Differential Susceptibility of Coleomegilla maculata and Scymnus creperus Larvae to Aggression by Solenopsis invicta Workers

SIMPLE SUMMARY: The biological control of aphids by aphid-eating lady beetles (coccinellids) is often hampered by aphid-tending ants or aggressive, invasive ants foraging for food; aggressive species, such as the imported fire ant, may attack and kill lady beetle larvae. This study tested the hypothesis that wax-secreting coccinellid larvae are less susceptible than non-wax-secreting coccinellid larvae to fire ant aggression. Laboratory experiments were set up using bird cherry-oat aphid nymphs and adults (as prey for coccinellids) on barley leaves with either coccinellid species with or without fire ant workers. Results revealed that the presence of fire ants reduced aphid predation by non-wax-secreting but not wax-secreting larvae. The frequency of fire ant attacks was greater on non-wax-secreting larvae; the mortality was significantly greater for non-wax-secreting larvae. The wax covering over coccinellid larvae reduced fire ant aggression. Unexpectedly, coccinellid larvae with the wax cover removed did not suffer greater fire ant attacks or increased mortality. In conclusion, the wax cover and, perhaps, the volatile or non-volatile compounds in the wax and on the integument of wax-secreting coccinellid larvae reduce fire ant aggression. Further research could identify the wax compounds and determine their role as potential repellents or irritants against fire ants. ABSTRACT: The biological control of aphids by aphidophagous coccinellids is often hampered by aphid-tending ants or aggressive, invasive ants foraging for food. Aggressive species such as the imported fire ant Solenopsis invicta Buren may attack and kill coccinellid larvae. This study tested the hypothesis that wax-secreting Scymnus creperus Mulsant larvae are less susceptible than non-wax-secreting Coleomegilla maculata (DeGeer) larvae to S. invicta aggression. Laboratory experiments were set up using bird cherry-oat aphid Rhopalosiphum padi (L.) nymphs and adults (as prey for coccinellids) on barley leaves in arenas with either coccinellid species and with or without S. invicta workers. The presence of S. invicta reduced aphid predation by C. maculata but not Sc. creperus. The frequency of S. invicta attack was greater for C. maculata than Sc. creperus; mortality was significantly greater for C. maculata than Sc. creperus. The wax covering on Sc. creperus reduced S. invicta aggression. Unexpectedly, Sc. creperus larvae with the wax cover removed did not suffer more S. invicta attacks or increased mortality. In conclusion, the wax cover and, perhaps, the volatile or non-volatile compounds in the wax and on the integument of Sc. creperus larvae reduce S. invicta aggression. Further research could identify the wax compounds and determine their functionality as semiochemicals against S. invicta.