The influence of aphid abundance on coccinellid egg predation in tallgrass habitats in Missouri

Populations of native coccinellids have been declining concurrent with the rise of exotic species.  Previously we documented a greater abundance and richness of native lady beetle species in agricultural tallgrass habitats as compared to natural tallgrass habitats in Missouri.  Given that the abundance of aphids, a common food source for predatory insects, was greater in agricultural than natural tallgrass habitats, we hypothesized that the presence of this alternative food source diminished the vulnerability of native lady beetle eggs to predation. To explore this potential mechanism in more detail, we manipulated aphid abundance in agricultural and natural tallgrass habitats through the application of an aphicide and compared the amount of predation on released clutches of eggs of a native lady beetle species, Hippodamia convergens, and an exotic species, Coccinella septempunctata.  In plots where aphid abundance was reduced, a lower proportion of eggs were consumed overall (both native and exotic).  Regardless of aphid abundance, eggs of the native species were consumed at a greater rate than those of the exotic species.  These findings are inconsistent with the previously established trend for a greater abundance of native lady beetles occurring in these tallgrass habitats.  Because many species of lady beetles are known to be polyphagous, it is possible that these habitats provide additional food resources throughout the growing season such as pollen, nectar and fungal spores that enable the native species to persist while the larger-bodied exotic species migrate to nearby habitats where food resources, such as aphids, are abundant and readily available.