Combined effects of insecticide and IGP on native and invasive ladybeetles in apple orchard

The Asian ladybeetle Harmonia axyridis Pallas is an invasive species commonly found in Canadian apple orchards. It threatens native ladybeetles through exploitative competition and intraguild predation (IGP), including the 12 spotted ladybeetle Coleomegilla maculata DeGeer (Gagnon, Heimpel and Brodeur, 2011). Both species are exposed to novaluron, a reduced risk insecticide used against the codling moth Cydia pomonella (L.), the primary pest in apple orchards. Previous research showed that novaluron is highly toxic for the invasive H. axyridis, but it is less toxic for the indigenous C. maculata. Novaluron reduces the adaptive value of IGP for H. axyridis but not for C. maculata. However, there is a lack of knowledge on the combined effect of novaluron and IGP outside laboratory conditions. This study explores under semi-field conditions the effect of novaluron residues and IGP on the survival of H. axyridis and C. maculata larvae. IGP was assessed considering third instar larvae as predators. Results showed that novaluron was toxic for first instar larvae of both species after 10 days of exposure. When H. axyridis was the intraguild predator, novaluron increased the survival of the intraguild preys. Novaluron did not change the outcome of the IGP when C. maculata was the intraguild predator. These results indicate that novaluron can change the competition outcome between the invasive H. axyridis and a native ladybeetle species. This is the first demonstration that intraguild predation and competition may be greatly altered in the presence of pesticides.