Sensitivity of heliothine larvae to wild-type and recombinant strains of asingle nucleocapsid (HzSNPV) and a multiple nucleocapsid (AcMNPV) baculovirus

Mid-stadium larvae of Helicoverpa tea were equally susceptible to either th e wild-type (Wt) or recombinant (RcHz-Lqh) strain of HzSNPV. There was, how ever, a significant difference (1.8-fold) in the rate of mortality between the two strains. The LT50 for the Wt and Re strain was 3.4+/-0.1 days and 1 .9+/-0.2 days, respectively. In contrast, there was a significant differenc e in both the rate and extent of mortality of mid-stadium larvae of H. vire scens fed either the Wt or Re strain of HzSNPV (LC50: Wt. 11.4 +/- 0.6 OB/c m(2); Re, 42.2 +/- 8.2 OB/cm(2). LT50: Wt, 4.1+/-0.1 days; Re, 2.8+/-0.2 da ys). An 8-fold increase in the viral rate of the WtHzSNPV was needed (800 O B/mm(2)) to match the effectiveness of the RcHzLqh strain at 100 OB/cm(2). Increasing the RcHzLqh rate above 100 OB/cm(2) did not significantly reduce the LT50 or larval feeding as measured by frass weight. All viral rates of the Re strains significantly reduced (ca. 6- to 12-fold) mid-stadium larva l feeding over that of the control. Differences between the Re and Wt strai ns of HzSNPV, using late-stadium H. zea larvae, showed the same tendency as that using mid-stadium larvae, but the differences were not as great (< 2- fold). Mid-stadium larvae of H. virescens were ca. 3-fold more susceptible to RcAcLqh than to either the WtAc or RcAcAa. Both RcAcL.qh and RcAcAa stra ins killed larvae quicker(1.5- to 1.7-fold) and reduced feeding more (1.6- to 1.9-fold) than the WtAc strain. A combination of RcAcLqh and RcAcAa at 5 OB/cm(2) had no additive effect on mortality of H. virescens larvae and th e combination was less effective than either Re strain at 10 OB/cm(2). Unex pectedly, the known level of susceptibility of semi-permissive larvae of H. zea to WtAcMNPV, as measured by percent mortality, LT50, and feeding, was significantly enhanced (ca. 2-fold) when H. tea larvae were fed the RcHzLqh strain. In contrast, expression of the Lqh-toxin by RcHzLqh did not signif icantly increase the susceptibility of H. subflexa, larvae previously repor ted as being highly resistant to WtHzSNPV.