Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/7/1997
Publication Date: N/A
Interpretive Summary: Naturally occurring insect viruses are being genetically modified to increase their effectiveness as safe viral insecticides. We wanted to know what influence a modified strain of virus had on rapidity of kill of different sizes of pest cabbage looper larvae. We found that regardless of the larval size, the modified viral strain killed cabbage looper larvae faster than the natural strain. Larvae of medium size, however, were not killed as quickly as smaller or larger cabbage loopers. The fact that a modified virus is able to kill all stages of cabbage looper quicker than the wild strain is certainly an advantage, especially if quicker kill means less feeding and damage. Although this may not be important for young loopers that cause less feeding damage it would be very important to growers wishing to reduce feeding damage caused by older loopers.
Technical Abstract: Six weight-classes (ranging from 0.3 to 95 mg/larva) of cabbage looper larvae, Trichoplusia ni, were exposed to a wild (AcC6) or recombinant strain (AcAaIT) of the nuclear polyhedrosis virus of Autographa californica (AcMNPV). Larval mortality, recorded on a daily basis, was never detected earlier than the 2nd day post-exposure (PE). The greatest difference in larval mortality between the two strains was detected on the 3rd day PE. By the 5th day PE, however, no differences in larval mortality between the strains could be detected. The recombinant AcAaIT, regardless of larval size, killed cabbage looper quicker than the wild AcC6 strain. Differences in lethal-time-50 (LT-50) between strains AcC6 and AcAaIT were significant for all weight classes except for larvae weighing 45 to 60 mg/larva (mid-instar larvae). The greatest difference (ca. 90%) in the LT-50 of the strains was for larvae weighing 0.3 to 0.5 mg/larva (1st-instar larvae) and dthe least difference (< 15%) was for 3rd-instar larvae weighing 45 to 50 mg/larva.