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 susceptibility of Helicoverpa zea and Heliothis virescens larvae to the Helicoverpa zea nucleo- polyhedrovirus (HzNPV). Larval H. zea and H. virescens treated with HzNPV on foliage from various hosts showed that H. virescens were significantly more susceptible to HzNPV on cotton and Carolina geranium than H. zea, whereas H. zea were significantly more susceptible to HzNPV on velvetleaf than H. virescens. Plant phenology had an effect on larval susceptibility to HzNPV. Vegetative or reproductive tissues of cotton, soybean, tomato, crimson clover, Carolina geranium, and velvetleaf were treated with HzNPV and fed to second-instar H. zea or H. virescens. Host phenology significantly affected the mortality of HzNPV-treated H. zea on all hosts except tomato. HzNPV-treated H. zea fed on vegetative tissues of crimson clover, Carolina geranium, velvetleaf, and soybean had significantly higher mortality than those fed on reproductive tissues. On the contrary, larval susceptibility to HzNPV on cotton was greater on reproductive than vegetative tissues. H. virescens susceptibility to HzNPV was greater on vegetative than reproductive tissue for crimson clover. In addition, we examined the effect of prior herbivory on larval susceptibility to the HzNPV. Cotton, soybean, tomato, and velvetleaf plants were either untreated or wounded with three fourth-instar H. zea or H. virescens. Neonates were reared on the respective treatments and then treated with HzNPV as second instars. Non-HzNPV-treated H. zea larvae fed on wounded foliage had up to 93.1% reduced weight gain compared with those fed on unwounded foliage. The weight reduction was greatest on cotton (93.1%), followed by velvetleaf (80.6%), tomato (78.4%), and soybean (54.1%). Prior herbivory on all plants significantly affected the larval growth of nontreated H. zea. In nontreated H. virescens, the reduction in weight was greatest on velvetleaf (63.1%), followed by soybean (29.2%) and cotton (21.8%). However, the effect of prior herbivory on cotton did not significantly affect the larval growth of nontreated H. virescena. Only virus-treated H. zea that fed on wounded virus-treated tomato foliage showed significantly enhanced larval mortality (47.7%) compared with larvae on unwounded foliage. For HzNPV-treated H. virescens, only prior herbivory on cotton significantly enhanced larval mortality (39.4%).