Intraguild Predation Responses in Two Aphidophagous Coccinellids Identify Differences among Juvenile Stages and Aphid Densities

(1) Intraguild predation (IGP) can occur among aphidophagous predators thus reducing their effectiveness in controlling crop pests. Among ladybirds, Coccinella septempunctata L. and Hippodamia variegata Goeze are the most effective predators upon Aphis gossypii Glov., which is an economically important pest of melon. Understanding their likelihood to engage in reciprocal predation is a key point for conservation of biological control. Here, we aim to investigate, under laboratory conditions, the level of IGP between the two above mentioned aphidophagous species. (2) Fourth-instars of the two species were isolated in petri dishes with combinations of different stages of the heterospecific ladybird and different densities of A. gossypii. The occurrence of IGP events was recorded after six hours. (3) C. septempunctata predated H. variegata at a higher rate than vice versa (70% vs. 43% overall). Higher density of the aphid or older juvenile stage of the IG-prey (22% of fourth instars vs. 74% of eggs and second instars) reduces the likelihood of predation. (4) To our knowledge, IGP between C. septempunctata and H. variegata was investigated for the first time. Results represent a baseline, necessary to predict the likelihood of IGP occurrence in the field.