Activity of Bacillus thuringiensis Isolates against Diaprepes abbreviatus (Coleoptera: Curculionidae)

A. A. Weathersbee III, S. L. Lapointe, R. G. Shatters Jr.

Abstract


A collection of Bacillus thuringiensis isolates plausibly active against coleopteran insects was obtained from the Agricultural Research Service Culture Collection. Each isolate was cultured, spores and d-endotoxin crystals were pelleted by centrifugation and lyophilized, and the resulting product was incorporated in insect diet for testing against Diaprepes abbreviatus neonates. A bioassay method was developed that utilized small amounts of insect diet and B. thuringiensis spores and d-endotoxin to treat single neonates confined to 0.2-mL clear polymerase chain reaction (PCR) tubes. The method was less expensive in terms of labor and materials as compared to previous methods and reduced control losses due to burrowing and aggressive behaviors of D. abbreviatus larvae confined together. Of 19 B. thuringiensis isolates screened for activity against D. abbreviatus with a discriminating dose of 250 ppm spores and d-endotoxin on diet, 5 were selected for further evaluation in dose-response experiments. Diaprepes abbreviatus larvae demonstrated a significant dose response to 4 of the 5 isolates tested. The most active isolates were those that expressed CryET33 and CryET34, or Cyt2Ca1 proteins. A wild-type B. thuringiensis strain that expressed Cyt2Ca1 generated the lowest LC50 value (50.7 µg/ml) and steepest slope (1.11) based on log10 probit analysis of the data. These B. thuringiensis d-endotoxins may have utility in transgenic approaches to citrus rootstock protection from D. abbreviatus.

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