Search

In Culex pipiens mosquitos from Southern France, resistance to the organophosphorus insecticide chlorpyrifos is due to the dominant allele ( Chl) of an autosomal gene. The Chl gene is localized between the a-Gpd and Est-2 loci at 26.8 and 5.8 units of crossing-over respectively. [ABSTRACT FROM AUTHOR]

Six quality-control laboratories in 4 countries independently bioassayed aliquots of a flowable formulation of Bacillus thuringiensis var. israelensis (B.t.i.) against the international standard powder IPS-82. All laboratories substantially followed World Health Organization or U.S. Department of Agriculture standard protocols. Significant differences were found in resulting potency values between laboratories. Factors that may have influenced results, such as age, stage, and strain of larvae used, amount and type of food provided to larvae, and processing of samples were examined. Use of different rearing temperatures, different strains of Aedes aegypti L., or late 3rd instars vs. the recommended early 4th instars did not explain the inconsistencies. The slope of the dose-response curve of the IPS-82 powder was influenced by particle size, which varied with the nature and duration of sample homogenization. Laboratories using low-intensity processing obtained a greater slope in the dose-response curve for the flowable product than for the powder standard. The type and quantity of food provided to larvae affected susceptibility. Larvae fed an excess of protein-rich food became 4th instars in 3 days and were less susceptible to B.t.i. than those fed smaller quantities of carbohydrate-rich food that became 4th instars in 5-7 days. Overall, deviations from standard protocols with regard to larval stage, holding temperature, and lighting regime may not be as important as differences in sample processing and pretest rearing conditions. The need to improve standardization in these areas, which are not clearly specified in current protocols, is discussed.

Only one Bacillus sphaericus strain, strain 2362, is currently used commercially to control Culex larval populations. A reliable methodology, easily used, was developed to identify new strains for field application. Larvicidal activities of 3 highly mosquitocidal strains, strains C3-41, Mal, and LB24, previously selected in the laboratory, were compared with that of strain 2362 in tropical and European countries. The following steps were performed: production and titration of acetonic powders from these 4 strains on local Culex species, survey of initial and residual activity under standardized indoor and outdoor conditions, and evaluation of the efficacy of liquid formulations of the 4 strains in natural breeding sites of Culex. In indoor conditions, strain C3-41 showed the highest activity on both Culex pipiens and Culex quinquefasciatus; strain Mal was the least active. The residual activity causing 80% mortality differed from 20 to 90 days according to the strains and the country. Outdoor experiments with powders (0.02-1.6 mg/liter) were performed and the initial toxicities were similar in all cases. Residual activities were very different, from 6 to 95 days posttreatment. Liquid formulations were applied to larval habitats (from 0.1 to 10 g/m2). In tropical countries, larval recolonization in cesspits or ponds occurred after 10-35 days. In Europe, higher doses were needed in polluted water than in clear water (from 3 to 10 liter/ha) for the same control, and the time before 80% residual activity was reached was less than 9-12 days. However, in cesspits, residual activity could be observed for 12 days to 5 mo. A strain 3-5 times more active than the others in bioassays is not significantly detectable from those strains in field trials.

Field Culex pipiens pipiens (L.) mosquitoes that were collected after a control failure with Spherimos in southern France developed high resistance (> 10,000-fold) to Bacillus sphaericus crystal toxin after < 8 generations of laboratory selection. We show that this resistance is encoded by a single major recessive gene on linkage group I at 22.1 recombination units from the sex locus, and that it is not associated with any loss of binding affinity between brush border membrane fractions and the B. sphaericus radiolabeled toxin. Thus, in Southern France, resistance differs from the high B. sphaericus resistance developed after laboratory selection of Californian C. p. quinquefasciatus. This demonstrates that at least 2 different mechanisms may confer high levels of resistance to B. sphaericus crystal toxin in mosquitoes of the C. pipiens complex. These results have important implications for mosquito control strategies.

Bacillus sphaericus was used to control Culex pipiens breeding in a water treatment settling basin near Montpellier, France. Four treatments with 4 liters/ha (3.6 lb/acre) of commercially available B. sphaericus formulation (Vectolex) reduced the larval population of Cx. pipiens with 50-600 spores of B. sphaericus/ml recorded in the treated portions of the water plant 14 days following the last treatment. Natural recycling of the bacteria was shown to take place at the water surface where germination of spores was evidenced. The appearance of vegetative forms of B. sphaericus may be linked to the passage through larval guts of several filter-feeding arthropods.

The primary powder R.153.78 of the serotype H.14 of B. thuringiensis has been evaluated in small field plots against larvae of Ae. detritus and Cx. pipiens.. The minimal concentration of the primary powder inducing a complete mortality was 0.1 mg/1 against Ae. detritus and 0,4 mg/1 against Cx. pipiens, corresponding at dosages of respectively about 310-420 and 1240-1680 International Toxic Units per litre. The residual larvicide effectiveness was close to nil in spite of the chemical stability of the delta endotoxin in neutral and acid waters. Experimental formulations derived from this primary powder were less effective than, or at the best as effective as, the primary powder itself. The above findings confirm that the serotype H.14 of B. thuringiensis could constitute the basis of very effective new larvicides and suggest that formulation research has a major role to play to maximize the potential of this promising biological control agent.

Est-2 and Est-3 linkage disequilibrium was investigated in 43 natural populations. An association between Est-2(0.64) and Est-3A alleles (or its reverse, Est-3Null and alleles other than Est-2(0.64) was not observed in 19 (1.2%) of the 1599 mosquitoes analyzed, whereas it should have been found in nearly 400 (25%) individuals if the two loci were in equilibrium. This observation is discussed in relation to organophosphate resistance and genetic distance of the two genes.

Esterase polymorphism and Dursban (O,O-dimethyl-2-pyridylphosphorothioate) sensitivity have been investigated in 12 natural populations and three laboratory strains of Culex pipiens pipiens. This mosquito has two esterase loci, Est-1 and Est-2, which were shown to code esterases of the B group (aliesterases) but not cholinesterases. No correlation between Est-1 polymorphism and Dursban sensitivity was found, but the increase of the Est-2(0.64) allele in the populations less sensitive to Dursban was highly significant (r = -0.9850 for 6 df).