Is intra-guild predation likely among lady beetles in maize and soybean?

Some insects that share prey also prey upon each other.  Predictions about this relationship, termed intra guild predation (IGP), suggest that intra guild predators should coexist only rarely.  However, this prediction is not consistently met in observations of nature.  If predators that prey upon each other in a laboratory setting could greatly limit their contact in natural habitats, IGP might actually be much rarer than expected. We evaluated this hypothesis by surveying lady beetles, known to interact via IGP, each week in maize and soybean over four summer seasons (2008-2011) to determine how often potential IG predators occurred on the same plant at the same time.  If the hypothesis that intra guild predators are able to avoid interaction in nature were true, we would expect observed contacts among lady beetles to be much less frequent than the number of contacts predicted assuming lady beetles were randomly distributed among plants. 

Our data did not support the hypothesis that IG predators, in our case lady beetles, are able to avoid contact with heterospecifics, making opportunities for IGP rare.  About two thirds of the time IG predators shared a plant less often than predicted based on their frequencies.  However, larvae, which are much more likely to fall victim to IGP, often faced higher than predicted levels of contact with IG predators.  Further, late in the season predators were in more contact than predicted. Since at this time prey are scarce, it is even more likely that contacts would result in IGP.  These survey methods are valuable in identifying species and life stages for which IG predation may be important to lady beetle population dynamics.