Jun 2, 2002 - 24° and 30°C in water-containing tires under conditions of intra- and interspecific competition and with or without leaf litter. ... (Skuse) in the U.S. that began in the 1980s (Hawley 1988, ..... Nasci, R.S., C.G. Hare and F.S. Willis.
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Journal of Vector Ecology
June, 2002
Does temperature affect the outcome of larval competition between Aedes aegypti and Aedes albopictus? L.P. Lounibos1, S. Suárez2, Z. Menéndez2, N. Nishimura1, R.L. Escher1, S.M. O’Connell1, and J.R. Rey1 1
University of Florida, Florida Medical Entomology Laboratory, 200 9th St. SE, Vero Beach FL 32962 USA 2 Departamento de Control de Vectores, Instituto de Medicina Tropical “Pedro Kouri” Apartado 601, Mariano 13, La Lisa, La Habana, Cuba Received 18 September 2001; Accepted 14 November 2001 ABSTRACT: The superior larval competitive ability of Aedes albopictus has been proposed to explain the recent displacement of Aedes aegypti by the former species in parts 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° and 30°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 (λ´) was significantly higher for both species at the higher temperature, and the proportional increases in λ´ did not differ between species. Therefore, our experiment predicts that by itself, temperatures between 24° and 30°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 of Ae. aegypti females was uniquely retarded by a high density of its own species. Journal of Vector Ecology 27(1): 86-95. 2002. Keyword Index: Competition, finite rate of increase, invasive mosquitoes, temperature, tires.
INTRODUCTION The establishment and spread of Aedes albopictus (Skuse) in the U.S. that began in the 1980s (Hawley 1988, Craig 1993) was associated with a reduction in abundance and range of the yellow fever mosquito Aedes aegypti (L.). The decline and local disappearance of Ae. aegypti that followed the spread of Ae. albopictus in the southeastern states has been well documented (Hobbs et al. 1991, Mekuria and Hyatt 1995, Nasci et al. 1989, O’Meara et al. 1995). In Florida, the yellow fever mosquito remains common only in urban sections of the southern portion of the state (O’Meara et al. 1995; O’Meara, personal communication). Various mechanisms have been proposed to explain the invader-mediated demise of Ae. aegypti in the U.S., including differential effects of a gregarine parasite introduced with Ae. albopictus (Craig 1993, Blackmore et al. 1995); a ‘satyr’ effect (Ribeiro 1988), whereby asymmetric cross-mating was proposed as causing female sterility of Ae. aegypti (Nasci et al. 1989); egg
hatching inhibition by Ae. albopictus larvae that differentially impacts Ae. aegypti (Edgerly et al. 1993); and larval resource competition (Juliano 1998). This last hypothesis is based on compelling evidence that larvae of Ae. albopictus are superior to those of Ae. aegypti in growth and survivorship under conditions of intra- or interspecific competition in the presence of limiting, litter-based resources (Barrera 1996, Juliano 1998, Daugherty et al. 2000). The laboratory studies that concluded Ae. albopictus is the superior competitor were run at 24.525.5°C (Barrera 1996) or 27°C (Daugherty et al. 2000). Field experiments by Juliano (1998) that demonstrated the superior competitive ability of larval Ae. albopictus were conducted outdoors in shaded tires in southern Florida where the average daytime temperature, measured mornings, was 24.8°C (Juliano, personal communication). Although natural populations of Ae. aegypti have been largely extinguished from such wooded habitats of southern Florida, this species persists in nearby urban sites (O’Meara, personal
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Journal of Vector Ecology
communication). In unshaded urban tires occupied only by Ae. aegypti in West Palm Beach, we have routinely recorded summertime water temperatures of 33-37°C (Lounibos & Rey, unpublished data). Many experimental studies have demonstrated that a change in temperature can reverse the outcome of interspecific competition between insects that occupy the same habitat. Classic research by Birch (1953) showed that the outcome of competition between grain beetles was reversed by a 3.2°C temperature change, Rhizopertha dominica predominating at 32.3°C and Calandra oryzae at 29.1°C. Between competing flour beetles, a temperature of 34°C and relative humidity of 70% favored Tribolium castaneum, but at 24°C and 30% RH, Tribolium confusum dominated T. castaneum (Park 1954). The predominant species in interspecific competition between closely related species of Drosophila was determined by whether experiments were conducted at 19° or 25°C (Ayala 1970). Russell (1986) reported that the native Australian mosquito Aedes notoscriptus had a slight competitive advantage over Ae. aegypti at 22°C, but the latter introduced species predominated at 28°C. Alto and Juliano (2001) showed experimentally that population growth of Ae. albopictus is directly proportional to temperatures from 22 to 26oC, although the carrying capacity (which may be more relevant to competitive ability) of this species was lower at the higher temperature. To date there have been no published reports comparing the outcome of interspecific competition between Ae. albopictus and Ae. aegypti at different temperatures, although the emergence rate of Ae. aegypti was claimed to be higher than that of Ae. albopictus at 30 o C but not different at a lower (unspecified) temperature (Huang 1997). The current study was designed specifically to test the effects of two temperatures experienced by larvae and pupae on correlates of population growth of Ae. aegypti and Ae. albopictus and to evaluate whether the former species might be a superior competitor at 30 oC compared to 24 oC. Species/density treatments and the presence of food, in the form of decaying leaf litter, were also manipulated in our experiments. These variables have been shown to interact with temperature in the population responses of other insect species (Davis et al. 1998). MATERIALS AND METHODS F1 progeny of Ae. aegypti and Ae. albopictus from 1999 collections of these species from discarded tires and cemetery vases in south Florida were used. Discarded golf cart tires (outer diameter = 40.6 cm, inner
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diameter = 19.7 cm, width = 20.3 cm) obtained from nearby golf courses were used as experimental containers. Before use, these tires were immersed in scalding water, scrubbed with a brush dipped in a dilute bleach solution, and thoroughly rinsed. Water for experiments (= pre-treated tire water) was suctioned from mosquito-containing automobile tires on the grounds of the Florida Medical Entomology Laboratory (FMEL). This water was sieved to remove most macrobiota and detritus and frozen at -20°C to kill remaining organisms. Fallen oak (Quercus virginiana) leaves were collected, oven-dried at 80°C for 48 hours, and weighed into one-gram aliquots for plus-litter treatments. Forty cleaned tires were rested vertically against the walls of each of two walk-in insectaries, one maintained at 24+1°C and the other at 30+1°C. These experimental temperatures were chosen based on the preliminary data, cited above, from wooded and urban habitats in southern Florida as well as the report of Huang (1997). Daily light:dark regimens were 12L:12D in both insectaries. One day before hatch of larvae, each tire received one liter of pre-treated tire water, and 20 tires at each temperature received one gram of dried oak leaves. The pre-treated tire water was judged to contain sufficient organic resources for the completion of larval development in the 20 tires that received no litter. Eggs of each Aedes species were hatched in deoxygenated water. First-instar larvae
