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IMPROVEMENT of biological pesticides through genetic modification has enormous potential and the insect baculoviruses are particularly amenable to this approach1,2. 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 toxins3–11, insect hormones12,13 or enzymes14,15. We report here the first, to our knowledge, field trial of a genetically improved nuclear polyhedrosis virus of the alfalfa looper, Autogmpha californica (AcNPV) that expresses an insectselective toxin gene (AaHIT) derived from the venom of the scorpion Androclonus australisl6–18. 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 pathogenicity6 and host range19. 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 v

Despite the blossoming interest in host-microparasite epidemiology, and in use of viruses in the biological control of insect pests, few empirical studies have attempted to quantify transmission and mortality rates under field conditions. We report a laboratory and field study in which the transmission parameter (u) and mortality rate (a) due to nuclear polyhedrosis virus (NPV) are measured in different larval instars of the cabbage moth, Mamestra brassicae (Lepidoptera: Noctuidae). Laboratory studies of food consumption and virus susceptibility were used to produce crude estimates of relative transmission rates in successive instars. Increased in the rate of feeding outstrip increases in virus resistance with instar, so we predict that transmission rates should increase in older larvae (assuming rate of intake of virus to be proportional to rate of feeding). This prediction was tested in a field experiment in which a constant initial density of susceptible and infected (moribund) larvae were reared together on cabbage plants for 2-8 d. Estimates of the linear transmission parameter (u) did not differ between instars and gave a mean value of 2.16 x 10^-^1^2 for all instars and time points. Causes for the discrepancy between predictions based on laboratory data and field measurements are discussed. Differences were found between instars in the time from infection to death (?) (equivalent to 1/a, where @a is the rate of mortality due to viral infection). Second-instar larvae died more rapidly once infected than third instars, which, in turn, died more rapidly than fourth instars. The effect of host population stage structure on patterns of viral infection can be pronounced and if recognized, may significantly increase the accuracy and predictive value of models of host pathogen systems

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology.Abstract. The advent of genetically modified organisms such as pathogens has raised ecological questions that need to be addressed in order to assess any risks involved in their use. The baculovirus Autographa californica nucleopolyhedrovirus (AcNPV), which infects a number of lepidopteran species, has been modified to express an insect-selective toxin. This genetic modification increases the speed with which it kills its host. However, in addition to this intended feature of the modified virus, there may be other consequences for the host-pathogen interaction. We report a field experiment in which transmission patterns of the wild-type and the genetically modified baculovirus are measured within and between a model target (susceptible) and nontarget (less susceptible) lepidopteran species. Two foliar feeders were chosen: Trichoplusia ni, the cabbage looper, is highly susceptible to this pathogen, while Mamestra brassicae, the cabbage moth, is semipermissive. These two species are used as both the source and the recipients of infection for both virus types. A series of models are fitted to determine the probabilities of infection (given survival from other sources of mortality) over a 7-d period within contained field cages. Fitting these models to data illustrates that a substantial fraction of the population escapes infection, and it is the size of the pathogen-free refuge that varies between treatments. When infected individuals from the less susceptible species die, the yield of virus is greater than from susceptible hosts, yet this does not significantly alter the risk of transmission to other hosts. In contrast, the genetically modified baculovirus always results in a lower risk of infection in the field compared to the wild type. This is because the recombinant virus causes paralysis, and as a result, the cadaver may fall from the plant before death and virus release. Hence the number of cadavers remaining on the foliage has a greater influence on transmission than the yield of virus from those cadavers.

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