Testing the interference competition hypotheses for native lady beetle decline

Lady beetles (Coccinellidae) provide biological control services in agricultural and semi-natural habitats. Recent population surveys show a dramatic decline in several native coccinellid species, coinciding with the establishment of exotic coccinellids. Interference competition via intraguild predation (IGP) of native coccinellid eggs and larvae by exotic coccinellids has been proposed as a mechanism to explain this decline. To test this hypothesis I compared levels of egg predation experienced by an exotic (Harmonia axyridis) and two native (Hippodamia convergens, and Colemegilla maculata) coccinellids within three habitats: alfalfa, soybean, and grassland. Egg IGP was assessed by determining the proportion of native and exotic lady beetle eggs removed by predators within 48 hours. To account for other contributors to egg loss such as wind and rain, eggs in an open (predator accessible) treatment were compared with an exclusion cage treatment. At each site, intraguild-predator populations were surveyed using sweep nets and yellow sticky cards. Aphid density was measured within a 0.25m2 quadrat. Surveillance cameras were used to directly observe the community of predators which attacked eggs of each lady beetle species. Once data collection is completed (September 2010) we will compare IGP among the three focal species and examine the influence of both habitat and landscape structure on predation. I expect to find that 1) predation of native coccinellid eggs will be significantly greater than exotics, 2) exotic coccinellids are a significant contributor to IGP of native species, and 3) aphid abundance within a habitat is negatively correlated with IGP.