Overwintering spruce budworm population density as predictor of following-year larval density and defoliation on balsam fir

Abstract

The Early Intervention Strategy (EIS) was developed to control spruce budworm (SBW) populations before an outbreak develops. It uses an action threshold population density originally based on feeding fourth-instar larvae (L4) sampled in spring. But in practice, in all SBW management strategies, it is the density of overwintering second-instar larvae (L2) sampled in the previous fall that is used in decision making. Those densities serve to predict defoliation and plan control operations for the following summer. Therefore, there was a need to establish a relationship between L2 and L4 (apparent survival) by which to adequately express treatment thresholds in terms of L2 rather than L4. This was done with samples collected in each life stage in several sites in Quebec and New Brunswick and over three different periods between 1999 and 2022. We found that the distribution of L2 is more aggregated than that of L4, and thus that sample sizes needed for precise density estimation in that stage are larger than at the L4. We also found that apparent survival between L2 and L4 varies at low and high L2 densities for different reasons. For example, movement of larvae from the inner crown, bole, or non-host trees affect apparent survival at the lowest population densities. In contrast, at very high densities survival of populations during these early larval stages is reduced by crowding effects. At densities in the mid-range where control decisions need to be made, apparent survival is very close to 100% and varies little among years. The current threshold density is 3.5 L4 per 45-cm branch tip and was established as 7 L2 per 75-cm branch by assuming 50% mortality. Considering the results presented here, we recommend that the current EIS intervention threshold be set at 3.5 L2 per 75-cm branch. In this density, defoliation typically ranges between 15 and 30% in the following summer in the absence of treatment.