Comoviruses: Transmission, Epidemiology, and Control

Abstract

Most comoviruses have been shown to be transmitted efficiently by leaf-feeding beetles in the families Chrysomelidae, Coccinellidae, Curculionidae, or Meloidae. Table I lists the comoviruses, with acronyms used in this chapter, and the beetle species that have been shown to transmit them. Beetle transmission of glycine mosaic virus (GMV), pea mild mosaic virus (PMiMV), and ullucus virus C (UVC) has not been demonstrated. Comoviruses can be acquired or transmitted by beetles immediately upon initiation of feeding, although higher frequencies of transmission occur with prolonged feeding. It was once thought that virus transmission by beetles is a simple mechanical process involving contamination of the mouthparts of the beetle (Smith, 1965). More recent evidence, however, indicates a complex type of interaction between plant viruses and their beetle vectors (Fulton et al., 1980; Gergerich and Scott, 1991). The interaction between beetles and the viruses they transmit has been characterized as circulative because many beetle-transmitted viruses move into the hemolymph of their vectors after ingestion (Freitag, 1956; Slack and Scott, 1971). In vector species such as the bean leaf beetle (Cerotoma trifurcata) and the spotted cucumber beetle (Diabrotica undecimpunctata howardi), southern bean mosaic sobemovirus (SBMV) moves from the digestive system into the circulatory system through the midgut (Wang et al., 1994). However, the comovirus bean pod mottle virus (BPMV) does not enter the hemolymph of these species, although it is efficiently transmitted by them (Wang et al., 1992). Moreover, neither SBMV nor BPMV move into the hemolymph of the Mexican bean beetle (Epilachna varivestis), an efficient vector of these viruses (Wang et al., 1992). As SBMV and BPMV are similar in size and shape, this selective movement through the midgut wall suggests a specific interaction between the virus and some component of the midgut that is required for virus movement through the midgut wall. Because ingested comoviruses are not always found in the hemolymph of their beetle vectors, it appears that virus circulation in the beetle is not invariably an important part of the transmission process, and that in some cases the relation between the beetle and the virus is similar to that found in the foregut-borne type of transmission described for viruses transmitted in the semipersistent manner by aphids (anthriscus yellows virus; Murant et al., 1976) and leafhoppers (maize chloro-tic dwarf virus; Nault and Ammar, 1989). The gradual decrease in the amount of BPMV in beetles during test feeding following virus acquisition (Ghabrial and Schultz, 1983) is taken to indicate that the virus does not multiply in the beetle.