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

Mid-stadium larvae of Helicoverpa zea were equally susceptible to either the wild-type (Wt) or recombinant (RcHzLqh) strain of HzSNPV. There was, however, a significant difference (1.8-fold) in the rate of mortality between the two strains. The LT50 for the Wt and Rc strain was 3.4±0.1 days and 1.9±0.2 days, respectively. In contrast, there was a significant difference in both the rate and extent of mortality of mid-stadium larvae of H. virescens fed either the Wt or Rc strain of HzSNPV (LC50: Wt, 11.4±0.6 OB/cm2; Rc, 42.2±8.2 OB/cm2. LT50: Wt, 4.1±0.1 days; Rc, 2.8±0.2 days). An 8-fold increase in the viral rate of the WtHzSNPV was needed (800 OB/mm2) to match the effectiveness of the RcHzLqh strain at 100 OB/cm2. Increasing the RcHzLqh rate above 100 OB/cm2 did not significantly reduce the LT50 or larval feeding as measured by frass weight. All viral rates of the Rc strains significantly reduced (ca. 6- to 12-fold) mid-stadium larval feeding over that of the control. Differences between the Rc and Wt strains 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 RcAcLqh and RcAcAa strains 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/cm2 had no additive effect on mortality of H. virescens larvae and the combination was less effective than either Rc strain at 10 OB/cm2. Unexpectedly, 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. zea larvae were fed the RcHzLqh strain. In contrast, expression of the Lqh-toxin by RcHzLqh did not significantly increase the susceptibility of H. subflexa, larvae previously reported as being highly resistant to WtHzSNPV.