Population dynamics of a thelastomatid nematode of cockroaches

Abstract

SummaryBlatticola monandros Zervos lives in the hind gut of the endemic New Zealand cockroach Parellipsidion pachycercum Johns. A field survey showed that infection prevalence was high (54 %) in small nymphs and increased further with cockroach size to a maximum (92 %) in penultimate nymphs. Variance to mean ratios and the Chi-square test for goodness of fit of a Poisson distribution provide strong evidence that the distribution of the nematode was not clumped (over-dispersed) or Poisson (random) but under-dispersed in most host size classes. The modal infrapopulation type consisted of 1 adult male and 1 adult female. Other infrapopulations were transitional to this type. Monogamous infrapopulations increased in prevalence with increased host size. Juvenile males were uncommon and probably develop faster into adults than juvenile females. Infrapopulations with 2 juvenile males were very rare and no cockroach contained 2 adult males, although some contained 2 adult females. Seasonal variations in prevalence and infrapopulation structure were slight over a 3 year period, except during one unusually dry summer. During the drought, prevalence of monogamous infrapopulations and of transitional infrapopulations were lowest, while prevalence of single-worm and single-sex infrapopulations were greatest. It is evident that competition between females reduces fecundity. When only 1 adult female nematode is present in an adult cockroach, more eggs are produced than the total produced if 2 adult females are present. Eggs were produced cyclically with short periods of high egg production interspersed with several days of very low or no egg production. Most eggs produced on any one day were attached to the outer surface of only 1 of the up-to-6 faecal pellets produced by the host/day. Mechanisms which may regulate infrapopulations and control reproductive competition and cyclical egg production are discussed. It is suggested that B. monandros regulates its own infrapopulation.