Insect-specific viruses can be highly effective natural controls of several caterpillar pests. Different strains of naturally occurring nuclear polyhedrosis virus (NPV) and granulosis virus are present at low levels in many insect populations. Epizootics can occasionally devastate populations of some pests, especially when insect numbers are high.
Insect viruses need to be eaten by an insect to cause infection but may also spread from insect to insect during mating or egg laying. In some cases, for example while searching for suitable hosts for egg laying, beneficial insects such as parasitoids may physically spread a virus through the pest population.
No threat to humans or wildlife is posed by insect viruses. Virus diseases of caterpillar pests may cause indirect mortality of some beneficial larval parasitoids if the host insects die before the parasitoids have completed development. Predators and adult parasitoids are not directly affected. Viruses can overwinter in the environment or in overwintering insects to re-establish infection in subsequent seasons.
The successful commercialization of insect-pathogenic viruses has been limited. Thus far, NPV strains have only been mass produced in living insects, a costly procedure. Viral insecticide development is further hindered by the fact that the viruses are specific to one species or genus, ensuring a relatively small market.
Nuclear polyhedrosis viruses have been registered in the United States for use against cotton bollworm, Douglas-fir tussock moth, gypsy moth, European pine sawfly, beet armyworm, and alfalfa looper, although none of these is being produced commercially. The U.S. Forest Service and USDA ARS produce and use an NPV for gypsy moth, but do not sell it. The only commercially available NPV in the United States is used primarily in the Netherlands for greenhouse grown flowers.
Most crops and habitats affected by caterpillar pests.
Naturally occurring viruses may affect many caterpillar pests. NPV affects alfalfa looper, corn earworm, imported cabbageworm, cabbage looper, cotton bollworm, cotton leafworm, tobacco budworm, armyworms, European corn borer, almond moth, spruce budworm, Douglas fir tussock moth, pine sawfly and gypsy moth. Preparations of granulosis virus have been isolated from several caterpillar species, including imported cabbageworm, cabbage looper, armyworm, fall webworm, and mosquitoes, among many others.
Mode of Action
Viruses invade an insect's body via the gut. They replicate in many tissues and can disrupt components of an insect's physiology, interfering with feeding, egg laying, and movement.
Different viruses cause different symptoms. NPV-infected larvae may initially turn white and granular or very dark. Some may climb to the top of the crop canopy, stop feeding, become limp, and hang from the upper leaves or stems, hence the common name "caterpillar wilt" or "tree top" disease. Victims of a granulosis virus may turn milky white and stop feeding. In both cases, the body contents of the dead larvae are liquified and the cuticle ruptures easily to release infectious viral particles. Death from a virus infection usually occurs within three to eight days.
A naturally occurring viral epizootic can seriously deplete a pest population. Transmission of the virus through the population may take days or weeks but, if conditions are suitable, the entire population may eventually collapse. In some instances, the combination of naturally occurring viruses and other natural enemies will maintain pest populations at acceptable levels. For example, a virus has been recorded as destroying up to 28% of imported cabbageworm populations in cole crops, with up to 55% of the remaining population parasitized by several parasitoids. Cabbage looper populations experienced up to 40% infection by virus in the same studies.
Mass reared viruses have been successfully applied in limited areas as microbial insecticides against pests. In trials in Maryland, releases of granulosis virus proved as effective as Bt against some caterpillars and, in trials against imported cabbageworm on cole crops in New York, a granulosis virus yielded results equivalent to applications of chemical insecticide. Pheromones (insect attractants) have been used to lure male corn earworm moths to NPV-contaminated bait stations to help spread the virus within a corn earworm population.
Infected caterpillars have been mashed into a water solution and applied to pest populations as a form of microbial insecticide. Viruses are adversely affected by ultraviolet radiation and are best applied in the late afternoon. Pest abundance and the general fitness of the pest population will affect its susceptibility to virus attack, and the effectiveness of different strains of the same virus can vary considerably. Viruses can be stored frozen for many years. Research to reduce the killing time of viruses is ongoing and includes the use of genetic engineering to improve the performance of viral insecticides.
Tanada, Y., and Kaya, H.K. (1993) Insect Pathology. Academic Press, Inc., San Diego. 666pp.
Weinzierl, R., and Henn, Tess. (1989) Alternatives in insect management: Microbial insecticides. Cooperative Extension, University of Illinois, Circular 1295. 12 pp.