Chapter 17
Viral Insecticides
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James L. Vaughn Insect Biocontrol Laboratory, Plant Sciences Insitute, Beltsville Agricultural Research Center-West, Agricultural Research Service, U.S. Department of Agriculture, Beltsville, MD 20705-2350
Viruses are potentially valuable tools for managing pest insect populations. As has been observed in countless natural epizootics, a virus can cause significant reductions in pest insect populations. Presently they are used on a large scale to control the pine caterpillar in Japan and the soybean looper in Brazil. Four are registered for use in the U.S. They are exempted from the requirement for residue tolerance and, therefore, are free of residue problems. Their host specificity makes them compatible with other natural enemies and safe to use in environmentally sensitive areas. Recent advances in genetic manipulation offer possible solutions to the problems of severely restrictive host range and slow activity and have increased the attractiveness of viruses as pest control agents. Although resistance is possible, the fact that the virus is capable of variation and change reduces this possibility. The technology for in vitro production has improved rapidly and appears to be economically feasible.
The control of insect pests with viruses has a long history. In 1892 there was a severe outbreak of the nun moth, Lymantria monacha, in the pine forests of Germany. A disease of this insect, believed to be caused by an organism called Hofinann's bacterium, had been observed. To control the pest, foresters attempted to cultivate the suspected causative organism on a mixture of horse meat and potatoes. Following an appropriate incubation, pieces of the meat were tied in the trees in threatened parts of the forest. After a time it was reported that larvae of the nun moth began to show
This chapter not subject to U.S. copyright Published 1993 American Chemical Society In Pest Control with Enhanced Environmental Safety; Duke, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
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typical signs of the disease. Eventually the population of nun moths collapsedand extensive damage to the forest was prevented. So impressive was this result that the technique was widely used by the German forest service for nun moth control, with highly variable results (7). We now know that the disease was caused by a virus and that the collapse of the population was due to naturally occurring virus not to the intervention of the foresters. The story does illustrate two important points, however. First, attempts to use viruses to control insect pests often have been made with little or at least incomplete understanding of the biology of the system being used. Second, when this was done, the results were often inconsistent and failures occurred frequently. The long standing interest in the use of viruses for controlling insect pests is not surprising. Many viruses cause acute diseases and the affect they can have in a dense pest population often is spectacular. In Ontario, Canada a virus of the European spruce sawfly, Gilpinia herecynia, that was introduced from Europe in 1950 causes annual epizootics and still maintains the insect population below a level that would cause economic damage (2). A similar virus periodically causes the collapse of gypsy moth, Lymantria dispar, populations in hardwood forests (3). A virus causes regular, naturally occurring epizootics in populations of the cabbage looper, Trichoplusia ni {4\ Over 1600 virus isolates have been recorded from diseased animals in about 1100 species of insects and mites (5). There are four characteristics that a virus must have to make it useful as a pest management tool. First, it should be lethal to the target insect or stop it from feeding. Second, the incubation period should be short. A prolonged incubation period in which the insect continues to feed may result in economic loss, even though the pest may be eventually controlled. Third, the virus must be safe to use. It must not infect vertebrates, plants, and nontarget insects. Finally, it must be stable enough to retain activity for a reasonable time between production and application. The viruses that cause diseases in insects are presently classified into several families shown in Table I. The Poxviridae, Reoviridae, and Baculoviridae have received most of the attention as biological pest control agents. One feature that these viruses have in common is that the virion is occluded in a protein crystal when the replication is completed giving a relative stability to the virion, Figure 1. Insect viruses belonging in the family Poxviridae have morphological and enzymatic characteristics very similar to other members of this family that have been isolated from vertebrates. However, the restriction enzyme nucleotide fragment patterns are quite different and the Entomopox viruses have not been shown to infect any animals other than insects (d). Pox viruses have been described in a number of pest insects including forest pests, beetles, and grasshoppers (as examples see refs. 7-9). These viruses develop slowly in the infected insect. For example, Henry, et al. (9) reported that death of infected grasshoppers occurred 20-30 days after inoculation. The similarity
In Pest Control with Enhanced Environmental Safety; Duke, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
17. VAUGHN
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Viral Insecticides
of these viruses to vaccinia, the long incubation time, and problems inproduction have discouraged their development for pest control.
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Table I. Viruses Infecting Insects
FAMILY
NUCLEIC ACID
INSECT PATHOGENS
BACULOVIRIDAE
