Plant module size and dose of gall induction stimulus influence gall induction and galler performance

Abstract

Several different hypotheses attribute large variations in gall abundance and galler performance to variations in plant growth rate and/or module size. The plant vigor hypothesis (PVH) predicts that galler performance will be greatest on large, fast-growing plant modules due to their large, rapidly dividing cells that allow them to make large galls, where survival and size of survivors are usually greater. The optimal module size hypothesis (OMSH) predicts that galler performance will be greatest on intermediate-sized modules, based on a tradeoff between ease of gall induction on small modules and increased resource availability in large modules. Here, we evaluate predictions of the PVH and OMSH during a two-year field study by examining gall induction and full gall development success, as well as subsequent galler performance in successfully developed galls, using a shoot galling adelgid, Adelges abietis, on white spruce Picea glauca. In addition, we tested whether increased dose of gall induction stimulus on different sized modules, achieved by observing differing A. abietis densities per bud, could increase the rate of successful gall induction, as predicted by the OMSH. Galls of A. abietis appeared to be induced by a dose-dependent stimulus from fundatrices in spring. Furthermore, the critical stimulus dose required to induce a gall appeared to increase with shoot length. These findings support the OMSH and not the PVH. Galler performance (gall volume and the number of gallicolae that emerged from each gall) in successfully developed galls was positively related to shoot length, supporting both the PVH and OMSH. We conclude that the PVH represents one component of the more inclusive OMSH, which considers both ease of gall induction on different-sized modules and the effect of plant module size on gall size.

Individual tree traits shape insect and disease damage on oak in a climate‐matching tree diversity experiment

“…It is possible that phenological asynchrony of Italian provenances with local galler populations may explain why Italian trees in our study had lowest gall abundance across sites and years. Some galler groups are also known to respond positively to plant module size (Flaherty & Quiring, ; Kopelke, Amendt, & Schönrogge, ). We found differential resource allocation to plant modules across provenances; local trees invested in longer primary shoots, nonlocal provenances produced more/longer lammas growth (Figures and ).…”

“…We tested the following hypotheses: Local and nonlocal provenances would differ in their tree phenotypic traits. Phenotypic traits measured were those with a known impact on insect herbivores and foliar pathogens: plant vigor (tree height and shoot length, both commonly used proxys of vigor (Flaherty & Quiring, ; Gripenberg, Ovaskainen, Morriën, & Roslin, )) and the number of lammas shoots produced (as a predictor of powdery mildew infection). Insect herbivore abundance and powdery mildew infection would be highest on local provenances (local adaptation hypothesis). Associational resistance to both insect herbivores and powdery mildew would occur in mixed species plots compared to single species plots. Differences in provenance susceptibility to insect herbivores and powdery mildew could lead to associational effects in mixed provenance plots, compared to provenance monocultures. Both tree vigor, and tree apparency, would be significant positive drivers of insect herbivory and powdery mildew infection. Insect herbivory and oak powdery mildew infection would be positively associated with the same trees. …”

Timing of growth determines fitness and performance of a galling insect on willow

“…Studies in other systems have shown trade‐offs between initiation success and plant vigour, suggesting an optimal plant module size for insect fitness and performance (Sopow et al , ; Morrison & Quiring, ). In such cases, the galler's performance is greatest on intermediate‐sized modules because of the conflict between the ease of gall initiation on small modules and the enhanced supply of resource in large modules (Flaherty & Quring, ). Such a trade‐off was not documented in the system examined in the present study: gall initiation was as successful at the site of maximum growth as at the sites with relatively less growth.…”

Plant module size influences the intra‐tree distribution and abundance of a shoot‐boring sawfly in young balsam fir

“…The ‘plant vigor’ hypothesis (PVH), for instance, predicts that herbivores should prefer and perform better on relatively large plant modules (e.g., shoots), which tend to possess relatively high concentrations of nutrients and water (Price, , ). However, recent studies suggest that some herbivores struggle to exploit the largest, most vigorously growing modules, despite their superior nutritional quality (Flaherty & Quiring, ). Such herbivores may instead favor intermediate‐sized shoots, balancing tradeoffs between utilizing relatively large, nutritious shoots vs. small, more easily exploited shoots (i.e., the ‘optimal module size’ hypothesis, OMSH) (Quiring et al., ; Flaherty & Quiring, ).…”

“…However, recent studies suggest that some herbivores struggle to exploit the largest, most vigorously growing modules, despite their superior nutritional quality (Flaherty & Quiring, ). Such herbivores may instead favor intermediate‐sized shoots, balancing tradeoffs between utilizing relatively large, nutritious shoots vs. small, more easily exploited shoots (i.e., the ‘optimal module size’ hypothesis, OMSH) (Quiring et al., ; Flaherty & Quiring, ).…”