Ontogenetic changes in immunity and susceptibility to fungal infection in Mormon crickets Anabrus simplex

Insects have innate immunity that may be weakened by resource allocation to growth. I measured enzymatic immunity, encapsulation response, and susceptibility to fungal infection in Mormon crickets of known age. Although the concentrations of circulating spontaneous and total phenoloxidase (PO) increased with age from the most recent molt in late instar nymphs (5th, 6th, and 7th) and 0-5 day old adults, mean values did not differ between stadia, indicating that circulating PO titers are knocked back with each molt. In contrast, encapsulation rate increased throughout nymphal development and adult maturation. No longer required to molt, adult PO titers increased steadily with age. Survivorship also increased with the age at which Metarhizium acridum fungus was applied to adults. I conclude that immunity relevant to defense against fungi continues to develop well into the adult stage. With each molt setting the insects back in circulating PO titers, very young adults are much like nymphs in enzymatic immunity.