Disease susceptibility and the adaptive nature of colony demography in the dampwood termite Zootermopsis angusticollis

The effect of demographic structure on disease resistance was studied in experimental colonies of the dampwood termite Zootermopsis angusticollis Hagen. Instar and group demography were found to be significant and independent predictors of susceptibility to infection after termites were exposed to 10(1)-10(7) spores/ml suspension of the fungus Metarhizium anisopliae (or a spore-free medium) and subsequently maintained in isolation, or in homogeneous and heterogeneous instar groups. Nymphs, which were the oldest and largest individuals, had the highest survivorship. The youngest termites (instars III and IV) had approximately 3.6 and 2.0 times the hazard ratio of death of nymphs and were the most likely to succumb to disease. Termites in instar V had a hazard ratio of death 1.2-fold that of nymphs; termites in instar VI did not differ significantly from nymphs in susceptibility. Analysis of the survivorship patterns of each instar showed that (1) isolated termites exposed to fungal spores were significantly more susceptible to infection than similarly exposed individuals maintained in same-instar social groups, and (2) spore-exposed termites living in same-instar groups were significantly more susceptible to disease than similarly exposed individuals maintained in mixed-instar groups. Survivorship indices of spore-exposed termites in mixed-instar groups were the highest among all experimental treatments, approaching those of unexposed controls. Colony demography thus had a significant influence on individual survivorship and reduced mortality risk. Z angusticollis colony demography results from the asynchronous development of termites which hatch during successive bouts of oviposition. We suggest that the demographic distribution of colony members produced by this overlap of generations reduces disease risk and is a previously unrecognized adaptive role of the caste distribution function.