ARS | Publication request: Ontogenetic changes in immunity and susceptibility to fungal infection in Mormon crickets Anabrus simplex

Submitted to: Journal of Insect Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 12, 2011
Publication Date: January 1, 2012
Repository URL: http://handle.nal.usda.gov/10113/55067
Citation: Srygley, R.B. 2012. Ontogenetic changes in immunity and susceptibility to fungal infection in Mormon crickets Anabrus simplex. Journal of Insect Physiology. 58: 342-347.

Interpretive Summary: We investigated whether the immune system of insects might be weakened by the need to allocate resources to growth. We found that concentrations of circulating phenoloxidase, a key enzymes involved in the immune system, increases with age of Mormon crickets following each molt. However, its concentration is knocked back at each molt such that average levels do not increase from one nymphal stage to another. This is probably due to the fact that phenoloxidase is also involved in cuticular tanning which reduces the circulating concentration at each molt. As a result, very young adults are much like nymphs in enzymatic immunity. Because adults no longer molt although they still gain weight, their phenoloxidase concentration can increase with age without interruption. In contrast, the ability of the insects’ humoral and cellular defenses to encapsulate a foreign object introduced into its abdomen increased steadily with age throughout the larval and adult stages. The ability of the Mormon crickets to survive contact with Metarhizium anisopliae, a pathogenic fungus, also increased with adult age. Thus the ability of Mormon crickets to defend against fungus continued to develop well into the adult stage. When managing Mormon cricket outbreaks with pathogenic fungi, this study indicates that, all else being equal, fungal application to nymphs will yield the best results.

Technical Abstract: 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 age at which Metarhizium anisopliae 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.