Two common invertebrate predators show varying responses to different types of sentinel prey in a mesocosm study

Sentinel prey (an artificially manipulated patch of prey) are widely used to assess the level of predation provided by natural enemies in agricultural systems. While a number of different methodologies are currently in use, little is known about how arthropod predators respond to artificially-manipulated sentinel prey in comparison to predation on free-living prey populations. We assessed how attack rates on immobilised (aphids stuck to cards) and artificial (plasticine lepidopteran larvae mimics) sentinel prey differed to predation on free moving live prey (aphids). Predation was assessed in response to density of the common invertebrate predators, a foliar active ladybird Harmonia axyridis (Coleoptera: Coccinellidae), and a ground active beetle Pterostichus madidus (Coleoptera: Carabidae). Significant increases in attack rates were found for the immobilised and artificial prey between the low and high predator density treatments. However, an increased predator density did not significantly reduce numbers of free living live aphids included in the mesocosms in addition to the alternate prey. We also found no signs of predation on the artificial prey by the predator H. axyridis. These findings suggest that if our assessment of predation had been based solely on the foliar artificial prey then no increase in predation would have been found in response to increased predator density. Our results demonstrate that predators differentially respond to sentinel prey items which could affect the level of predation recorded where target pest species are not being used.