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

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.

Keywords: aphids; biological control; defense mechanism; ladybird beetles; predation; semiochemicals.

Figure 1

(A–E) Images of R. padi nymphs and adults on barley leaf (A) and other R. padi individuals consumed by 3rd instar C. maculata (B). Image of S. invicta worker attacking 3rd instar C. maculata (C) in the arena. The blue arrow illustrates S. invicta biting C. maculata; the orange arrow illustrates a drop of hemolymph exuding from the integument of C. maculata after being stung by S. invicta. Images of 3rd instar Sc. creperus not being attacked by S. invicta workers (D,E).

Figure 2

The mean ± SE number of aphids killed by 3rd instar C. maculata in the presence or absence of S. invicta (A), and a scatterplot of the number of aphids killed by C. maculata versus the time (min) required for S. invicta to kill C. maculata (B). The mean ± SE number of aphids killed by 3rd instar Sc. creperus in the presence or absence of S. invicta (C). The sample size was 40 observations for the test with C. maculata and the test with Sc. creperus. The scatterplot represented 13 observations.

Figure 3

The mean ± SE proportional frequency of attack by S. invicta and mortality of C. maculata or Sc. creperus (A,B). The sample size was six observations for proportional attack frequency and proportional mortality estimates for C. maculata or Sc. creperus with or without S. invicta. The mean ± SE proportional frequency of attack by S. invicta and mortality of C. maculata, Sc. creperus, or non-wax covered Sc. creperus (C,D). The sample size was nine observations for proportional attack frequency, and mortality estimates for C. maculata, Sc. creperus, or non-wax-covered Sc. creperus with or without S. invicta.