Interest in the biological control of insect pests has arisen as a result of the problems associated with the use of conventional chemical pesticides. Chemical pesticides generally affect beneficial as well as non-beneficial species. Pest insects tend to acquire resistance to such chemicals so that new pest insect populations can rapidly develop that are resistant to these pesticides. Furthermore, chemical residues pose environmental hazards and possible health concerns. The biological control of insect pests presents an alternative means of pest control which can play a role in integrated pest management and reduce dependence on chemical pesticides.
The primary strategies for biological control include the deployment of naturally occurring organisms which are pathogenic to insects (entomopathogens), and the development of crops that are more resistant to insect pests. In recent times, much attention has been focused on the development of insect-resistant crop species by genetic engineering techniques. There is considerable potential for the improvement of biological control strategies through a variety of approaches. These include the identification and characterization of insect genes or gene products which may serve as suitable targets for insect control agents, the identification and exploitation of previously unused microorganisms (including the modification of naturally occurring non-pathogenic microorganisms to render them pathogenic to insects), the modification and refinement of currently used entomopathogens, and the further development of genetically engineered crops which display greater resistance to insect pests.
Viruses that cause natural epizootic diseases within insect populations are among the entomopathogens which have been developed as biological pesticides. Baculoviruses are a large group of viruses which are infectious only in arthropods (L. K. Miller, Virus Vector for Genetic Engineering in Invertebrates, in "Genetic Engineering in the Plant Sciences", N. Panopoulous, Ed., Praeger Publ., N.Y., pp. 203-224, 1981; Carstens, "Baculoviruses--Friend of Man, Foe of Insects?," Trends and Biochemical Science, 52:107-110, 1980; Harrap and Payne, "The Structural Properties and Identification of Insect Viruses" in Advances in Virus Research, M. A. Lawfer, F. B. Bang, K. Maramorosh and K. M. Smith, Eds., Vol. 25, pp. 273-355, Academic Press, New York, 1979). Many baculoviruses infect insects which are pests of commercially important agricultural and forestry crops. Such baculoviruses are therefore potentially valuable as biological control agents. In fact, four different baculoviruses have been registered for use as insecticides by the U.S. Environmental Protection Agency. Among the advantages of baculoviruses as biological pesticides is their host specificity. Not only are baculoviruses as a group found solely in arthropods, individual baculovirus strains are usually restricted in their replication to one or a few species of insects. Thus, they pose no risk to man or the environment and can be used without detriment to beneficial insect species.
Baculovirus subgroups include nuclear polyhedrosis viruses (NPV), granulosis viruses (GV), and nonoccluded baculoviruses. In occluded forms of baculoviruses (NPV and GV subgroups), the virions (enveloped nucleocapsids) are embedded in a crystalline protein matrix. This structure, referred to as an inclusion or occlusion body, is the form found extraorganismally in nature and is responsible for spreading the infection between organisms. The characteristic feature of viruses of the subgroup NPV is that many virions are embedded in each occlusion body. The occlusion bodies are quite large (up to 5 micrometers). Occlusion bodies of viruses of the GV subgroup are smaller and contain a single virion each. The crystalline protein matrix of the occlusion bodies of both forms is primarily composed of a single 25,000 to 33,000 dalton polypeptide which is known as polyhedrin or granulin. Baculoviruses of the nonoccluded subgroup do not produce a polyhedrin of granulin protein and do not form occlusion bodies.
In nature, infection is initiated when an insect ingests food contaminated with baculovirus particles, typically in the form of occlusion bodies. The occlusion bodies dissociate under the alkaline conditions of the insect midgut, releasing individual virus particles which invade epithelial cells lining the gut. Within the cell, the baculovirus migrates to the nucleus where replication takes place. Initially, certain specific viral proteins are produced within the infected cell via the transcription and translation of so-called "early genes". Among other functions, these proteins are required to allow replication of the viral DNA, which begins 4 to 6 hours after the virus enters the cell. Extensive viral DNA replication proceeds up to about 12 hours post-infection (pi). From about 8 to 10 hours pi, the infected cell begins to produce large amounts of "late viral gene products". These include components of the nucleocapsid which surround the viral DNA during the formation of progeny virus particles. Production of the progeny virus particles begins around 12 hours pi. Initially, progeny virus migrate to the cell membrane where they acquire an envelope as they bud out from the surface of the cell. This nonoccluded virus can then infect other cells within the insect. Polyhedrin synthesis begins from 12 to 18 hours after infection and increases to very high levels by 24 hours pi. At that time, there is a decrease in the number of virus particles budding from the cell and progeny virus begin to become embedded in occlusion bodies. Occlusion body formation continues until the cell ultimately dies or lyses. Some baculoviruses infect virtually every tissue in the host insect so that at the end of the infection process, the entire insect is liquified, releasing extremely large numbers of occlusion bodies which are then responsible for spreading the infection to other insects. (Reviewed in: "The Biology of Baculoviruses," R. G. Granados and B. A. Federici, Eds., Vol. I and II, CRC Press, Boca Raton, Fla., 1986.)
One of the significant disadvantages to using baculoviruses as pesticides is the length of time which elapses between ingestion of the virus and the time the insect finally dies. During this time, the pest insect will continue to feed and damage the crop. Because the grower is unlikely to apply the pesticide until after an infestation becomes apparent, it is critical that the extent of feeding during this lag period be kept to a minimum.
What is needed is a biological pesticide which reduces the the adverse effects of the pesticide. A biological pesticide is preferred because it creates less of an environmental hazard than a chemical pesticide. A pesticide that causes insect death more rapidly is additionally needed. What is also needed is the identification and isolation of a gene that codes for a protein which will control insect development. Such a gene or its protein product could then be incorporated into various organisms for the improved biological control of insect pests.