An improved method to determine the biological activity (LC₅₀) of the granulovirus PoGV in its host Phthorimaea operculella
Granulovirus (PoGV) is a promising candidate to substitute for chemical insecticides in integrated pest management (IPM) of the potato tuber moth, Phthorimaea operculella (Zeller) (Lepidoptera: Gelechiidae). A procedure to assess pathogenicity (LC₅₀) and its standardization is crucial for research into the use of pathogens as biocontrol agents. In the present study, an egg-dip bioassay method was developed and its precision tested in a series of six bioassays over a time period of 18 months. Bioassays were carried out at 25°C incubation temperature. The probit model as a statistical analysis method for the interpretation of concentration responses proved better than the exponential model. LC₅₀ values ranged from 2.3 × 10⁶ to 10⁷ granules ml⁻¹ with a mean of 5 × 10 ⁶ granules ml⁻¹ and regression slopes varied between 0.73 and 1.05 with a mean of 0.84. LC₅₀ values and slopes displayed no significant differences according to their 95% confidence limits. The rate of harvested infected larvae showed a linear increase with increasing concentration on a log-log scale. The mean natural mortality was 15% (SE ± 1.8%) and can be considered as a main factor producing variability among bioassays. The advantages of this bioassay method and its application in PoGV research are discussed. © 2005 The Netherlands Entomological Society.
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