Environmental factors that affect the susceptibility of Simulium vitattum larvae to insecticidal proteins produced by Bacillus thuringiensis subsp. israelensis
Iburg, Joseph Paul
MetadataShow full item record
Application of insecticidal crystalline proteins (ICPs) produced by Bacillus thuringiensis subsp. israelensis (Bti) to the larval habitats is the method of choice for black fly (Diptera: Simuliidae) suppression. There have been occasional reports of less than optimum control when applying the Bti ICPs to certain larval habitats. The effects of antibiotics, seston, and time on larval susceptibility to ICPs were investigated. Simulium vittatum Zetterstedt larvae were exposed to enrofloxacin, tylosin, sulfamethoxazole, and trimethoprim followed by exposure to Bti ICPs. Anthropogenic concentrations of a mixture of these antibiotics and individual antibiotic concentrations 10,000-80,000 times higher than those found in contaminated rivers were unable to reduce the activity of ICPs. Some components of seston from problematic sites impaired the effectiveness of ICPs. Exposure of larvae to clay minerals prior to ICP exposure resulted in no significant decrease in mortality. Exposure of larvae to cellulose, viable diatoms, and purified diatom frustules prior to Bti ICP exposure resulted in significant reductions in mortality. Larvae were exposed to various materials and water-insoluble Neon Red particles (NRP) to determine if selected materials interfered with larval feeding. The quantity of NRP consumed by larvae in each medium was determined by spectrophotometric analysis. The number of times the larvae extended and retracted their cephalic fans per min (flick rate) in each medium was calculated. The flick and ingestion rates of larvae were not significantly affected by clay material or viable Chlorella vulgaris Pratt cells. Scenedesmus quadricauda (Turp) Bréb colonies, Cyclotella meneghiniana Kütz cells, and frustules caused significant decreases in larval flick and ingestion rates. The time necessary for ICPs to cause mortality in larvae is poorly understood. Larvae were exposed to Bti ICPs and mortality was monitored over time. Larvae exposed to operational concentrations of ICPs exhibited maximum mortality after 4 h. Exposure of larvae to 1/3 of that concentration resulted in similar mortality after 8 h. Clay material had no effect on larval mortality or time to achieve maximum mortality. When cellulose was present in the medium the time to maximum mortality was increased 50% and overall mortality was reduced by 40%.