BEHAVIOURAL, CHEMICAL, AND MORPHOLOGICAL DEFENCES OF INSECTS AGAINST PARASITES AND PATHOGENS

Candidate: Timothy Smith
Supervisor: Dr. Janet Koprivnikar

Abstract:

The threat of parasitism and how it is affected by host traits and defences in insects is an important but understudied field. I investigated the mechanisms of two different host defences, chemical by way of quinone secretions and behavioural avoidance, in the red flour beetle (Tribolium castaneum) against two parasitic threats, the fungus Beauveria bassiana and the nematode Steinernema carpocapsae. I found that both benzoquinones making up the secretion equally contributed to inhibiting B. bassiana growth but showed reduced efficacy after 24 hours, losing all antimicrobial effect after 72 hours. When S. carpocapsae was exposed to these compounds, the higher concentrations reduced nematode survival after four days. However, behavioural assays done over 48 hours indicated no effect of exposure on nematode mobility, and sublethal exposure up to 72 hours did not reduce nematode lethality to greater waxwing moth (Galleria mellonella) larvae, indicating that surviving nematodes remain capable of finding and infecting hosts. To test behavioural avoidance by T. castaneum, I used choice tests representing combinations of the infectious stages of either parasite, and conspecific cadavers that were either euthanized or parasite-killed, with cues tested both singly and in mixtures. I found that red flour beetles generally did not show avoidance of these parasite threat cues, except for avoiding fungi when the alternative was nematodes. This might reflect a heavier reliance on chemical defences by these hosts. Interestingly, I also found a general trend of beetle attraction towards S. carpocapsae-infected cadavers, potentially indicating that infection masks warning signals to conspecifics, or a potential parasite manipulation to bring potential hosts close by. I also investigated how select host traits reflecting their ecology and life history broadly correspond to their use by parasites using a comparative analysis with larval Lepidoptera (moths and butterflies) finding that aspects of their habitat and morphology correspond to their infection by certain parasite taxa. Such work extends our understanding of how a wide suite of insect host traits influence their interactions with different parasites in order to defend themselves against infections, with relevance in both natural and applied settings.