Host Plant Quality and Local Adaptation Determine the Distribution of a Gall-Forming Herbivore

Abstract

Herein we report results of transplant experiments that link variation in host plant quality to herbivore fitness at the local scale (among adjacent plants) with the process of local (demic) adaptation at the landscape scale to explain the observed distribution of the specialist gall former Belonocnema treatae (Hymenoptera: Cynipidae) within populations of its host plant, Quercus fusiformis. Field surveys show that leaf gall densities vary by orders of magnitude among adjacent trees and that high-gall-density trees are both rare (< 5%) and patchily distributed. B. treatae from each of five high-gall-density trees were reared on (1) the four nearest low-gall-density trees, (2) the four alternative high-gall-density trees, and (3) their natal trees (control). Each treatment (source X rearing site) was replicated three times. Nine components of performance that sequentially contribute to fitness were evaluated with over 21000 galls censused across the 25 experimental trees. When reared on their natal trees and compared with low-gall-density neighbors, transplanted gall formers had higher gall initiation success (P < 0.05), produced more (P < 0.001) and larger galls (P < 0.001), and produced a higher proportion of galls that exceeded the threshold size for natural enemy avoidance (P < 0.05). Comparison of gall-former performance on natal vs. alternative high-gall-density trees demonstrated significant (P < 0.001) differences in six performance measures with five differing in the direction predicted by the hypothesis of local adaptation. Overall, these linked experiments document direct and indirect effects of host plant variation on gall-former performance and demonstrate convincingly that (1) high-gall-density trees equate to high-quality trees that are surrounded by trees of relatively lower quality to the herbivore and (2) gall-former populations have become locally adapted to individual trees.

Leaf-Miners Co-opt Microorganisms to Enhance their Nutritional Environment

“…Decrease in foliar nutritional quality with leaf age due to maturation and senescing processes constrains many phytophagous insects to a single generation in a season (e.g., Feeny 1970;Potter and Kimmerer 1989). Additionally, positive effects of carbohydrate, protein and/or amino acid leaf content on offspring (survival, development time, size) and on adults (realized fecundity) performances as well as behaviors (feeding and oviposition preferences) frequently are observed (Egan and Ott 2007;Scheirs et al 2001;Stiling et al 1999). Variations in foliar nutritional quality ranging from 1.1 to 1.5 fold have been found to be sufficient to significantly affect important offspring performance factors, such as survival and pupal size in the leafminer Chromatomyia milii on creeping velvet grass (Scheirs et al 2001).…”

The cabbage moth or the sorrel moth (Lepidoptera: Noctuidae)?

“…Long-term adaptation represents an important foundation of evolutionary theory in plant-herbivore interactions (Via, 1990). In this scenario, repeated exposure to a plant over the course of many generations physiologically "conditions" a herbivore strain or genotype to that plant species, enabling it to outcompete conspecifi cs which are physiologically naïve to the plant (Witham & Slobodchikoff, 1981;Strauss & Karban, 1994;Egan & Ott, 2007). In our study, the lab strain of M. brassicae developed much faster on R. acestosa without any negative effect on pupal mass, even though it had been continually reared on cabbage for over 20 years and approximately 200 generations (given that it completes ~10 generations a year under lab conditions).…”

Galling Insect Community Associated with Copaifera langsdorffii (Fabaceae): The Role of Inter- and Intra-annual Host Plant Phenology

“…Thus, insects using these resources during unfavorable phenological stages may suffer reduction on their performance (Forister 2005 ). For instance, galling insect emerging after leaf maturation, and even those using late-produced foliage, may have insuffi cient resource to ideal development before leaf abscission (Egan and Ott 2007 ;Hood and Ott 2010 ). Thereby, synchronization of consumers with the resources upon which they depend is of fundamental importance to consumer fi tness.…”

“…However, the nature of this phenological window, being it predictable or variable, broad or narrow, has a large impact on the evolutionary history of plantherbivore interactions (Yukawa 2000 ;Egan and Ott 2007 ). In fact, the selection would favor galling insects that attack early bud-breaking plants if plants present high bud-breaking synchronism (e.g.…”

“…Yukawa and Akimoto 2006 ). Alternatively, asynchronic bud-breaking within a plant population results in temporal isolation between herbivore allowing the formation of demes and sympatric gall speciation (see Mopper 2005 ;Egan and Ott 2007 ).…”