Susceptibility of Phthorimaea operculella Zeller (Lepidoptera; Gelechiidae) to its granulovirus PoGV with larval age

1 Granulovirus PoGV is a strong candidate to substitute for chemical insecticides in integrated pest management (IPM) of the potato tuber moth Phthorimaea operculella Zeller (Lepidoptera: Gelechiidae). Generally, susceptibility to baculoviruses decreases with increasing larval age. For effective field applications, knowledge of the age-related change in susceptibility is crucial. 2 The relative susceptibility of various instars of P. operculella larvae to PoGV was studied by leaf-disc bioassays in the laboratory at 26 °C incubation temperature. The LC₅₀ increased from 4 x 10⁶ granules/mL for neonate larvae to 1.2 x 10⁷, 1.1 x 10⁸, 4.4 x 10⁸, and 4.2 x 10⁹ granules/mL, signifying resistance ratios of 3, 29, 110, and 1051, for 5, 6, 7, and 9-day-old larvae, respectively. 3 The relationship between log-LC₅₀ values and log-larval weight was significantly linear. A logistic function described significantly the relation between larval weights and physiological age measured in degree-days (DD > 13.4 °C). Both functions allowed prediction of the activity ratio of PoGV for different larval ages by using temperature summation to express physiological age. 4 PoGV was found to be highly active against P. operculella larvae up to a physiological age of approximately 50 DD (>13.4 °C) (i.e. the first third of the total larval development time). Thereafter, the virus rapidly lost its activity against older larvae. Prospects for applying this knowledge in the field are discussed.