Aerial application of nucleopolyhedrovirus induces decline in increasing and peaking populations of Neodiprion abietis

The potential role of diseases in generating population cycles has often been advocated but has received little experimental support from the field. We introduced a nucleopolyhedrovirus (NeabNPV) into field populations of Neodiprion abietis (Harris), the balsam fir sawfly, to determine its role in the collapse of outbreaks and examine its potential for biological control. This was accomplished through the use of aerial applications of NeabNPV on increasing, peaking, and declining populations of its host. Results indicate that N. abietis densities were distinctly lower in the generation following an aerial application of NeabNPV, but only when treatments were directed against increasing or peaking populations. When directed against declining populations, NeabNPV applications apparently did not influence the natural collapse of outbreaks. Although the artificial introduction of NeabNPV did not consistently affect densities of the treated generation, it had an effect on host biology in the weeks following the treatment as the incidence of NeabNPV infection increased and frass production (concomitant with larval feeding) decreased in treated areas. This study supports the hypothesis that NeabNPV epizootics initiate the decline of N. abietis populations. Our results also indicate that NeabNPV may provide an effective tactic to suppress increasing or peaking population outbreaks of N. abietis through the use of aerial applications of NeabNPV at rates as low as 1 × 109 polyhedral occlusion bodies per hectare.