Field evaluation of survival rates of Harmonia axyridis (Coleoptera: Coccinellidae) and other aphidophagous species

The multicoloured Asian ladybird Harmonia axyridis Pallas (Coleoptera: Coccinellidae), previously introduced as a biological control agent against aphids, is now frequently considered as an intraguild predator, consuming other aphids natural enemies. Interactions between this exotic ladybird and the other aphidophagous species present in Belgian agro-ecosystems such as Coccinella septempunctata, Episyrphus balteatus, Chrysoperla carnea are asymmetric to the benefit of H. axyridis. Many studies have already shown its predatory behaviour.
This study focuses on the survival rate of three aphidophagous species Harmonia axyridis (Ha), Coccinella septempunctata (C7) and Episyrphus balteatus (Eb) in biological fields of potato and sugar beet. During June and July, hermetic cages were disposed on the crops with 10 larvae (second stage) of each aphidophagous. There were 4 combinations of insects placed inside the cages (1) Ha+Eb+C7+ aphids, (2) Ha+Eb+C7, (3) Eb+C7+ aphids and (4) Eb+C7. Each combination was repeated three times. Aphids (Aphis fabae) were added in excess when needed.
In sugar beet fields, E. balteatus was the species with the highest mortality rate. The two coccinellid species had a better survival rate than E. balteatus. All aphidophagous species had higher mortality rates in absence of aphids. In potato, the survival rate of all aphidophagous was lower than sugar beet when aphids were present in excess probably because A. fabae reproduced less in potato. Nevertheless, we have observed in potato no mortality of C. 7-punctata during all its development cycle. This suggests that C. 7-punctata could use more efficiently aphids as food sources than H. axyridis. In potato and sugar beet, H. axyridis had the longest development cycle, being still at the larval stage when C. 7-punctata and E. balteatus reached the pupae stage. We can suppose that pupae of E. balteatus and C. 7-punctata were an alternative food (intraguild prey) to H. axyridis when aphids were lacking. Pupae are immobile and therefore more sensitive to the attack of predators.