INFLUENCE OF INTERSPECIFIC AND INTRASPECIFIC HOST-PLANT VARIATION ON THE SUSCEPTIBILITY OF HELIOTHINES TO A BACULOVIRUS

In this paper we report on the effect of host plant variation on the s usceptibility of Helicoverpa zea and Heliothis virescens larvae to the Helicoverpa tea nucleopolyhedrovirus (HzNPV). Larval H. tea and H. vi rescens treated with HzNPV on foliage from various hosts showed that H . virescens were significantly more susceptible 60 HzNPV on cotton and Carolina geranium than H. tea, whereas H. tea were significantly more susceptible to HzNPV on velvetleaf than H. virescens. Plant phenology had an effect on larval susceptibility to HzNPV. Vegetative or reprod uctive tissues of cotton, soybean, tomato, crimson clover, Carolina ge ranium, and velvetleaf were treated with HzNPV and fed to second-insta r H. tea or H. virescens. Host phenology significantly affected the mo rtality of HzNPV-treated H. tea on all hosts except tomato. HzNPV-trea tedH. tea fed on vegetative tissues of crimson clover, Carolina gerani um, velvetleaf, and soybean had significantly higher mortality than th ose fed on reproductive tissues. On the contrary, larval susceptibilit y to HzNPV on cotton was greater on reproductive than vegetative tissu es. H. virescens susceptibility to HzNPV was greater on vegetative tha n reproductive tissue for crimson clover. In addition, we examined the effect of prior herbivory on larval susceptibility to the HzNPV Cotto n, soybean, tomato, and velvetleaf plants were either untreated or wou nded with three fourth-instar H. tea or H. virescens. Neonates were re ared on the respective treatments and then treated with HzNPV as secon d instars. Non-HzNPV-treated H. tea larvae fed on wounded foliage had up to 93.1% reduced weight gain compared with those fed on unwounded f oliage. The weight reduction was greatest on cotton (93.1%), followed by velvetleaf (80.6%), tomato (78.4%), and soybean (54.1%). Prior herb ivory on all plants significantly affected the larval growth of nontre ated H. tea. In nontreated H. virescens, the reduction in weight was g reatest on velvetleaf (63.1%), followed by soybean (29.2%) and cotton (21.8%). However, the effect of prior herbivory on cotton did not sign ificantly affect the larval growth of nontreated H. virescens. Only vi rus-treated H. tea that fed on wounded virus-treated tomato foliage sh owed significantly enhanced larval mortality (47.7%) compared with lar vae on unwounded foliage. For HzNPV-treated H. virescens, only prior h erbivory on cotton significantly enhanced larval mortality (39.4%). (C ) 1998 Academic Press.