The Effect of Larval Density on Adult Longevity of a Mosquito, Aedes sierrensis: Epidemiological Consequences

Abstract

SUMMARY (1) A 3-year field study of larval populations of Aedes sierrensis, a common North American tree-hole mosquito, produced no evidence for density-dependent larval mortality, but revealed that pupal weight of females was inversely correlated with larval density. (2) Sampling of a wild adult population indicated that, in general, larger adults lived longer than smaller ones. The interval between blood meals in nature was estimated as 8 days. (3) The total expectation of infective life (for transmission of a nematode, Dirofilaria immitis) for females was estimated within the range of larval densities observed in nature. As these estimates may be maximal at intermediate population densities, reduction of larval density may result in an increase in the capacity of the adult population to transmit disease. Accurate assessment of the longevity of adult female mosquitoes has long been recognized as prerequisite to quantification of the ability of a species to transmit disease (MacDonald 1957; Garrett-Jones & Grab 1964; Garrett-Jones 1964). In no case, however, has it been shown that intraspecific variation in adult longevity may be related in a simple fashion to larval density, and that this relationship has important epidemiological consequences. The present study shows that for a wild mosquito population (i) pupal size is inversely related to larval density and (ii) adult longevity is positively correlated with adult size. Equations describing these relationships are linked by an equation relating pupal weight to adult size, then used in conjunction with MacDonald's (1957) equation describing adult survivorship to explore the relationship between the total expectation of infective life of females arising from a given population and the larval density of that population. The treehole-breeding Aedes sierrensis (Ludlow) is an appropriate species for this kind of study because: (i) it is common and easily identified; (ii) as its larval habitat is confined, sampling is simplified; (iii) it is univoltine, allowing easy estimation of larval mortality; (iv) it is the only common mosquito inhabiting tree-holes throughout most of its range (Zavortink 1985); and (v) it is a vector of a nematode parasite of canids, Dirofilaria immitis (Leidy) (Weinmann & Garcia 1974; Walters & Lavoipierre 1982). Life history of A. sierrensis Aedes sierrensis is prevalent on the west coast of North America from southern California to British Columbia. Winters in this region are mild and wet, while summers are