Advantages of a mixed diet: feeding on several foliar age classes increases the performance of a specialist insect herbivore

Abstract

Two field studies were carried out to determine the influence of Abies balsamea foliage age on the preference and performance of larvae of Neodiprion abietis, a specialist Diprionid sawfly. Preference was determined by examining N. abietis defoliation on all age classes of foliage. Performance was estimated using larval survival, cocoon weights and the percentage of adults that were females. Neodiprion abietis preference for, and performance on, current-year foliage was very low, peaked on 2 or 3-year-old foliage, and declined on older foliage. Thus, sawfly feeding preference was adaptive. However, survival and cocoon weight were highest when sawflies were allowed to feed on all age classes of foliage, demonstrating that an insect specialist may perform better when feeding on several age classes of foliage from a single host plant species. These results indicate that either different larval instars have different nutritional requirements, or that food mixing provides the best diet, permitting the herbivore to obtain needed nutrients while avoiding ingestion of toxic doses of secondary metabolites. In addition, our results suggest that limited availability of varied foliage has more negative consequences for N. abietis females than for males, as the percentage of survivors that were females decreased when juvenile mortality was high. Our results emphasize the importance of considering non-linear changes in foliar quality as leaves age on herbivore preference and performance, and demonstrate how a herbivore can use this variability to maximize its fitness.

Influence of pre-commercial thinning of balsam fir on defoliation by the balsam fir sawfly

“…2) are similar to those described in contemporaneous studies carried out in the same region (Moreau et al, 2003;Parsons et al, 2003). Early instar larvae of N. abietis are gregarious and usually initiate feeding on the 1-year-old foliage on which they emerged, but disperse to nearby shoots of similar or older age as they develop (Anstey et al, 2002).…”

Relationship between balsam fir sawfly density and defoliation in balsam fir

“…Other variations in defoliation not attributable to sawfly density may have resulted from differences in the actions of natural enemies (Moreau, 2004), the availability of different age-classes of foliage (Moreau et al, 2003) and foliage quality (e.g., Krause et al, 1993;Wagner and Zhang, 1993;Géri et al, 1993;McMillan and Wagner, 1997;Moreau, 2004).…”

“…Newly emerged larvae feed gregariously but become more solitary in later instars. Larvae feed on all age-classes except current-year foliage, which they only consume occasionally (Moreau et al, 2003;Parsons et al, 2003). At the end of the fourth-instar (males) or fifth-instar (females), larvae spin cocoons and pupate.…”

Effects of invasive seaweeds on feeding preference and performance of a keystone Mediterranean herbivore

“…However, such compensatory feeding of C. racemosa was not enough to provide higher inputs of organic matter, carbon and nitrogen than those provided by the native seagrass, with negative consequences on herbivore performance (e.g., Fink and Von Elert 2006). In the field, where other food is available, diet mixing could be a strategy that generalist herbivores employ to avoid unbalanced nutrition and diminish toxicity effects of a particular species (e.g., Cruz-Rivera and Hay 2000; Moreau et al 2003;Box et al 2009). …”