Microbial Viral Insecticides

Abstract

Microbial viral insecticides are pathogens that attack insects and other arthropods. Baculoviruses (BV) are parasitically replicating microscopic elements. Baculoviruses are extremely small and are composed primarily of double-stranded DNA required for the virus to establish itself and reproduce. The genus Baculoviruses contains three subgroups of viral types: nuclear polyhedrosis viruses (NPVs), granulosis viruses (GVs) and nonoccluded viruses. NPVs and GVs differ in the number and structure of the protective protein coat and are both relatively large and complex in structure in comparison to many other types of viruses. While little information is available for viruses from the third subgroup, several aspects of the infectivity and mode of action of NPVs and GVs have been studied. The most common route of entry into an insect is by ingestion. The primary site of infection is the midgut cells by membrane function. However, two distinct mechanisms of virus uncoating occur among the baculovirus, that is, NPVs uncoat within the nucleus, whereas GVs uncoat within the nuclear pore complex. NPVs may pass through the intestinal epithelium immediately after ingestion, thereby establishing a systematic infection of the haemocoel prior to virus replication in the midgut cells. The GVs do not appear to pass through midgut cells as rapidly as NPVs, and the developmental cycle of GVs is longer than that of NPVs.

The NPVs are mass produced in larval hosts grown on artificial diet or host plant. Usually third to fourth instar larvae of Helicoverpa armigera are infected with the viral food. The definitive phase of viral disease occurs over a period of 5–10 days. Once the complete infection of the virus in the larvae is completed, the larvae start ‘putrefying’ releasing billions of polyhedra. In commercial production, larvae are being harvested before purification to keep bacteria at a low level in the final product. After the larval production phase is complete, the larvae are collected and formulated. NPVs are ideal candidates for use where a single lepidopteran species is the major pest. NPVs are being used against H. armigera and Spodoptera litura on cotton, corn, sorghum, tomatoes and chrysanthemum. It is also being used against Anticarsia gemmatalis of soybean. One of the most important successes in commercial production and use of a GV is Cydia pomonella GV (CpGV) on apples and pears. Advantages in using microbial viruses are safety for humans and other nontarget organisms, reduction of pesticide residues, little or no development of resistance by the target organism, no secondary pest outbreak and no preharvest interval is required. Though there are many advantages, some disadvantages are also there, for example, host specificity is a double-edged sword; it is an advantage as well as a disadvantage. Moreover, long period of lethal infection is required, and the virus gets inactivated by environmental factors like ultraviolet light, extreme temperature, etc. In this chapter, an attempt has been made to cover the commercially available BVs for the control of agricultural pest particularly in India. The objective of this chapter was to briefly cover the aspects like importance of baculoviruses in pest control, history, genome and the products available in the Indian market.