Left: Cabbage looper larva killed by nuclear polyhedrosis virus. J.Ogrodnick Center: Cabbage looper infected with nuclear polyhedrosis virus. J.Ogrodnick Right: Imported cabbageworm infected with granulosis virus (above) and healthy (below). J.Ogrodnick
Viruses
- Primary hosts: caterpillars
- Key characters: dead larvae hang as limp, semi-liquified, black,
decomposing sacs, sometimes attached to upper leaves
- Crops: many
- Commercially available: for greenhouse grown flowers
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.
Habitat (Crops)
Most crops and habitats affected by caterpillar pests.
Pests Attacked
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.
Symptoms
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.
Relative Effectiveness
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.
Taken from:
Hoffmann, M.P. and Frodsham, A.C. (1993)
Natural Enemies of Vegetable Insect Pests. Cooperative Extension,
Cornell University, Ithaca, NY. 63 pp.
Additional References
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.
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