Colonization of apple orchards by predators of Dysaphis plantaginea: sequential arrival, response to prey abundance and consequences for biological control

Episyrphus balteatus (DeGeer) (Diptera: Syrphidae), Adalia bipunctata (L.) (Coleoptera: Coccinellidae) and Aphidoletes aphidimyza (Rondani) (Diptera: Cecidomyiidae) are the three most abundant natural enemies of Dysaphis plantaginea Passerini (Homoptera: Aphididae) in Asturian (NW Spain) apple orchards. They attack this aphid in sequence: E. balteatus arrived first, followed by A. bipunctata and then by A. aphidimyza. The cecidomyiids arrived too late to have a regulating effect. The syrphids laid an average of 2.3 ± 1.7 eggs per aphid colony and the coccinellids 18.4 ± 9.9 regardless of the degree of the infestation rates of the apple shoots. This value corresponds to the size of an egg batch laid by one female. Therefore, these aphid predators did not respond numerically to the abundance of the pest. The results of this study indicate that natural populations of syrphids and ladybird beetles are unable to control D. plantaginea, and therefore a more complex strategy than waiting for natural enemies is required.