Nutritional immunology of a specialist herbivore: Food plant quality mediated effects on the immune system of Manduca sexta (Lepidotera: Sphingidae)

Top down pressure from natural enemies and bottom up from plants are believed to be the main drivers of herbivore diet breadth, but only recently has it been found that diet quality affects immunocompetence and susceptibility towards parasites. Butterflies and moths reared on more toxic plants tend to have suppressed immune activity, but, surprisingly, are more resistant to parasites. To investigate this contradiction further we used Manduca sexta to examine host performance and immune activity across a gradient of plant species. Caterpillar performance was assayed by measuring wet weight at seven days and immunocompetence was determined by measuring the encapsulation response during the mid-third instar. We found that larval growth was significantly influenced by food plant, with individuals generally preforming better on wild versus agricultural species. Interestingly, encapsulation did not seem to vary between food plants, with all individuals encapsulating artificially implanted beads but rarely depositing melanin on their surface. Our findings require further investigation, but one explanation for this might be that M. sexta is able to compensate for differences in plant quality that might affect immune capacity by trading off with growth rate.