The toxicity of dursban, reldan and abate were tested against the larvae of Culex pipiens L. and Aedes aegypti (L.), in addition to evaluating effect of two bacterial toxins (Bacillus thuringiensis H-14, and B. sphaericus 1593) against C. pipiens larvae.

The synergistic effects of bacterial toxin-insecticide combinations were calculated and the new LCp-lines were determined by using a sublethal toxin dose of B. thuringiensis and B. sphaericus in combination with the three tested insecticides against Culex pipiens larvae, which was the most tolerant mosquito species.

Bacterial toxins potentiated the three used insecticides at different levels of potentiation, depending on the chemical structure of the insecticide used.

Motile zoospores, the only infective stage of Pythium species, attached, encysted and infected 100% second instar larvae of Aedes aegypti, Aedes africanus, Aedes simpsoni, and Anopheles gambiae. Motile zoospores variably attached and infected Culex spp. and Eretmopodites but did not attack field collected Toxorhynchites sp. We observed mycelial development within the exposed larvae after exposure indicating that mortality was due to the fungus. The fungus infected 75 ± 10% (SD) second instar larvae of Aedes aegypti, under aseptically reared conditions. It infected 65 ± 12.8% (SD) second instar larvae of Culex quinquefasciatus in aseptic conditions in the laboratory. All field collected larvae were heavily infested with vorticella but after treatment with 5% sodium hypochlorite, control mortalities ranged from 0 to 8.6% (mean = 4.3%).

The fungus was most infective between 20–27.5°C; at 30°C many zoospores were attached to the cuticle but did not infect. Mortality rates were as low as 51.5% at 30°C, with no infection at all at 35°C. pH range from 6–8 was optimum for infection, but pH above 8 inhibited infection. Infection level of 100% was obtained in aseptically reared larvae at higher densities of 1600 larvae of second instar larvae of Aedes aegypti and 97% infection for 1600 second instar larvae of C. quinquefasciatus when exposed to fungal infection. Phosphate, borate and acetate buffers inhibited zoospore production.

The larvicidal activity of two entomopathogenic bacteria Bacillus thuringiensis H-14 and B. sphaericus 1593 against mosquito larvae of Culex pipiens has been evaluated. According to LC50 values obtained, B. thuringiensis (4.6 x 105 spores/ml) proved to be highly pathogenic to fourth instar larvae than fi. sphaericus (4.2 x 106 spores/ml).

Larval treatment with LC50 levels of both pathogens affected the reproductive potential of surviving adults during two successive gonotrophic cycles. These treatments caused insignificant changes in some basic morphometric characteristics of mosquito females that developed from surviving larvae.

Three polyvinyl chloride (PVC) formulations of malathion were prepared and tested as controlled-release formulations against mosquito larvae of Culex pipiens L. Non-expanded formulations gave effective control, with 100% larval mortalities, for 23–51 days, followed by expanded (16–33 days) and foamed (14–27 days) formulations. The results indicate that increasing the percentage of active ingredient in the present formulations increased their effective control against mosquito larvae for longer periods.

Larval treatments with these formulations of malathion led apart from lethal action to a decrease in the reproductive potential and in longevity of mosquito adults which survived from these treatments.

Coelomomyces indicus was reisolated from six larvae of Anopheles gambiae in Uganda. A crustacean copepod Cyclops sp. was found to be infected (63%). This indicated that the copepod could be an intermediate host for the fungus. All the four instars were highly susceptible to C. indicus but fourth instar Aedes aegypti was fairly susceptible (94.2%), while second instar An. gambiae (100%) and second instar C. fatigans (100%) were the most susceptible.

Laboratory tests were conducted to compare reproductive capacity and longevity of An. gambiae, C. fatigans and Ae. aegypti when infections of C. indicus were induced in each of the different larval instars. Infections of C. indicus sharply reduced the reproductive capacity and longevity of An. gambiae, C. fatigans and Ae. aegypti, regardless of the instar that were exposed and became infected. The reductions were highest when first and second instars were exposed. The tests indicated that introduction of C. indicus into laboratory populations of An. gambiae, C. fatigans and Ae. aegypti consisting of mixed larval instars, could significantly reduce the number and fertility of eggs and thus result in an average reduction of about 65.4% for An. gambiae, about 46.7% for C. fatigans and 58.7% for Ae. aegypti of the F1 progeny from those females that became infected. The importance of these results in biological control is discussed.

The larvicidal activity of the pathogen Bacillus thuringiensis H-14 against mosquito larvae of Aedes aegypti (L.) and Culex pipiens L. has been evaluated.

The results showed that the susceptibility of culicine larvae to this pathogen was closely correlated with mosquito species tested and its larval instars. The toxicity curves indicated that the serotype H-14 had a higher pathogenicity against A. aegypti than C. pipiens. LC50 values showed that, for both mosquito species tested, the second instar larvae were more susceptible to the pathogen used than the fourth instar larvae.

Apart from lethal action, larval treatments with B. thuringiensis H-14 led to a decrease in the reproductive potential of female mosquitoes that emerged from these treatments.

A large pond, up to 2000 m2 area, adjacent to the sea at Jimbo, near Vanga, on the south coast of Kenya was studied for salinity and the presence of mosquito larvae 1980–1981. Rainfall peaked in November–December 1980 and again in March–May 1981, while salinity of the pond-water fluctuated inversely from dryness in February, more than 100% seawater equivalent in months before main rains, to below 13% seawater after the onset of rain in March.

Five types of mosquito larvae occurred in the pond: Aedes albocephalus mostly at 21–30% salinity (range 13–30%); Culex sitiens mostly at 31–40% salinity (range 13–54%); Culex tritaeniorhynchus and Culex thalassius mostly at 41–50% salinity (range 13–50%) and the Anopheles gambiae complex from 13 to 54% salinity, at least those at higher salinities being Anopheles merus. No larvae were present when salinity surpassed 54% through evaporation, or fell below 13% after rain. The most reliable way to limit or control these pest and vector mosquitoes at Jimbo would be to drain the pond into the sea after rainfall, so that mosquito populations could not increase by breeding sequentially, while salination proceeds.