The bionomics of the malaria mosquito Anopheles gambiae sensu lato in Southeast Tanzania : adult size variation and its effect on female fecundity, survival and malaria transmission

<p>Size of adult mosquitoes is known to affect both population dynamics as well as disease transmission. Studies devoted to this topic have given different results for different species. For example in some mosquito species, large size was found to be associated with high fecundity and longer survival (Steinwascher, 1982; Nasci, 1986a; 1986b; 1987) but in others large size did not result in longer survival (Walker <em>et al</em> ., <em></em> 1987; Landry <em>et al</em> ., <em></em> 1988; Pumpuni & Walker, 1989). Similar data were found for disease transmission. Some results indicated that smaller mosquitoes transmit Japanese Encephalitis, West Nile and La Cross viruses more efficiently than larger mosquitoes (Takahashi, 1976; Baqar <em>et al</em> ., <em></em> 1980; Grimstad & Haramis, 1984), while other results did not show any difference between small and larger mosquitoes in their ability to transmit viral diseases (Kay <em>et al</em> ., <em></em> 1989). With <em>Plasmodium</em> parasites, Ichimori (1989) did not find any relationship between <em>Anopheles stephensi</em> Liston female size and the number of <em>P.</em><em>yoelii nigeriensis</em> oocysts developed, whereas Kitthawee <em>et. al.</em> (1990) showed that large <em>An. dirus</em> Peyton and Harrison developed more <em>P.</em><em>falciparum</em> oocysts than small ones.<p>Variation in mosquito adult size is associated with the type of breeding sites used by a species. Several studies have shown that temporary habitat breeders are more variable in size than permanent habitat breeders (Haramis, 1983; 1985; Fish, 1985; Nasci, 1987). <em>An. gambiae</em> Giles, the subject of this study, breeds preferably in temporary water bodies and is one of the most size variable anophelines in the sub-Saharan region. No work has previously been undertaken to study the effect of adult size on the bionomics of this mosquito, information which could elucidate our understanding of the biology of this important mosquito.<p>The present research study was initiated in order to answer the following general questions: (1) what causes adult size variation in <em>An. gambiae</em> and (2) how does adult size affect important female characteristics such as reproduction, survival duration and malaria transmission. The specific aims of the study were:<br/>- to investigate the effects of temperature and larval density on development and survival of immature <em>An. gambiae</em> and on the size of adults.<br/>- to investigate the effects of adult size on blood feeding and on fecundity.<br/>- to investigate the effects of adult size on survival.<br/>- and to find out the relationship between adult size and malaria parasite infections.<p>In the course of this research, a total of 50,321 female <em>An. gambiae s. 1.</em> were caught using various sampling methods, and 11,097 wings were measured, wing length being an accepted measurement of body size (Christophers, 1960; Haramis, 1983).<p>Factors affecting larval development and adult size were studied in the laboratory as well as in the field. Mosquitoes were reared in the laboratory under various constant temperatures and densities (chapter 2). In the field, larvae were reared at constant densities under natural fluctuating temperatures (chapter 3). Developmental times and survival rates of immatures under different conditions were monitored and the wing length of emerged females compared. Rate of larval development and immature survival as well as size of adults were determined by the interaction between density and temperature.<p>Natural breeding sites were monitored to determine relative densities and survival of immature stages, and the size of emerging adults. Mortality of immatures was very high (on average 95%), caused mainly by pathogens and by predators, as well as weather conditions. Large, semi-permanent breeding sites produced larger sized females than the small temporary puddles (chapter 4). Spatial and temporal differences in adult size were investigated in field populations of <em>An. gambiae s. 1.</em> There was a significant variation in adult size of populations from different localities, and also a seasonal variation in size of mosquitoes collected from the same locality over a two year period, with larger females being caught during the cooler months of the year (chapter 5). Density of female <em>An. gambiae</em> inside houses peaked towards the end of the rainy season in May, which was accompanied by an increase in entomological inoculation rates (the number of infective bites per person per night). Thus, the intensity of malaria transmission was higher towards the end of the rainy season (chapter 6).<p>Effect of adult size on pre-gravidity and on fecundity was examined for blood fed indoor resting mosquitoes and for newly emerged wild females fed in the laboratory. Females which developed eggs after a single blood meal were larger than those which required more than one meal to produce one batch of eggs, and produced more eggs per batch (chapter 7). Survival of adults was investigated by comparing the size of newly emerged females from field collected pupae with that of nulliparous and parous host seeking females (chapter 8). Newly emerged females were significantly smaller than the host seeking females. There was no difference in mean size between nulliparous and parous host seeking mosquitoes. Small- sized females were equally likely to be infected during blood feeding as were large-sized females, but large females produced more oocysts. The proportion of mosquitoes with sporozoites, however, was highest in intermediate sized females (chapter 9). Finally, the effect of adult mosquito size on the overall malaria transmission was examined using a malaria transmission model described by Koella (1991), (chapter 10). The model predicts that mosquito size has little effect on malaria transmission. Possible reasons for this are discussed.<p>The conclusions from these studies are:<br/>(1) Environmental factors, notably the temperature of breeding water, and the density of larvae directly affect the amount of food available to larvae and influence the development and survival of immatures and the size of adult <em>An. gambiae s. 1.</em><br/>(2) Adult size affects time of first reproduction and fecundity of An. <em>gambiae s. I.</em> , <em></em> hence fitness of individual females.<br/>(3) Small sized mosquitoes die early in adult life and do not contribute much to the bionomics, of the species.<br/>(4) Large-sized mosquitoes produced many oocysts, but they did not survive well enough to transmit the parasites (probably due to their heavy oocyst infections).<br/>(5) The effect of adult size on the overall malaria transmission is negligible due to the effects of co-variation in the transmission parameters