Effect of certain newer generation insecticides on the developmental stages of mosquitoes

Abstract

Mosquitoes collected from five cattle sheds and human dwellings in and around Thrissur were identified into 27 species coming under seven genera in three subfamilies. Only one genus each could be found in the subfamilies of Toxorhynchitinae and Anophelinae, while there were five genera in the subfamily Culicinae. The genus Culex constituted 67.1 per cent of the total collection. Culex tritaeniorhynchus was identified as the most prevalent species in cattle sheds while in human dwellings it was Cx. quinquefasciatus.

Culex tritaeniorhynchus was successfully colonized in the laboratory. The development of adults from eggs took 7-18 days under laboratory conditions. The average number of eggs laid by a female was 42. The average survival time was 45 days. The male mosquitoes always emerged first from the pupae.

Dissection of 1001 mosquitoes collected from cattle sheds did not reveal any helminth larvae. Meanwhile, 1019 mosquitoes collected from human dwellings in the vicinity of dog houses when dissected revealed helminth larvae in two mosquitoes. The larvae were identified as those of Dirofilaria repens based on morphology.

Six different insecticides namely, diflubenzuron, methoprene, Bti, azadirachtin, ethofenprox and deltamethrin were evaluated for their effect on the eggs, 4th stage larvae and pupae of Cx. tritaeniorhynchus. The 4th stage larvae were more susceptible to all the insecticides with the least concentration of LC50 and LC90.

The IGRs used in the study were most efficient. In terms of LC50 concentration on larvae, the various insecticides in the order of efficacy were diflubenzuron (0.0002288 ppm), methoprene (0.0006738), deltamethrin (0.0020853) ethofenprox (0.0082146), azadirachtin (4.6867982) and Bti (6.6076021). The IGRs -diflubenzuron and methoprene- are recommended for use in mosquito larval control as only minimum concentration need to be used. Methoprene had no effect on the eggs whereas several multiples of the other insecticides were needed to produce the same effect on eggs or pupae. In case of azadirachtin, the LC50 on larvae and pupae were almost same.

When MRP alone was considered, ethofenprox was found to be the cheapest insecticide, costing Rs.0.03/- per one lakh litres of water followed by deltamethrin (Rs. 0.51/-). The most expensive one was Bti (Rs. 2179/-) followed by azadirachtin (Rs. 1630/-). Among the IGRs diflubenzuron was much cheaper (Rs. 1.38/-) than methoprene (Rs. 13.87). To avoid selection pressure on immature stages of mosquitoes, deltamethrin and ethofenprox should not be used for larval mosquito control as they are invaluable adulticides.