Interguild interactions between a non‐native phloem feeder and a native outbreaking defoliator

Abstract

1. Competitive and synergistic interactions directly or indirectly drive community dynamics of herbivorous insects. Novel interactions between non-native and native insects are unpredictable and not fully understood.2. We used manipulative experiments on mature red spruce trees to test interactions between a non-native phloem feeding insect, the brown spruce longhorn beetle (BSLB), and an outbreaking native defoliator, the spruce budworm. We subjected treatment trees to defoliation by three densities of spruce budworm larvae. Treatment trees were: stressed by (i) girdling (to mimic beetle feeding) or (ii) girdling + BSLB before spruce budworm larvae were introduced on branches in sleeve cages. Budworm larvae then fed on foliage and developed to pupation. We assessed all branches for budworm performance, defoliation, shoot production and shoot growth.3. Shoot length did not differ in response to stress from girdling or BSLB infestation. Neither stress from girdling, nor interactions with BSLB feeding affected spruce budworm performance or defoliation. Intraspecific impacts on performance and defoliation in relation to budworm density were stronger than the effects of tree stress.4. Prior infestation of red spruce by BSLB in our experimental set-up did not influence spruce budworm performance. BSLB is a successful invader that has blended into its novel ecological niche because of ecological and phylogenetic similarities with a native congener, Tetropium cinnamopterum. 5. Outbreaks by BSLB will not likely impede or facilitate spruce budworm outbreaks if they co-occur. It would be useful to evaluate the reverse scenario of BSLB success after defoliation stress by spruce budworm.

Inverse size‐dependent phenology of female spruce budworms along reproductive and behavioral classes: an empirical study

“…In addition, it is very challenging to follow individual budworm larvae through development on their host trees due to larval movements within and between branches combined with high levels of disappearance (mortality). An alternative approach to identify ecological determinants of body size consists in rearing budworm larvae into sleeve cages attached to branch section of host trees and quantify parameters statistically related to body size (Moise et al, 2019; MacDonnell et al, 2020; Stead et al, 2021). Although this approach is valid in theory, caged budworms yield ‘out of range’ small female budworms with forewings on average 1.5–4.0 mm shorter than feral budworms.…”