Increased abundance, size, and longevity of food-deprived mosquito populations exposed to a fungal larvicide.

To determine whether the quantity of food available to mosquitoes in their aquatic environment limits the effectiveness of microbial pathogens as biological control agents, experimentally well-nourished and malnourished larval Aedes aegypti (Linn.) were exposed to graded inocula of the entomopathogenic fungus, Metarhizium anisopliae. First instar larvae were provided access either to 3 or to 5 mg of food, and lots from each food regimen were inoculated with 20, 40, 60, or 80 micrograms of fungal spores/ml water. Application of the fungus to well-nourished larvae reduced the proportion developing to the adult stage, and increased the size of those adults that developed; their survival was not affected. In the case of malnourished larvae, such applications appeared not to effect the proportion of larvae that matured, and, paradoxically, increased the size and longevity of these resulting adults. By destroying a portion of the larvae, the pathogen apparently reduced competition among malnourished larvae, thereby enhancing their survival, as well as the size of the resulting adults. Thus, biocontrol agents may fail when used as larvicides against such nutrient-deprived mosquitoes as frequently occur in nature.

Soil Applications of Steinernematid and Heterorhabditid Nematodes for Control of Colorado Potato Beetles, Leptinotarsa decemlineata (Say).

Three strains of Steinernema feltiae Filipjev (All, Mexican, and Breton strains) and one of Heterorhabditis heliothidis (Khan, Brooks, and Hirschmann) were evaluated for their potential to control Colorado potato beetle (CPB), Leptinotarsa decemlineata (Say), larvae and pupae in the soil. In laboratory studies, H. heliothidis and S. feltiae (Mexican strain) produced the highest mortality (6 days posttreatment) of CPB when applied to the surface of a soil column containing mature CPB larvae 5 cm below. Mortality ranged from 80 to 90% at rates of 79-158 nematodes/cm(2). Similar results were seen in a field microplot study with all four nematodes; S. feltiae (Mexican strain) and H. heliothidis were most effective. Adult CPB emergence was reduced 86.5-100% after application of 31-93 H. heliothidis/cm(2) and 88.4-100% with 93-155 S. feltiae (Mexican strain)/cm(2). The All strain of S. feltiae was moderately effective (ca. 80% reduction at 93-155 nematodes/cm(2)), while the Breton strain was ineffective (< 40% reduction at 155 nematodes/cm(2)). In small plots of potatoes enclosed in field cages, application of H. heliothidis and S. feltiae (Mexican strain) at rates of 93-155 nematodes/cm(2) before larval CPB burial in the soil resulted in 66-77% reduction in adult CPB emergence. Soil applications of these nematodes show potential for biological control of CPB.

Paradoxical effects of simulated larviciding on production of adult mosquitoes.

We determined whether simulated mortality of larval mosquitoes in food-deprived bodies of water increases emergence of adult females as well as their size by releasing the remaining larvae from competition for food. Removal of larval Aedes aegypti from rearing containers with suboptimal amounts of food increased the number of adults that emerged. Thus, larvicidal measures do not inevitably lead to reduced numbers of mosquito vectors of disease. In food-limiting breeding sites, partially effective anti-larvae measures can be counterproductive, perhaps increasing transmission of vector-borne disease.