were used. Eight treatments including a soybean-only control were designed to test this three-way interaction. Organisms were allowed to interact in situ, with damage and population estimates recorded daily. On the final day of the experiment, organism survival was determined and plants from each cage were analyzed for leaf damage using ImageJ. When survival of spiders and clover worms was analyzed, no treatment effects were found. When proportion of leaf damage was compared, the spider-only, spider with ant, and spider with clover worm treatments had significantly less damage than the ant-only, ant with clover worm, and ant with clover worm and spider treatments. These results indicate that spiders decrease plant damage through non-consumptive pathways while ant presence increases plant damage in a similar fashion. This study indicates that ants, in some instances, can inhibit biological control services even when the same species in a different system provide substantial levels of pest suppression.