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

Abstract 1 Granulovirus Po GV 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 Po GV was studied by leaf‐disc bioassays in the laboratory at 26 °C incubation temperature. The LC 50 increased from 4 × 10 6 granules/mL for neonate larvae to 1.2 × 10 7 , 1.1 × 10 8 , 4.4 × 10 8 , and 4.2 × 10 9 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‐ LC 50 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 Po GV for different larval ages by using temperature summation to express physiological age. 4 Po GV 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.