Predator responses to novel haemolymph defences of Western corn rootworm (Diabrotica virgifera) larvae

Authors: Welch, Kelton; Lundgren, Jonathan

Submitted to: Entomologia Experimentalis et Applicata
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/7/2014
Publication Date: 10/6/2014
Citation: Welch, K.D., Lundgren, J.G. 2014. Predator responses to novel haemolymph defences of Western corn rootworm (Diabrotica virgifera) larvae. Entomologia Experimentalis et Applicata. 153:76-83.

Interpretive Summary: Pest insects that feed on crop plants often have toxic or foul-tasting chemicals in their bodies to discourage predators from attacking them. Predators may learn to recognize these chemicals and avoid attacking these pests, which would reduce the effectiveness of the predators in pest control. It has recently been discovered that Western corn rootworm (WCR), the most destructive pest of corn, uses chemical defenses to deter predators, which is one possible reason why it is such a destructive pest. Here, we conducted feeding experiments in the laboratory to determine whether several types of predators learned to avoid WCR chemical defenses. We compared predator responses to WCR with predator responses to an undefended prey, house fly maggots. In our experiments, different predators responded differently to WCR chemical defenses. Crickets were repelled by the chemical defenses, but nevertheless showed no difference in their willingness to attack WCR or maggots, showing that they did not learn to recognize and avoid the chemical defenses. Centipedes were not repelled by WCR chemical defenses, and attacked both WCR and maggots with equal frequency. Ants were also repelled by WCR chemical defenses, but altered their attack strategy after their first experience with WCR, suggesting that they were learning to overcome WCR defenses. These experiments show that predators can still be useful in controlling WCR, despite its chemical defenses. They also show that chemical defenses do not affect all predators in the same way, which makes it difficult to predict how biological control will be affected.

Technical Abstract: Many herbivorous arthropods use defensive chemistry to discourage predators from attacking. This chemistry relies on the ability of predators to rapidly learn to recognize and avoid offensive stimuli. Western corn rootworm (WCR) employs multifaceted chemical defences in its haemolymph, which may contribute significantly to its success as a major economic pest. Here, we test the hypothesis that agrobiont predators can rapidly learn to recognize and avoid WCR larvae, and thereby reduce their contribution to WCR suppression. In controlled feeding assays, the effectiveness of WCR haemolymph defences varied across three predator taxa (crickets, centipedes and ants). Centipedes were only minimally affected by WCR defences, but crickets reduced the time spent feeding on WCR relative to undefended control prey, maggots. We uncovered no evidence suggesting that crickets are capable of rapidly learning to avoid WCR larvae, indicating that WCR larval defences offer few, if any, survival benefits. However, colonies of Lasius neoniger ants switched from solitary foraging tactics in initial attacks on WCR to group foraging tactics in subsequent attacks, indicating an attempt to overcome, rather than avoid, WCR haemolymph defences. These results suggest that a diverse community of natural enemies will show an array of behavioural responses to toxic pest prey, and highlight the importance of behavioural diversity in driving the function of natural enemy communities.