Background/Question/Methods  Immune defenses confer protection from wounding and invasion. A background level of constitutive immunity is maintained, but wounding or invasion may induce further defenses. A conflict between allocation of resources to growth or defense is well known in many plants, but it is much less explored in insects. Mormon crickets (a katydid) form large migratory bands that march over rangeland in the western United States. They begin to migrate as early as the 4th nymphal instar and continue through their adult lives. Immune defense is particularly relevant to survival in migratory bands, but little is known about whether Mormon cricket immunity changes with growth and development. Our goal was to measure the enzymatic immunity of Mormon crickets during nymphal development and adult maturation. Mormon crickets were collected in Lodgegrass, Montana and fed ad lib in the laboratory. We recorded molt dates; and as an indicator of their capacity to defend against nematodes, parasitoids, or fungi and heal wounds, we measured phenoloxidase (PO) and prophenoloxidase (proPO) activity and total protein in late instar nymphs (5th, 6th, and 7th) and teneral adults between 0-13 days post-eclosion. We also measured the rate that they encapsulated glass rods introduced into their abdomens.

Results/Conclusions  PO, proPO and total circulating protein did not differ significantly between nymphal instars, but PO and total protein increased significantly with time post molt (log PO: n=56, P=0.037; log proPO: n=73, P=0.087; sq. root protein: n=73, P<0.0001). Comparing nymphs and adults between 0-5 days post molt, total protein did not change with age in adults, whereas it increased in nymphs. PO and proPO also increased with age but did not differ significantly between adults and nymphal stages in these first few days of adulthood. From this we conclude that very young adults are much like nymphs in enzymatic immunity. The rate of encapsulation was not significantly different between nymphs and adults within one day of molting. Adults do not mature until 10-12 days post molting, and PO and proPO activity continued to increase between 0-13 days (log PO: n=40, P=0.028; log proPO: n=50, P=0.0029), independent of sex. Total protein did not differ significantly with adult age or sex. Moreover, rate of encapsulation increased with adult age (P=0.016), independent of sex (P=0.39). Our results indicate that each molt sets the insects back in terms of circulating PO, proPO, and rate of encapsulation. A conflict between molting and enzymatic immunity is apparent.