Reduced efficacy of Helicoverpa armigera nucleopolyhedrovirus (HearNPV) on chickpea (Cicer arietinum) and other legume crops, and the role of organic acid exudates on occlusion body inactivation

Abstract

Baculoviruses, such as the nucleopolyhedroviruses (NPV), are used widely as commercial biopesticides, however, their efficacy is reduced on some crops including chickpea (Cicer arietinum) due to the inactivation of the baculoviruses on the leaf surface. Here, we report for the first time, that a reduction in infectivity of the baculovirus, Helicoverpa armigera nucleopolyhedrovirus (HearNPV), also occurs on cowpea (Vigna unguiculata) and pigeonpea (Cajanus cajan), though at much lower levels than that recorded in chickpea. The isoflavonoids formononetin and biochanin A, known to be present in chickpea, pigeonpea and cowpea, were associated with reduced infectivity of HearNPV occlusion bodies (OB) in vitro comparable to that seen on cowpea and pigeon pea, but did not explain the greatly higher level of reduced infectivity recorded on chickpea in vivo. Exposure of OB to malic and oxalic acids, two organic compounds produced naturally at high concentrations from chickpea leaf trichome exudates, dramatically reduced the infectivity. The activity of baculovirus OB is known to be reduced by low pH equal to that recorded on chickpea leaf surfaces by malic and oxalic acid. Thus, we hypothesise that chickpea leaf surface acids, and in particular oxalic acid, are the primary leaf surface factors responsible for the reduced infectivity of OB on chickpea reported earlier in field and laboratory studies. The mechanism of this inactivation is not yet completely resolved, although electron microscopy suggests that the acids do not affect the OB structural integrity. The implications for the use of baculovirus insecticides on cowpea, pigeonpea and chickpea in the light of these findings are discussed.