Persistence and coexistence of engineered baculoviruses

Introduction to the Use of Baculoviruses as Biological Insecticides

Baculoviruses are widely used both as protein expression vectors and as insect pest control agents. This section provides an overview of the baculovirus life cycle and use of baculoviruses as insecticidal agents. This chapter includes discussion of the pros and cons for use of baculoviruses as insecticides, and progress made in genetic enhancement of baculoviruses for improved insecticidal efficacy. These viruses are used extensively for control of insect pests in a diverse range of agricultural and forest habitats.

Transmission of wild-type and recombinant HaSNPV among larvae of Helicoverpa armigera (Lepidoptera: Noctuidae) on cotton

Horizontal transmission of insect viruses is a key factor in their cycling in agro-ecosystems. Here we study the transmission of the baculovirus HaSNPV among larvae of Helicoverpa armigera (Hübner) in cotton. Transmission of three HaSNPV genotypes was studied from larvae infected with a single virus genotype and from larvae infected with two different genotypes. Genotypes included a wild-type virus, an ecdysteroid UDP-glucosyltransferase (egt) deletion mutant (HaSNPV-LM2) with slightly enhanced speed of kill, and an egt-negative genotype that expresses a neurotoxin gene derived from the scorpion Androctonus australis Hector (HaSNPV-4A). The latter genotype has a substantially increased speed of kill. In three field experiments, the wild-type and egt deletion virus variants and a mixture of the two had similar rates of transmission. Transmission increased with density of infector insects and decreased with time lapsed since the inoculation of the infector larvae. Transmission of the neurotoxin expressing virus was lower than that of the other two genotypes in a glasshouse experiment. The studied genotypes of HaSNPV have significant differences in time to kill and virus yield, but we found no significant differences in rates of virus transmission at the crop level in the case of the egt deletion variant HaSNPV-LM2. Transmission of the transgenic virus genotype HaSNPV-4A was significantly reduced. Overall, differences in transmission between virus genotypes were subtler, and more difficult to detect with statistical significance, than effects of other factors, such as density of infectors and time delay between release of infectors and recipient caterpillars on the plant.

Mixed infections and the competitive fitness of faster-acting genetically modified viruses

Faster-acting recombinant baculoviruses have shown potential for improved suppression of insect pests, but their ecological impact on target and nontarget hosts and naturally occurring pathogens needs to be assessed. Previous studies have focused on the fitness of recombinants at the between-hosts level. However, the population structure of the transmission stages will also be decided by within-host selection. Here we have experimentally quantified the within-host competitive fitness of a fast-acting recombinant Autographa californica multicapsid nucleopolyhedrovirus missing the endogenous egt gene (vEGTDEL), by means of direct competition in single- and serial-passage experiments with its parental virus. Quantitative real-time PCR was employed to determine the ratio of these two viruses in passaged mixtures. We found that vEGTDEL had reduced within-host fitness: per passage the ratio of wild type to vEGTDEL was on average enhanced by a factor of 1.53 (single passage) and 1.68 (serial passage). There is also frequency-dependence: the higher the frequency of vEGTDEL, the stronger the selection against it is. Additionally, the virus ratio is a predictor of time to host death and virus yield. Our results show that egt is important to within-host fitness and allow for a more complete assessment of the ecological impact of recombinant baculovirus release.

Moderation of pathogen-induced mortality: the role of density in Bacillus thuringiensis virulence

Virulence in pathogens may be increased or decreased in order to maximize reproduction and transmission. We investigated how reproduction and virulence in the entomopathogen Bacillus thuringiensis (Bt) changed with bacterial density. We predicted that virulence would be moderated at high pathogen densities because extended time to death allows more growth in hosts. We found that pathogen reproduction (spores produced per cadaver) peaked at an intermediate time to death and was lowest in hosts that die early. Manipulating spore density (spores per unit area of leaf) by combining pathogenic Bt spores with a non-pathogenic mutant confirmed our prediction: larval 5-day mortality was reduced at higher pathogen densities. Pathogen reproduction increased with the density of pathogenic Bt. We hypothesize that more effective reproduction at high density is a consequence rather than a cause of density-dependent virulence.