Possible selective advantage of Anopheles spp. (Diptera: Culicidae) with the oxidase- and acetylcholinesterase-based insecticide resistance genes after exposure to organophosphates or an insect growth regulator in Sri Lankan rice fields

Treatment of rice fields in Sri Lanka with monocrotophos at 10 mg/litre or pirimiphos-methyl at 0·1 mg/Iitre gave a selective advantage to larvae of Anopheles subpictus Grassi and A. nigerrimus Giles carrying the oxidaseand acetylcholinesterase-based resistance genes, respectively. However, this selective advantage was apparent for less than ten days after spraying. There was no mortality of any larval instar with monocrotophos 12 days after spraying, and no fourth-instar larval mortality with pirimiphos-methyl 17 days after spraying. The chemical degredation curves for these compounds indicated that this short duration of efficacy was due to the instability of the compounds in water under field conditions. In contrast, the new growth regulator S-31183 (2-[1-methyl-2-( 4-phenoxyphenoxy )ethoxy ]pyridine) sprayed at 0·1 mgllitre conferred no selective advantage to larvae with either resistance mechanism and had a total efficacy period of at least 71 days. This difference can be attributed to the greater toxicity of the growth regulator and its slower chemical degradation under field conditions when compared to the organophosphates. Introduction Insecticide resistance is becoming a major problem in many areas. In Sri Lanka, there is broad spectrum resistance in Anopheles nigerrimus Giles and A. subpictus Grassi throughout the island, while in some locations malathion-specific resistance is developing in A. culicifacies Giles. Many of the Culex species in Sri Lanka also have some organophosphateand carbamate-resistance, although this has been well documented mechanistically only in C. quinquefasciatus Say (Hemingway eta/., 1986; Hemingway eta/., 1987; Herath et a/., 1987; Villani et a/., 1983). Resistance may be due to either selection by insecticide control measures aimed directly at the mosquito population, such as house spraying with malathion against A. culicifacies, or exposure of the mosquito population to 472 J. HEMINGWAY, B. C. BONNING, K. G. I. JAYAWAROENA ET AL. compounds used for agriculture, such as the spraying of rice fields against pests. The selective advantage of the resistance genes have rarely been measured under field conditions. The present study was undertaken to determine the level and duration of protection conferred by oxidaseand acetylcholinesterase-based resistance genes after l~ryi~ding of rice fields in Sri Lanka. The two organophosphates monocrotophos and pmnnphos-methyl were used, as these were commonly used by farmers for rice treatment in Sri Lanka. New ~ompounds to which there is no cross-resistance from existing mechanisms, will be valuable m future control programmes. Such compounds need evaluating in terms of their ~fficacy, sta~e sp~cificity, environmental effect and cos~, such that they can be successfully Integrated mto msect control programmes as effectively as possible. A new growth regulator, S-31183 (2-[1-methyl-2-(4-phenoxyphenoxy)ethoxy]pyridine), has been developed to which the acetylcholinesterase resistance gene should, in theory, give no crossresistance (Hatakoshi et al., 1986). The effectiveness of this growth regulator against organophosphateand carbamate-resistant and susceptible mosquitoes, and the duration of active control was, therefore compared to that of the two organophosphates. Materials and methods Monocrotophos and pirimiphos-methyl were supplied as 60% emulsifiable concentrates (e.c.) and were sprayed on 40 x 20-m rice-field plots at dosages of 10 and 0·1 mg active ingredient/litre, respectively. The insect growth regulator was supplied as a 10% e.c. and was sprayed at 0·1 mg active ingredient/litre. Before spraying, water samples were taken from each plot and assayed with firstand fourth-instar larvae of A. nigerrimus and A. subpictus. There was no mortality 24 h after the start of each test and only low level mortality ( <10%) when the larvae were maintained in the water with light feeding until they either died or successfully developed into adults. Larvae were found in all the rice field plots immediately prior to the insecticide treatment. Mosquitoes used were as follows: (a) Susceptible and resistant colonies of A. nigerrimus, established by selection and single family rearing. As this species does not readily mate in cages, the colonies were actively maintained by hand-mating a large number of females every generation. The F6 and F1 generations, which bred true for susceptibility and resistance by WHO bioassay, were used in these experiments. The resistance mechnanism in this species is an altered acetylcholinesterase (AChE), which gives broad spectrum organophosphate and carbamate resistance (Hemingway et al., 1986). (b) TheFt progeny of wild-caught females of A. subpictus were also used. Eggs were obtained by isolating single females in vials. The families were classified as resistant, mixed or susceptible by WHO bioassay susceptibility tests on half the family as one-day-old adults. Larval resistance in this species is controlled by two genes, an oxidase-based mechanism, which gives broad spectrum organophosphate resistance, and an esterase mechanism, which gives some carbamate resistance and possibly enhances the organophosphate resistance (Hemingway et al., 1987). Water samples were taken from all rice plots plus the untreated control plot at regular intervals after spraying. Water (1000 ml) was collected from top right and bottom left sectors of each plot and assayed separately to determine homogeneity of treatment. Larvae of each test species of each resistant type were used for two replicate bioassays. Bioassays from organophosphate-treated plots were run for 24 h, after which mortality was scored. Firstor fourth-instar larvae were used in all tests. Where possible, 25 larvae were used in each replicate, but in a few cases lower numbers of one particular genotype were available for testing; the minimum number in any one replicate was ten larvae. Bioassays on water samples from the plots treated with growth regulator were run until all larvae had either died or developed into adults. All tests used early fourth-instar larvae of each species and genotype. Controls were always run for both type of test with water from the untreated rice field, and occasional controls were run with di~tilled water. I ! i