IMPROVEMENT of biological pesticides through genetic modification has enormous potential and the insect baculoviruses are particularly amenable to this approach(1,2). A key aim of genetic engineering is to increase their speed of kill, primarily by the incorporation of genes which encode arthropod or bacterially derived insect-selective toxins(3-11), insect hormones(12,13) or enzymes(14,15). We report here the first, to our knowledge, field trial of a genetically improved nuclear polyhedrosis virus of the alfalfa looper, Autographa californica (AcNPV) that expresses an insect-selective toxin gene (AaHIT) derived from the venom of the scorpion Androctonus australis(16-18). Previous laboratory assays with the cabbage looper, Trichoplusia ni, demonstrated a 25% reduction in time to death compared to the wild-type virus, but unaltered pathogenicity(6) and host range(19). In the field, the modified baculovirus killed faster, resulting in reduced crop damage and it appeared to reduce the secondary cycle of infection compared to the wild-type virus.