Efficacy of flowable concentrate formulations of Bacillus thuringiensis var. israelensis against black flies (Diptera:Simuliidae)

Seven flowable concentrate formulations of Bacillus thuringiensis (H-14), Teknar wdc, auto-dispersible Teknar, Teknar 2X aqueous concentrate, Teknar 2X oil base concentrate, Vectobac AS, Bactimos FC and Skeetal F, were evaluated in small streams against Simulium vittatum. There was no significant difference in efficacy among the formulations with the exception of the Teknar 2X aqueous concentrate, which required considerably less formulation (5 mg/liter/1 min) than the others to produce 95% mortality in penultimate instars of S. vittatum. The field determined LC-95 for the other formulations ranged from 10.6 to 15.9 mg/liter/1 min. There was no significant difference between the efficacy of excessively diluted and undiluted formulations.     

Laboratory study of the influence of water temperature and pH on Bacillus thuringiensis var. israelensis efficacy against black fly larvae (Diptera: Simuliidae)

An experimental formulation of Bacillus thuringiensis var. israelensis was used in the laboratory to assess the influence of water temperature and pH on the relationship between concentration, duration of exposure, and mortality of the northern black fly species Simulium decorum and Prosimulium mixtum/fuscum group. Mortality increases in both species with increases in duration of exposure, concentration, temperature and pH. Onset of death is shortened by increase in concentration and temperature. As temperature rises, the concentration of B.t.i. required to induce mortality decreases; the sharpest decline occurring between 12 and 18 degrees C for S. decorum, and between 4 and 8 degrees C for P. mixtum/fuscum larvae. Lower pH induces a loss of efficacy of the B.t.i. formulation on S. decorum larvae at 4 and 12 degrees C. Dialysis of the B.t.i. formulation at pH 11 for 2 h increases its potency against S. decorum larvae, suggesting an effect of an extralarval alkaline hydrolysis on the B.t.i. efficacy. An alkaline prehydrolysis of the paracrystalline bodies could therefore be used in cold and acidic environments to compensate for loss of efficacy.     

The lack of effect of low temperature and high turbidity on operational Bacillus thuringiensis subsp. israelensis activity against larval black flies (Diptera: Simuliidae)

Black fly suppression programs are conducted across a wide range of environmental conditions, targeting a variety of pest species with diverse life histories. Operational applications of Vectobac 12AS (Bacillus thuringiensis subsp. israelensis) were conducted during times characterized by water temperature and turbidity extremes. Applications were conducted in the Yellow River in central Wisconsin targeting Simulium annulus and S. johannseni when water temperatures were 1-2 degrees C. Applications were conducted in the Green River in western North Carolina targeting the S. jenningsi group after a rain event, when portions of the treatment zone experienced turbidities of 276 nephelometric turbidity units. Excellent larvicidal activity was observed in both programs, with 97% mortality or greater being observed at distances over 5 km downstream of a treatment site. Mortality data for larval black flies in 2 operational suppression programs conducted in 2011 demonstrated a negligible effect of near-freezing water temperatures and exceptionally high turbidity on Bti activity.     

Efficacy of Bacillus thuringiensis var. israelensis against larvae of the southern buffalo gnat, Cnephia pecuarum (Diptera: Simuliidae), and the influence of water temperature.

Susceptibility of southern buffalo gnat larvae to Bacillus thuringiensis var. israelensis (B.t.i.) was studied during 1986-88. Tests were conducted using Vectobac 12AS in a mini-gutter aquatic bioassay system. Mini-gutter tests using a 4.5 ppm B.t.i. concentration and a 10 min exposure period confirmed a positive correlation between water temperature and B.t.i. effectiveness. Significantly lower larval mortality occurred as water temperature decreased below 9 degrees C, indicating that proper timing of B.t.i. application is essential to maximize larval control. Even with this temperature limitation, larval control using B.t.i. should provide an economically effective means of preventing outbreaks of the southern buffalo gnat.     

A simple technique for determining relative toxicities of Bacillus thuringiensis var. Israelensis formulations against larval blackflies (Diptera: Simuliidae)

A laboratory system for assaying the potency of Bacillus thuringiensis var. israelensis formulations against larval blackflies was developed. An orbital shaker was used to create a water current in 250-ml Erlenmeyer flasks containing the test larvae. This system produced dose-mortality relationships with acceptable statistical parameters.     

Persistence of mermithidae (Nematoda) infections in black fly (Diptera: Simuliidae) populations

Data on the persistence of mermithid infections in black fly populations over several years indicate mermithid infection rates are relatively stable over several years in a stream. These data were from specific sites in a variety of streams all having the same simuliid fauna present each season. This suggests that the suitability of the stream bed for the free living phase is very important in regulating mermithid populations.     

Development of the first black fly (Diptera: Simuliidae) management program in Argentina and comparison with other programs.

In response to increasing pest populations of black flies, the government of Argentina initiated a pilot program to evaluate the effectiveness of larval control in the irrigation system of the Negro River Valley, an 18,240-km2 area in Patagonia. The extensive system of irrigation canals, drainage ditches, and natural waterways not only provides water for agriculture and general habitation but also affords habitat for immature black flies. Three species (Simulium bonaerense, Simulium wolffhuegeli, and Simulium nigristrigatum) are primary pests of humans and animals in this area. Trials were conducted using Vectobac AS in representative irrigation canals, a drainage ditch, and a medium-sized river. Most trials resulted in effective larval mortality and insecticide carry. Based on a comparison with 11 black fly suppression programs throughout the world, a suppression program for the Negro River Valley has a projected cost of approximately $1,623,360/year. This study represents the initial steps in development of the 1st areawide black fly suppression program in Argentina.     

Effects of algae on the efficacy of Bacillus thuringiensis var. israelensis against larval black flies

Personnel from several black fly control programs have reported that the efficacy of Bacillus thuringiensis var. israelesis (Bti) is reduced during periods when algal concentrations are high in the waterways. Although the reduction in Bti-induced mortality in black fly larvae is presumed to be related to the presence of algae, no scientific data support this theory. In this study, 4 genera of algae (Microcytis, Scenedesmus, Dictrosphaerium, and Chlorella) commonly detected in Pennsylvania rivers where Bti-induced mortality in black fly larvae has been reduced were assessed to determine their respective effects on Bti-induced mortality by using an orbital shaker bioassay with laboratory-reared black fly larvae (Simulium vittatum cytospecies IS-7). A significant reduction in Bti-induced mortality was observed when Scenedesmus was present in the flasks at concentrations > or = 16,000 cells/ml. The Bti-induced mortality of larvae was not significantly reduced when Chlorella, Dictyosphaerium, or Microcytis was present in the flasks, even at concentrations > or = 250,000 cells/ml. These results indicate that the presence of certain types of algae can reduce the mortality of black flies exposed to Bti. Although not clearly defined, the mechanisms involved may be related to algal morphology due to overall size and structures associated with certain types of algae, and possible interference with feeding.     

Larvicidal persistence of formulations of Bacillus thuringiensis var. israelensis to control larval Aedes aegypti

After detection of resistance to the organophosphate temephos in populations of Aedes aegypti in Brazil, corncob granule (CG) and water-dispersible granule (WDG) formulations of Bacillus thuringiensis var. israelensis (Bti) were introduced in routine focal treatments. Larvicidal persistence and the influence of exposure to sunlight on VectoBac formulations of Bti were compared in 250-liter fiberglass water containers. Production of pupal Ae. aegypti in containers was used to indicate control. In untreated containers, survival of larvae was always above 95%. A temephos sand granule formulation used as reference treatment maintained 100% control throughout the 12-wk period in all situations. Under sunlight exposure, control dropped below the 90% level in the 2nd week after treatment at both dosages of VectoBac CG (1 and 2 g/50 liters) and VectoBac tablet (T) formulation at 1 tablet/100 liters. VectoBac T at 1 tablet/50 liters provided 2 wk of 100% control. VectoBac WDG at dosages of 1 and 2 g/500 liters provided 100% control for 3 wk. Without sunlight exposure (covered containers), VectoBac CG provided 9 wk of continuous 100% control and 5 wk of continuous 100% control, respectively, at 1 and 2 g/50 liters. The VectoBac T formulation at both dosages initially provided 2 wk of 100% control. After this period, the control level fluctuated between 96 and 100%. VectoBac WDG provided continuous 100% control for 7 wk for the lower dosage and for 6 wk for the higher dosage. At both dosages of WDG, 100% control was achieved in 11 wk out of the 12-wk period.     

A broad evaluation of B.t.i. for black fly (Diptera: Simuliidae) control in a Michigan river: efficacy, carry and nontarget effects on invertebrates and fish

Efficacy for black fly control, carry and nontarget effects of B.t.i. (Teknar HP-D), applied in the Betsie River, Michigan, were studied in June 1988. Black fly mortality was high (approximately 100%) for a 2,200 m stretch downstream from the application site, declined to 30% at 3,200 m, and was nil at 4,500 m. Drift of black flies greatly increased after application at a downstream site, but did not change at an upstream site. There were no detectable nontarget effects of B.t.i. application on: 1) invertebrate macro- or micro-drift; 2) numbers of invertebrates in benthic Surber samples; 3) mortality or feeding of drifting and nondrifting insects; 4) growth or mortality of caged Stenomena sp. larvae; 5) invertebrate functional group composition; 6) mortality or weight change of caged rock bass; or 7) fish numbers, species composition, length-weight (rock bass only) relationships or rock bass diet. Sampling of Rheotanytarsus sp. midges on natural substrates indicated low (27%) mortality owing to B.t.i. at only 100 m downstream from the application site, with negligible mortality at all other downstream and upstream sites. This information, combined with no pronounced changes in numbers of midges in macro-drift after application, indicated that midge populations were not adversely affected by B.t.i. in the study.     

Bacterial control of mosquito larvae: investigation of stability of Bacillus thuringiensis var. israelensis and Bacillus sphaericus standard powders

Bacillus thuringiensis var. israelensis and Bacillus sphaericus products were assayed against their respective reference powders IPS82 and SPH88. Since their production in 1982 and 1988, the potency and larvicidal activity of these standard powders have been regularly checked on their test insects Aedes aegypti (for IPS82) or Culex pipiens (for SPH88). Over the 16-year evaluation period of IPS82 and 10-year evaluation period of SPH88, their potencies were considered stable. The global mean of each year's mean showed a coefficient of variation of less than 20%. Larval rearing was the most important factor in the reproducibility of the bioassay, although some variation also originated from the person performing the bioassay. This study demonstrated that the SPH88 standard could be kept in a stock suspension at 4 degrees C for 3 years without loss of potency. Moreover, after 9 years of storage in suspension, only a 2-fold decrease in the potency of SPH88 was detected.     

Comparison of morning and evening larvicide applications on black fly (Diptera: Simuliidae) mortality

The downstream movement, or drift, of larval black flies as well as many other aquatic insects has been well documented. This phenomenon has most often been associated with the dusk-to-overnight time frame. Operationally, larvicide applications targeting black flies are typically initiated early in the day and can extend until near sunset. To determine if larvicide applications conducted late in the day would be affected by the drift behavior of larval black flies, 6 larvicide applications were conducted. Three applications were conducted at midmorning and 3 during the evening. Larvicidal applications of Bacillus thuringiensis subsp. israelensis insecticidal proteins targeting Simulium appalachiense demonstrated no difference in larval mortality between morning and evening applications. These findings indicate that the larvae responded in a similar manner to the larvicide during the late morning to early afternoon and evening to night. The drift behavior of larval black flies does not appear to be an impediment to black fly suppression activities.     

Susceptibility of Aedes aegypti larvae to temephos and Bacillus thuringiensis var israelensis in integrated control.

The susceptibility of field collected Aedes aegypti larvae was evaluated in terms of median lethal time (LT50) and final mortality, when treated with temephos, Bacillus thuringiensis var israelensis as well as mixtures of these two agents. Third instar larvae were shown to be more susceptible than early and late fourth instar ones to the entomopathogen. Survival of some individuals when exposed to temephos suggest possible resistance. Temporal synergism in early fourth instar larvae was detected when they were exposed to mixtures of Bti-temephos. The possibility of this integrated treatment is commented on.     

Susceptibility of black fly larvae (Diptera: Simuliidae) to lawn-care insecticides individually and as mixtures

Urban and suburban watersheds have the potential to be highly impacted by chemicals, especially insecticides to control insect pests on lawns, ornamental plants, and home gardens. Three of the most common lawn-care insecticides detected in urban watersheds, carbaryl, chlorpyrifos, and malathion, have been evaluated using an acute orbital shaker toxicity test to determine their respective concentrations that produce 50% mortality (LC50) in Simulium vittatum Zetterstedt cytospecies IS-7 larvae. Results of the 48-h LC50 tests show chlorpyrifos to be the most toxic to black fly larvae (LC50 = 0.28 microg/L) followed by carbaryl (LC50 = 23.72 microg/L) and malathion (LC50 = 54.20 microg/L). These insecticides were also tested as binary and ternary mixtures using the toxic unit (TU) approach. Toxicity was shown to be greater than additive for the ternary mixture of chlorpyrifos-carbaryl-malathion (LC50 = 0.56 TU) and the binary mixtures of chlorpyrifos-malathion (LC50 = 0.72 TU) and carbaryl-malathion (LC50 = 0.78 TU). The binary combination of chlorpyrifos and carbaryl was shown to be additive (LC50 = 0.98 TU). These results indicate that aquatic invertebrate populations in urban and suburban streams may experience a higher-than-expected increase in toxicity-related effects when all three chemicals are present in the waterway.     

Field efficacy and nontarget effects of the mosquito larvicides temephos, methoprene, and Bacillus thuringiensis var. israelensis in Florida mangrove swamps

We compared the efficacy and nontarget effects of temephos, Bacillus thuringiensis var. israelensis (B.t.i.), and methoprene applied by helicopter to control mosquito larvae in mangrove swamps on Sanibel Island, FL, in May 1997. Three sites per treatment and 3 untreated sites were used. Temephos (Abate) was applied at 37 ml/ha (43% active ingredient [AI]), B.t.i. granules (Vectobac G) were applied at 5.606 kg/ha (200 International Toxic Units/mg), and methoprene (Altosid ALL) was applied at 213 ml/ha (5% AI). Efficacy was quantified by monitoring the survival of caged and uncaged larval Aedes taeniorhynchus. We quantified mortality of sentinel nontarget amphipods (Talitridae) at all sites, monitored the effect of temephos on flying arthropods using light traps, and collected dead insects in tarps suspended under mangroves in areas treated with either temephos or methoprene. Each pesticide showed good overall efficacy but occasional failures occurred. No detectable mortality of amphipods or flying insects attributable to pesticides was found. The inconsistent field efficacies of the pesticides indicate a need for reinspection of treated sites in this habitat.     

Impact of environmental factors on the toxicity of Bacillus thuringiensis var. israelensis IPS82 to Chironomus kiiensis

To determine the feasibility of microbiological control of chironomid larvae in water sources for the city of Shenzhen, China, the toxicity characteristics of Bacillus thuringiensis var. israelensis (Bti) IPS82 on Chironomus kiiensis Tokunaga were studied. Tests on fermentation products of IPS82 showed good correlations between toxicity, cell density, dissolved oxygen, and the spore-forming phase. In this study, bioassays were carried out with different stage larvae. Results showed that the LC50s (24 h postexposure) were 8.2, 15.2, 24.7, and 38.6 mg/liter for the 1st, 2nd, 3rd, and 4th instars, respectively. Tests on environmental factors influencing toxicity of Bti to C. kiiensis showed that sunlight is the most important factor, shortening the half-life of Bti from 21 days in dark to 10 days under sunlight. Temperature variations (15-30 degrees C) caused no impact on toxicity, but a 16% increase in larval mortality was observed at 35 degrees C. The toxicity of IPS82 was greatest at a pH of 7. In field trials, dosages above 100 mg/liter were effective in the control of C. kiiensis. Our study indicated that it is feasible to use Bti to control C. kiiensis in city source water.     

Larval gut pH profile in pestiferous Chironomus crassicaudatus and Glyptotendipes paripes (Chironomidae: Diptera) in reference to the toxicity potential of Bacillus thuringiensis serovar israelensis

Gut pH and pH optimum of amylase were measured in 4th-stage larvae of 2 pestiferous chironomid species, Chironomus crassicaudatus and Glyptotendipes paripes. The gut pH in both species was close to neutral, varying from 6.7 to 7.4 and 6.9 to 7.6 pH units for C. crassicaudatus and G. paripes, respectively. In both species, slightly alkaline pH values were measured in the anterior part of the mesenteron, whereas slightly acidic pH values were recorded in gastric caecae and the posterior part of mesenteron. Amylase pH optimum in both chironomid species remained between pH values of 5.5 and 7, being closer to pH value 7 than 5.5. This profile was consistent with the measured values of the gut pH. The recorded larval gut pH profile in the 2 species of chironomids in the present study is much lower than gut pH reported for mosquitoes or Lepidoptera larvae. This could be the reason for the relatively much lower susceptibility of chironomid larvae to Bacillus thuringiensis serovar israelensis toxin proteins than some other nematoceran Diptera, specifically mosquitoes.