Larvicidal Activity of Bacillus Pathogens Against Toxorhynchites Mosquitoes (Diptera: Culicidae)

Abstract

Bacillus sphaericus and Bacillus thuringiensis (H-14) were tested for larvicidal activity against Toxorhynchites spp. under laboratory conditions. In the presence of Aedes aegypti larvae, 24- and 48-h-old Tx. rutilus rutilus responded with 48 to 97% and 37 to 76% mortality, respectively, when exposed to 0.1 to 1.0 mg/litre of a lyophilized spore preparation of B. sphaericus (2013-4). Toxorhynchites theobaldi, Tx. amboinensis , and Tx. brevipalpis 48-h-old larvae were not significantly affected. A positive correlation between concentration of B. thuringiensis (H-14; IPS-78) and mortality was observed in 4th instars of Tx. amboinensis and Tx. brevipalpis in the presence of prey. At 10 mg/litre of IPS-78 in the presence of Ae. aegypti larvae, Tx. amboinensis and Tx. brevipalpis responded with 77.7 and 88.9% mortality, respectively. In the absence of prey at the same concentration, no significant mortality was observed. No linear correlation between number of prey and mortality was observed when Tx. brevipalpis was exposed to 5 mg/litre of IPS-78 in the presence of variable numbers of prey larvae. A curvilinear relationship between prey density and mortality was apparent for Tx. amboinensis . Lower mortality was observed with 1, 2, 50, and 100 prey, and higher mortality was elicited in the presence of 5 to 20 prey. No significant differences in mortality response were noted when Tx amboinensis was exposed to 5 mg/litre of IPS-78 with 20 Culex quinquefasciatus , Ae. aegypti , or Anopheles quadrimaculatus larvae present. The use of Toxorhynchites spp. and Bacillus pathogens in integrated vector control is discussed.