The comparative susceptibilities of Pieris brassicae and P. rapae to a granulosis virus from P. brassicae

A comparison was made of the dosage-mortality responses of larvae of Pieris brassicae and P. rapae to infection by P. brassicae granulosis virus (GV). Bioassays with first, second, third, and fourth-instar larvae of both species revealed a marked difference in susceptibility between instars and between species. Median lethal dosages (LD₅₀s) for P. rapae larvae ranged from five capsules for the first instar to 662 capsules for the fourth instar. With P. brassicae, this range extended from 66 capsules to 2.3 × 10⁷ capsules. The time-mortality responses of the two species were similar when fed virus dosages equivalent to an LD₉₀. Median lethal times (LT₅₀s) ranged from 5 days for first-instar larvae to 7–8 days for fourth-instar larvae. A comparison between a long-established laboratory stock of P. brassicae and a stock recently acquired from the field showed no significant difference in their susceptibility to GV. The implications of the pronounced species differences in susceptibility to GV infection are discussed in relation to the potential field control of P. rapae and P. brassicae.