Biolarvicides in vector control: challenges and prospects.

Biolarvicides, based on mosquitocidal toxins of certain strains of Bacillus sphaericus and Bacillus thuringiensis var israelensis H-14 (Bti) are highly effective against mosquito larvae at very low doses and safe to other non-target organisms. During past two decades various biolarvicide formulations produced in India and abroad have been tested at Malaria Research Centre and some formulations have undergone large-scale operational trials. Biolarvicide formulations of B. sphaericus are useful in the control of Culex and certain Anopheles spp, such as An. stephensi and An. subpictus, but not much effective against An. culicifacies and almost ineffective against Aedes aegypti. Repeated application of B. sphaericus in the same habitat, however, results in the development of resistance in larvae of target mosquitoes. In view of its low specificity for An. culicifacies and the potential for resistance in An. stephensi, B. sphaericus has limited prospects for control of malaria vectors. However, with some resistance management, B. sphaericus can still be used against Culex mosquitoes. On the other hand Bti formulations, which have broader spectrum of activity against Aedes, Culex and Anopheles spp, have not shown significant development of resistance in mosquitoes but their activity in field, particularly against surface feeding anopheline larvae is affected by various bioenvironmental factors, thus requiring weekly application in most habitats. To overcome this problem development of slow release formulations and genetically engineered biolarvicides by transplanting mosquitocidal toxin genes of Bti and B. sphaericus in some other environmentally compatible organisms have been investigated by different scientists.