Factors affecting the distribution and numbers of the neotropical mud-wasps Zeta abdominale (Drury) in Jamaica and Z. canaliculatum (Oliv.) in Trinidad are discussed. Rainfall was a limiting factor in both aspects of the ecology of the two species. In Jamaica, Z. abdominale was virtually absent from rainfall zones >2540 mm m.a.p. (mean annual precipitation), whereas in Trinidad, Z. canaliculatum hardly inhabited those localities of > 2250 mm m.a.p. Furthermore, the local populations were progressively smaller and more scattered in wetter areas, because consistent rainfall and overcast conditions may result in reduced nesting activity. However, in a favourable locality, numbers may be temporarily increased after rainfall which boosts food supply (caterpillars).
Developmental mortality, at 57.68% and 57.2896 respectively was remarkably similar in Z. abdominale and Z. canaliculatum. The important predators within the cells were Melittobia sp. (hawaiiensis complex) (Eulophidae) and Amobia fioridensis Townsend (Miltogramminae). In Zeta abdominale total developmental mortality was positively correlated with log,0 (no. of cells) at different sites; averaging only 33.3% for small populations, but 77.4% for large ones.
Regulation at low numbers (Z. abdominale) was probably achieved by harsh physical factors, causing low fecundity, but great survival in the cells and high emigration rate. Where Z. abdominale was numerous, the physical environment was favourable, but developmental mortality was so high, that it is necessary to postulate the immigration of many nesting females.