Induced tree response to herbivory influences the foraging preference and performance of a conifer‐defoliating sawfly

Abstract

Herbivory often alters the growth and development of woody plants and can thereby render hosts less susceptible to subsequent herbivores. We carried out field surveys and experiments to investigate how previous herbivory influences adult egg lay, larval feeding preference, and associated survival of the yellowheaded spruce sawfly, Pikonema alaskensis (Rohwer) (Hymenoptera: Tenthredinidae), within crowns of black spruce, Picea mariana (Mill.) Britton et al. (Pinaceae). Pikonema alaskensis females laid nearly twice as many eggs, and late‐instar larvae consumed nearly twice as much foliar biomass, on undefoliated vs. defoliated branches. The major factor driving the lower incidence of egg lay on defoliated branches was a significant reduction in the availability of preferred size classes of shoots. In general, adult females preferred medium‐sized shoots, whereas late instars preferred large shoots; previous defoliation generally decreased the availability of medium and large shoots. In a field bioassay, late‐instar survival generally declined with increasing defoliation, which again corresponded to significant reductions in shoot length on defoliated branches. The tendency of P. alaskensis to avoid shoots on severely defoliated branches is likely to increase the time lag between herbivory and the feedback effects of induction and could thus contribute to fluctuations in sawfly population density during outbreak. Our results differ from numerous past studies of closely related diprionid sawflies in conifers, most of which have reported weak or positive effects of previous herbivory on host susceptibility; we attribute the observed responses of spruce to the unusual sawfly feeding preference of P. alaskensis for developing conifer foliage.

Pathways for plant‐mediated negative feedback to insect herbivores: accounting for non‐linear effects of larval density on plant quality and quantity

“…Several models predict that plant induced resistance can influence insect population dynamics (Edelstein‐Keshet & Rausher, ; Lundberg et al., ; Underwood, ; Vos et al., ; Elderd et al., ). The form of density dependence mediated by induced resistance is a key assumption of these models, but although plant‐mediated negative feedback across several insect densities has been shown in a few systems (Karban, ; Brown et al., ; Rotem & Agrawal, ; Underwood, ; Johns & Leggo, ), there are virtually no data on whether density effects are linear or non‐linear (but see Morris, ; Underwood, ). In this study, we found that over the natural range of insect densities we considered, S. carolinense mediates linear, negative density‐dependent effects on at least two fitness correlates likely to influence L. juncta population growth: adult oviposition and larval performance.…”

“…One step toward linking these models with data is thus determining the shape of plant‐mediated negative feedbacks to herbivores. However, although previous empirical work has shown that induced resistance can produce negative feedbacks (Karban, ; Brown et al., ; Rotem & Agrawal, ; Underwood, ; Johns & Leggo, ), most of these studies used a small number of densities, making it difficult to determine whether density effects are linear or non‐linear.…”