Retention and adaptiveness of photoperiodic EGG diapause in Florida populations of invasive Aedes albopictus.

Female Aedes albopictus, F2-F3 descendents from individuals collected as immatures at 6 geographic sites in the USA during 2008, exposed to short daylengths (10 h of light and 14 h of darkness at 21 degrees C) laid eggs in diapause, whose frequency depended upon population origin. Diapause responses in northern Florida and Illinois were strong, as had been reported approximately 10 years earlier for Ae. albopictus from these regions. For southern Florida, the diapause response was polymorphic, and its mean incidence decreased at 2 of 3 collection sites compared to 10 years earlier. Exposure in the field for 2- to 4-wk intervals in Vero Beach (lat 27 degrees 35'N) during January 2009 revealed that eggs laid by short-day females had significantly higher survivorship, even though <50% were estimated, from laboratory results, to be in diapause. Enhanced desiccation resistance may select for retention of diapause in southern Florida.

Differential Survivorship of Invasive Mosquito Species in South Florida Cemeteries: Do Site-Specific Microclimates Explain Patterns of Coexistence and Exclusion?

Within 2 yr of the arrival of the invasive container mosquito Aedes albopictus (Skuse), the previously dominant invasive mosquito Aedes aegypti (L.) disappeared from many Florida cemeteries. At some cemeteries, however, Ae. aegypti populations seem stable despite Ae. albopictus invasion. We sought to understand this variation in the outcome (exclusion, coexistence) of this invasion, given that previous experiments show that Ae. albopictus is the superior larval competitor. We tested experimentally the hypothesis that climate-dependent egg survivorship differs between exclusion and coexistence cemeteries and that differences in invasion outcome are associated with microclimate. Viability of eggs oviposited in the laboratory and suspended in vases at six cemeteries was significantly greater for Ae. aegypti than for Ae. albopictus, and greater in 2001 than in 2006. Cemeteries differed significantly in egg survivorship of Ae. albopictus, but not of Ae. aegypti, which is consistent with the hypothesis that Ae. albopictus suffers site-specific, climate-driven egg mortality that mitigates the competitive superiority of larval Ae. albopictus. Principal component (PC) analysis of microclimate records from vases during the experiments yielded three PCs accounting for >96% of the variance in both years of experiments. Multivariate analysis of variance of the three PCs revealed significant microclimate differences among the six cemeteries and between exclusion versus coexistence cemeteries. Stepwise logistic regression of egg survivorship versus microclimate PCs yielded significant fits for both species, and twice as much variance explained for Ae. albopictus as for Ae. aegypti in both years. Higher mortalities in 2006 were associated with high average daily maximum temperatures in vases, with lethal thresholds for both species at ≈40°C. From 1990 to 2007, vase occupancy by Ae. albopictus increased and that by Ae. aegypti decreased, with increasing seasonal precipitation at one well-sampled cemetery. Results support the hypothesis that locally variable climate-driven mortality of Ae. albopictus eggs contributes to patterns of exclusion of, or coexistence with, Ae. aegypti.

Does temperature affect the outcome of larval competition between Aedes aegypti and Aedes albopictus?

The superior larval competitive ability of Aedes albopictus has been proposed to explain the recent displacement of Aedes aegyptiby the former species inparts of the southeastern U.S. Ae. aegypti persists, however, in sympatry with Ae. albopictus in urban areas of southern Louisiana, Florida, and Texas, and the impact of larval competition between these species has not been investigated at higher temperatures that may be characteristic of these urban environments. We compared growth and survivorship of the two species at controlled temperatures of 24 degrees and 30 degrees C in water-containing tires under conditions of intra- and interspecific competition and with or without leaf litter. When other variables were controlled statistically, the estimated finite rate of increase (lambda') was significantly higher for both species at the higher temperature, and the proportional increases in lambda' did not differ between species. Therefore, our experiment predicts that by itself, temperatures between 24 degrees and 30 degrees C would not alter the outcome of larval competition. Overall, response measures of Ae. albopictus were more sensitive than those of Ae. aegypti to the litter and species/density variables, although the development ofAe. aegypti females was uniquely retarded by a high density of its own species.

Egg maturation in neotropical malaria vectors: one blood meal is usually enough.

The requirement of more than one blood meal to complete a gonotrophic cycle is reported here to be infrequent among field-collected Anopheles (Nyssorhynchus), including Anopheles darlingi, South America's most important malaria vector. The overall frequencies of host-seeking females that did not develop eggs after one blood feeding were low for Anopheles darlingi (6.6%), Anopheles rondoni (5.0%), and Anopheles triannulatus (2.2%); the majority of wild-caught females that did not develop eggs after one blood meal were nulliparous. Laboratory-reared Anopheles albimanus and Anopheles albitarsis, other species of the same subgenus, were grown on enriched and impoverished larval diets to yield a range of adult sizes and to examine relationships between egg maturation, body size, and blood meal intake. 10.7% of nulliparous An. albimanus and 22.9% of An. albitarsis failed to mature eggs after sugar and one blood meal, and shorter wings and smaller blood meals predisposed females to require multiple blood feeds for egg maturation. Unmated An. albitarsis were significantly more likely than mated females not to develop eggs. Multiple blood meals within a single gonotrophic cycle appear to be less important in the life histories of neotropical Nyssorhynchus species compared to afrotropical malaria vectors.

Long-term dynamics of a predator used for biological control and decoupling from mosquito prey in a subtropical treehole ecosystem.

We identified, staged and counted the immature stages of mosquitoes from 1,826 censuses (with replacement) of the aquatic contents of ten treeholes surveyed every 2 weeks between 1978 and 1993. These time series were used to examine the population dynamics and effect on prey of the predatory mosquito Toxorhynchites rutilus. The mean annual frequency of occurrence of T. rutilus ranged from 0.02 to 0.67 among holes, and no fourth instars were recovered during a 30-month dry period. Oviposition and pupation by this species were recorded in all months, but most commonly in the spring. Overwintering larvae of the predator increased in weight during the prolonged fourth instar that preceded pupation in the spring. Time series analyses showed that the presence of a fourth instar T. rutilus significantly reduced the abundances of late-stage Aedes triseriatus mosquitoes. Pupal numbers of this prey species were more negatively affected by T. rutilus than were numbers of fourth instar A. triseriatus. Long-term declines in mean annual abundance of A. triseriatus prey during 16 years of observations on two holes were not correlated with increases in the mean annual frequencies of T. rutilus. Local extinctions of the aquatic stages of A. triseriatus within treeholes were common, but in most holes not significantly associated with the presence of T. rutilus, suggesting that predation does not routinely drive mosquito prey locally extinct in this ecosystem. The decoupling of T. rutilus and A. triseriatus, as revealed through these complete and long-term censuses, is contrasted with other reports of generalist predators causing extinctions of mosquito prey. Discrepancies among reported outcomes probably result from differences in duration of sampling periods and statistical procedures along with real differences in the intensity of predation among systems and sites.

Seasonal abundance, age composition, and body size of salt-marsh Anopheles (Diptera: Culicidae) in south Florida.

Five CO2-baited light traps operated fortnightly between November 1989 and May 1991 near a salt marsh in Vero Beach, FL, caught 1,961 Anopheles crucians Wiedemann (s.l.) and 691 An. atropos Dyar & Knab. Both species were most abundant in winter and least abundant in summer; their numbers correlated inversely with lagged mean ambient temperatures. Annual parous rates were 55% for An. crucians and 58% for An. atropos and varied little seasonally. Wing lengths of female An. crucians measured for eight consecutive months were negatively correlated with ambient temperatures, and parous females of this species had significantly longer wings than nullipars. Parous rates and wing length classes were positively correlated among An. crucians, confirming that larger mosquitoes are longer-lived.