The effect of predatory fish exudates on the ovipostional behaviour of three mosquito species: Culex quinquefasciatus, Aedes aegypti and Culex tarsalis.

Three mosquito species, Culex tarsalis Coquillett, Culex quinquefasciatus Say and Aedes aegypti L. (Diptera: Culicidae), were examined in laboratory binary choice experiments to investigate whether fish exudates from the mosquitofish, Gambusia affinis (Baird & Girard) (Cyprinodontiformes: Poecilliidae), deter oviposition and whether the responses of these mosquito species to fish exudates in oviposition sites are consistent with the risk of predation from fish experienced by each species in their respective natural breeding habitats. Culex tarsalis was deterred significantly from egg laying by the presence of fish exudates in oviposition cups, consistent with high levels of predation by fish in natural breeding sites. Egg laying by Cx quinquefasciatus was slightly reduced in water with fish exudates, but was not consistently deterred by water conditioned by mosquitofish, consistent with the species' relatively low risk of fish predation in natural habitats. Oviposition by container-breeding Ae. aegypti was not deterred by the presence of fish exudates in oviposition cups, consistent with a low risk of predation by fish in natural habitats.

Evaluation of vegetation management strategies for controlling mosquitoes in a southern California constructed wetland.

The abundance of mosquito larvae and adult production were measured in 3 vegetation treatments and 2 species of emergent macrophytes in replicated wetland mesocosms (12 x 80 m). During the 8-wk study, no significant differences were found in abundances of larvae and emerging adult mosquitoes among the vegetation treatments: 100% of the surface area in emergent vegetation, 50% of the surface area in emergent vegetation in 5-m-wide rows, and 50% of the surface area in emergent vegetation in 10-m-wide rows. Mosquito larvae (predominantly Culex tarsalis and Anopheles hermsi) were significantly more abundant in inundated bulrush (Schoenoplectus californicus) than in inundated cattail (Typha sp.). Adult emergence from vegetated zones containing bulrush also was significantly greater than from cattail. The failure of reduced emergent vegetation coverage to provide a significant reduction in mosquito production from the vegetated zones of the wetlands might have been caused by favorable conditions for mosquito oviposition and larval development after vegetation management and by the ineffectiveness of mosquito predators in emergent vegetation. A 50% reduction of vegetation did not significantly reduce the water quality of the wetland effluent; however, narrower rows (<5 m wide) of vegetation may be required to reduce mosquito production from vegetated regions of the treatment wetlands. Even though the abundance of mosquito larvae in open water is typically less than in emergent vegetation, creation of open-water zones in shallow treatment wetlands (<1 m depth) by drying the wetland followed by removal of emergent vegetation with heavy equipment is unlikely to provide a significant long-term reduction of mosquito production.

Cyt1Ab1 and Cyt2Ba1 from Bacillus thuringiensis subsp. medellin and B. thuringiensis subsp. israelensis Synergize Bacillus sphaericus against Aedes aegypti and resistant Culex quinquefasciatus (Diptera: Culicidae).

The interaction of two cytolytic toxins, Cyt1Ab from Bacillus thuringiensis subsp. medellin and Cyt2Ba from Bacillus thuringiensis subsp. israelensis, with Bacillus sphaericus was evaluated against susceptible and resistant Culex quinquefasciatus and the nonsensitive species Aedes aegypti. Mixtures of B. sphaericus with either cytolytic toxin were synergistic, and B. sphaericus resistance in C. quinquefasciatus was suppressed from >17,000- to 2-fold with a 3:1 mixture of B. sphaericus and Cyt1Ab. This trait may prove useful for combating insecticide resistance and for improving the activity of microbial insecticides.

Lack of cross-resistance to Cry19A from Bacillus thuringiensis subsp. jegathesan in Culex quinquefasciatus (Diptera: Culicidae) resistant to cry toxins from Bacillus thuringiensis subsp. israelensis.

Culex quinquefasciatus mosquitoes with high levels of resistance to single or multiple toxins from Bacillus thuringiensis subsp. israelensis were tested for cross-resistance to the Bacillus thuringiensis subsp. jegathesan polypeptide Cry19A. No cross-resistance was detected in mosquitoes that had been selected with the Cry11A, Cry4A and Cry4B, or Cry4A, Cry4B, Cry11A, and CytA toxins. A low but statistically significant level of cross-resistance, three to fourfold, was detected in the colony selected with Cry4A, Cry4B, and Cry11A. This cross-resistance was similar to that previously detected with B. thuringiensis subsp. jegathesan in the same colony. These data help explain the toxicity of B. thuringiensis subsp. jegathesan against the resistant colonies and indicate that the Cry19A polypeptide might be useful in managing resistance and/or as a component of synthetic combinations of mosquitocidal toxins.

Emergence patterns of Culex mosquitoes at an experimental constructed treatment wetland in southern California.

The emergence patterns of mosquitoes inhabiting a 0.1-ha experimental wetland in southern California were monitored using emergence traps during the late summer and autumn of 1996. Culex erythrothorax was the largest contributor to emerging populations, comprising 94% of the total emerged adults with an average emergence rate of 59 adults/day/m2. None of the Culex species exhibited a pattern of emergence associated with water depth (range: 5-60 cm). Culex quinquefasciatus and Cx. tarsalis did not show a pattern of emergence associated with the inflow-outflow gradient; however, Cx. erythrothorax emerged in higher numbers along a transect at the middle of the wetland than from near the inflow and outflow. Additionally, the number of emerged Cx. erythrothorax was positively correlated with the density of vegetation below emergence traps. The comparatively large number of adults emerging from the middle of the wetland was most likely caused by a trade-off between an increasing gradient of resource abundance and a decreasing gradient of toxic compounds from the inflow to the outflow of the small wetland.

Cyt1A from Bacillus thuringiensis synergizes activity of Bacillus sphaericus against Aedes aegypti (Diptera: Culicidae).

Bacillus sphaericus is a mosquitocidal bacterium recently developed as a commercial larvicide that is used worldwide to control pestiferous and vector mosquitoes. Whereas B. sphaericus is highly active against larvae of Culex and Anopheles mosquitoes, it is virtually nontoxic to Aedes aegypti, an important vector species. In the present study, we evaluated the capacity of the cytolytic protein Cyt1A from Bacillus thuringiensis subsp. israelensis to enhance the toxicity of B. sphaericus toward A. aegypti. Various combinations of these two materials were evaluated, and all were highly toxic. A ratio of 10:1 of B. sphaericus to Cyt1A was 3, 600-fold more toxic to A. aegypti than B. sphaericus alone. Statistical analysis showed this high activity was due to synergism between the Cyt1A toxin and B. sphaericus. These results suggest that Cyt1A could be useful in expanding the host range of B. sphaericus.

Cyt1A from Bacillus thuringiensis restores toxicity of Bacillus sphaericus against resistant Culex quinquefasciatus (Diptera: Culicidae).

The 2362 strain of Bacillus sphaericus, which produces a binary toxin highly active against Culex mosquitoes, has been developed recently as a commercial larvicide. It is being used currently in operational mosquito control programs in several countries including Brazil, France, India, and the United States. Laboratory studies have shown that mosquitoes can develop resistance to B. sphaericus, and low levels of resistance have already been reported in field populations in Brazil, France, and India. To develop tools for resistance management, the Cyt1A protein of Bacillus thuringiensis subsp. israelensis De Barjac was evaluated for its ability to suppress resistance to B. sphaericus in a highly resistant population of Culex quinquefasciatus Say. A combination of B. sphaericus 2362 in a 10:1 ratio with a strain of B. thuringiensis subsp. israelensis that only produces Cyt1A reduced resistance by >30,000-fold. Resistance was suppressed completely when B. sphaericus was combined with purified Cyt1A crystals in a 10:1 ratio. Synergism was observed between the Cyt1A toxin and B. sphaericus against the resistant mosquito population and accounted for the marked reduction in resistance. However, no synergism was observed between the toxins against a nonresistant mosquito population. These results indicate that Cyt1A could be useful for managing resistance to B. sphaericus 2362 in Culex populations, and also provide additional evidence that Cyt1A may synergize toxicity by enhancing the binding to and insertion of toxins into the mosquito microvillar membrane.

Comparative efficacy of the threespine stickleback (Gasterosteus aculeatus) and the mosquitofish (Gambusia affinis) for mosquito control.

The effectiveness of the threespine stickleback as a mosquito control agent was compared to that of the mosquitofish in 28-m2 earthen ponds during 26-wk experiments where the 2 fish were stocked alone and together. Relative to ponds without fish, the stickleback was not effective for controlling larval mosquito populations; however, sticklebacks reduced the abundance of Culex pupae. Mosquitofish provided significant levels of control whether stocked alone or concurrently with the stickleback. As compared to mosquitofish alone, mosquito control was not significantly enhanced when both fish were stocked together. Mortality of adult sticklebacks was related to a gradient of increasing water temperature across the ponds rather than the direct effects of other abiotic factors such as low dissolved oxygen concentrations or biotic interactions with the mosquitofish. The stickleback exhibited a lower thermal tolerance and slower population recruitment as compared to the mosquitofish populations,which reproduced successfully in water > 33 degrees C and grew rapidly. Stickleback biomass either declined or increased slightly (approximately 50% of initial stocking weight). Mosquitofish biomass increased 33- to 38-fold at rates averaging between 0.079 and 0.095 g wet weight/g/day and total wet weight per pond at 6 wk after stocking did not differ significantly between the 2 mosquitofish treatments.

Dispersal, survivorship, and host selection of Culex erythrothorax (Diptera: Culicidae) associated with a constructed wetland in southern California.

Three mark-recapture studies were carried out at a constructed wetlands facility in San Jacinto, CA, to examine the dispersal and population ecology of the most abundant host-seeking mosquito, Culex erythrothorax Dyar, collected in carbon dioxide-baited traps. Recapture rates were 0.3, 7.4, and 13.9% for August, September, and October, respectively. The mean distance traveled per night was approximately 0.5 km, and females were not recaptured farther than 2 km from the release site. Most marked individuals (> or = 99.5%) were recaptured within 0.5 km of the release point. Marked individuals were recaptured for 33 d after release. Horizontal estimates of survival calculated using recapture data were 0.89, 0.87, and 0.84/d for August, September, and October, respectively. Temporal differences in the recapture rate were attributed to the effects of blood meal acquisition on host-seeking activity versus effects of mortality and strong developmental site fidelity on weekly recapture rates. Partially engorged females collected by CO2-baited traps at the wetland fed predominantly on cattle indicating that host-seeking females were using hosts at dairies surrounding the wetland and were returning to the wetland for resting before seeking an additional blood meal. Estimates of the gonotrophic cycle length and survivorship (vertical estimates) were problematical because of the low parity rates for females collected by CO2-baited traps. Limited dispersal and long survival of Cx. erythrothorax are important factors in the development of large populations at constructed wetlands.

Variable cross-resistance to Cry11B from Bacillus thuringiensis subsp. jegathesan in Culex quinquefasciatus (Diptera: Culicidae) resistant to single or multiple toxins of Bacillus thuringiensis subsp. israelensis.

A novel mosquitocidal bacterium, Bacillus thuringiensis subsp. jegathesan, and one of its toxins, Cry11B, in a recombinant B. thuringiensis strain were evaluated for cross-resistance with strains of the mosquito Culex quinquefasciatus that are resistant to single and multiple toxins of Bacillus thuringiensis subsp. israelensis. The levels of cross-resistance (resistance ratios [RR]) at concentrations which caused 95% mortality (LC95) between B. thuringiensis subsp. jegathesan and the different B. thuringiensis subsp. israelensis-resistant mosquito strains were low, ranging from 2.3 to 5.1. However, the levels of cross-resistance to Cry11B were much higher and were directly related to the complexity of the B. thuringiensis subsp. israelensis Cry toxin mixtures used to select the resistant mosquito strains. The LC95 RR obtained with the mosquito strains were as follows: 53.1 against Cq4D, which was resistant to Cry11A; 80.7 against Cq4AB, which was resistant to Cry4A plus Cry4B; and 347 against Cq4ABD, which was resistant to Cry4A plus Cry4B plus Cry11A. Combining Cyt1A with Cry11B at a 1:3 ratio had little effect on suppressing Cry11A resistance in Cq4D but resulted in synergism factors of 4.8 and 11.2 against strains Cq4AB and Cq4ABD, respectively; this procedure eliminated cross-resistance in the former mosquito strain and reduced it markedly in the latter strain. The high levels of activity of B. thuringiensis subsp. jegathesan and B. thuringiensis subsp. israelensis, both of which contain a complex mixture of Cry and Cyt proteins, against Cry4- and Cry11-resistant mosquitoes suggest that novel bacterial strains with multiple Cry and Cyt proteins may be useful in managing resistance to bacterial insecticides in mosquito populations.

Effect of marsh design on the abundance of mosquitoes in experimental constructed wetlands in southern California.

The species composition and abundance of larval mosquitoes were studied in the vegetated regions of 2 types of experimental constructed wetlands: one-phase marshes, which have continuous vegetation throughout the marsh, and 3-phase marshes, which have 2 vegetated regions separated by a region of comparatively deeper open water. Larvae of Culex spp. were significantly more abundant in one-phase marshes than in 3-phase marshes. Larval populations in one-phase marshes also contained proportionately more older larval instars (stages III and IV) than did populations in 3-phase marshes. Mortality rates of larvae increased during the summer and were higher in 3-phase marshes than in one-phase marshes during the initial 6 wk of the study. Differences in mortality rates between marsh types were related to predator abundance during the first 6 wk of the study and thereafter were not strongly associated with predator populations. An infusion of decaying bulrush (Schoenoplectus californicus) stimulated oviposition by gravid Culex stigmatosoma more than by gravid Culex quinquefasciatus and Culex tarsalis. Culex erythrothorax was the most abundant host-seeking species collected in CO2-baited traps; however, larvae were rarely collected during routine dip sampling and egg rafts were never collected in oviposition studies.

Effectiveness of control measures against mosquitoes at a constructed wetland in southern California.

The effectiveness of larvicide and adulticide treatments against mosquitoes at a constructed wetland in San Jacinto, California was assessed with larval surveys, trapping of emerging adults, and collections of host-seeking females by carbon dioxide-baited traps. Bacillus thuringiensis var. israelensis (Bti, Bactimos pellets) applied at a rate of 19 kg/ha did not demonstrably affect Culex larval and emergent adult populations. Larval populations in the seven marshes of the wetland decreased from approximately one third-fourth instar larva/dip to undetectable levels following two applications of Bacillus sphaericus (Vectolex CG) at a rate of either 19 or 23.6 kg/ha. The largest decline in the number of adult mosquitoes emerging per day from vegetated regions of the wetland occurred after B. sphaericus treatments. The Culex erythrothorax host-seeking population declined about 80-fold during September beginning three weeks after the first treatment with B. sphaericus; however, the Culex tarsalis host-seeking population did not decline abruptly until mid-October 1997. This result suggests that immigration of females from other developmental sites might be an important factor influencing the Cx. tarsalis host-seeking population at the wetlands. Safety concerns required that insecticide applications were carried out during daylight hours, and two daytime applications of adulticide (Pyrenone) in early August were ineffective against mosquitoes resting in the thick vegetation.

Distribution of Culex tarsalis larvae in a freshwater marsh in Orange County, California.

The distribution and abundance of mosquito larvae in a freshwater marsh were determined during a low water period in the late summer. Culex tarsalis larval abundance was significantly associated with the percent cover by Typha spp. root masses and Typha spp. stem density in 1-m2 quadrants. Larval mosquito abundance was not significantly related to bulrush (Scirpus californicus) stem density, position in the marsh (transect compass direction), position in the inundated vegetation on each transect or several physicochemical factors. As water levels decline seasonally, mosquito developmental sites in San Joaquin Marsh change from mats of decaying vegetation to cattail root masses.

Ecology of Culex tarsalis (Diptera: Culicidae): factors influencing larval abundance in mesocosms in southern California.

Colonization and succession of mosquitoes and macroinvertebrate predators were studied in 30-m2 ponds (mesocosms) during summer and fall 1987. Larval abundance of Cx. tarsalis Coquillette was lower during the hot, summer months than during the fall. In all studies, larval populations declined markedly 2-3 wk after habitat flooding. Although predator abundances differed in these studies, sometimes by an order of magnitude, the common predators colonized mesocosms in the following order: Triops, hydrophilid beetle larvae, dytiscid beetle larvae, mesoveliids, dragonfly and damselfly naiads, and notonectids. The similarity of the colonization phenologies probably resulted from the vagility of the adult insects and species-specific developmental rates. Stepwise multiple regression was used to identify factors potentially affecting larval mosquito populations. For most studies, coleopteran larvae were related inversely to per capita change in the entire larval population and the third- and fourth-instar subpopulation (i.e., large coleopteran larval populations were associated with large declines in the Cx. trasalis larval population). Maximum water temperatures and pond age (days after flooding) also were identified as significant factors affecting larval abundance and per capita change of mosquitoes. Potentially lethal water temperatures (greater than or equal to 35 degrees C) occurred during the summer; however, the declines in larval abundance of Cx. tarsalis were not restricted to (or obviously associated with) periods of high water temperature. Our results indicated that predation by coleopteran larvae and factor(s) associated with pond age, such as mosquito ovipositional preferences, significantly affected Cx. tarsalis larval populations.

Rapid evolution of a life history trait.

The copepod Diaptomus sanguineus begins diapause in permanent ponds in late March as an adaptation to avoid summer fish predation. During a study of copepod populations in two Rhode Island ponds, a severe drought dried one pond killing all fish. The second (control) pond did not dry, and no fish were killed. Before the drought copepods in the two ponds entered diapause on nearly the same date. After the drought, the timing of diapause shifted to later in the year in the pond that had lost its fish, while no shift occurred in the control pond. The direction of this shift in the onset of diapause is that expected had the copepods been released from natural selection for early spring diapause imposed by summer fish predation.