James E. Milstead

The field persistence of Bacillus thuringiensis spores

Abstract A method for measuring viable spore counts on leaf surfaces is described and utilized to provide data for estimating the field persistence of four Bacillus thuringiensi formulations. The estimates of persistence are based on the assumption of constant average force of mortality acting on the spores over time and are expressed in terms of viable spore half-life. For one of the formulations, persistence is compared at two differing environmental settings.

Heterorhabditis bacteriophora as a vector for introducing its associated bacterium into the hemocoel of Galleria mellonella larvae

Abstract The nematode Heterorhabditis bacteriophora serves as a vector enabling its bacterial associate to reach the hemocoel of its host, the seventh-instar larva of Galleria mellonella . At 28.5°C, the LD 50 s of the orally introduced nematode-bacterial complex and the intrahemocoelically injected bacteria are three to six nematodes and one to two cells, respectively.

The field persistence of Bacillus thuringiensis spores on Cercis occidentalis leaves

Abstract The field persistence of viable spores of four Bacillus thuringiensis formulations, Amdal ® , Biotrol ® BTB 183, Thuricide ® HP, and Thuricide ® 90TS, was measured and compared on leaves of Cercis occidentalis . For Amdal, Biotrol BTB 183, and Thuricide 90TS the field persistence was compared also at two locations, Auburn and Sacramento, California, which differed in altitude and climate. The comparisons of field persistence were based on a segmented linear model of the decay of average viable spore count on a logarithmic scale, because much of the field data strongly rejected the simple log linear model. No significant difference in field persistence of viable spores was found between the two locations. Significant differences were found in both magnitude and pattern of field persistence compared to previously reported measurements of Thuricide 90TS, where leaves of Quercus agrifolia were the substrate, and a log linear pattern of decay of viable spore count was found. The persistence half-life of Thuricide 90TS on Q. agrifolia leaves at Monterey, California, was 3.9 days, compared to a persistence half-life during the first 3 days of 0.63 day on leaves of C. occidentalis with the pooled Auburn/Sacramento data (two-sided P The persistence half-life for Thuricide HP during the first 3 days was 1.85 days, which was significantly different from the corresponding result of 0.58 day for Amdal and 0.63 day for Thuricide 90TS ( P

Effect of tree species on the coverage and field persistence of Bacillus thuringiensis spores

Abstract The effect of leaves of Quercus agrifolia, Cercis occidentalis, Eucalyptus globulus, and Juglans regia on the initial deposit and subsequent rate of decay of viable spores of Bacillus thuringiensis was investigated. Significant differences in the size of the initial viable spore deposit were found between the various species, with E. globulus markedly lower than the others. The viable spore decay patterns of the various species were analyzed using a segmented linear model and significant differences in initial rates of decay were also measured. Thus it appears that the characteristics of the leaves of the treated plants may have an effect on the coverage and field persistence of viable B. thuringiensis spores. Some possible mechanisms for these effects are discussed.

Pathophysiological influences of the Heterorhabditis bacteriophora complex on seventh-instar larvae of the Greater Wax Moth, Galleria mellonella: Changes in the hemolymph refractive index

Abstract Shortly after penetration into the hemocoel of seventh-instar larva of Galleria mellouella , Heterorhabditis bacteriophora begins feeding upon the fat body. Disruption of the organ is associated with an increase in turbidity and hemolymph refractive index. Intrahemocoelic injection of the associated bacterium alone results in a depression of the refractive index.

Methods for assessing field persistence of Bacillus thuringiensis spores.

Field persistence was investigated for three concentrations of a Bacillus thuringiensis formulation, Dipel, on leaves of Cercis occidentalis. The application levels were 0.0025%, 0.025%, and 0.25% wv. Logarithmically transformed viable spore counts were analyzed using a segmented linear model. No biologically significant differences were found in rates of viable spore decay for the three levels of application. Statistical methods used in the analysis are described in detail. A model for assessing total effective dose delivered by a B. thuringiensis application is proposed. Application of the total dose model indicates that for typical field application levels more than 90% of total dose is delivered to the insect population during the first day following application.

Development of a model for prediction of target insect mortality following field application of a Bacillus thuringiensis formulation

Abstract A model for predicting mortality of Schizura concinna larvae on Cercis occidentalis trees following application of Bacillus thuringiensis formulations is based on an estimation of the effective dose. This is calculated from the initial application level and field persistence characteristics of the pathogen and from the feeding rate of the target insects. Using the estimated effective dose and the time the pathogen was on the host plant, the probability of larval mortality is calculated by means of a multiple logistic dose-response model. Predicted mortalities for B. thuringiensis applications at concentrations 0.00025, 0.0025, 0.025, and 0.25%, w v , were 7.7, 23.3, 46.2, and 87.1%, respectively. Corresponding, observed mortalities adjusted for larval loss due to forces unrelated to the B. thuringiensis application were 6, 54, 66, and 100%.

Viable spore count as an index of effective dose of Bacillus thuringiensis

Leaf samples from Cercis occidentalis trees, collected 5–75 hr after application of varying concentrations of a Bacillus thuringiensis formulation, Dipel, were used as dose sources for third instar Schizura concinna larvae. Dose, defined as logarithmically transformed, estimated viable spore counts, and duration of time the formulation was on the leaf prior to sampling were considered in combination as predictors of insect mortality. By use of the multiple logistic model both factors were found to be significant predictors of insect mortality (P < 0.0001). Available methods for assessing goodness of fit indicate that the multiple logistic model adequately fits these data.

Changes in weight and hemolymph total solids resulting from starvation in the sixth instar of Peridroma saucia and Pseudaletia unipuncta (Lepidoptera: Noctuidae)

Abstract Food and water deprivation during the active feeding phase results in an initial rise in the concentration of total solids in the plasma of summer laboratory generations of sixth-instar larvae of Peridroma saucia and Pseudaletia unipuncta reared at a constant temperature of 25°C. It is felt that this change is a characteristic symptom associated with starvation in these species.

Pathophysiological influences of the Heterorhabditis bacteriophora complex on seventh-instar larvae of Galleria mellonella: effect on silk production.

Abstract Oral introduction of invasive juveniles of Heterorhabditis bacteriophora results in an inhibition of silk production by seventh-instar larvae of Galleria mellonella . The inhibition can be duplicated by intrahemocoelic injection of its bacterial associate alone. The inhibition varies directly with dose and is correlated with a progressive increase in hemolymph bacterial titer.