Rob Johns

Variable Effects of Plant Module Size on Abundance and Performance of Galling Insects

We conducted a review of published literature reporting relationships between the size of plant modules and the abundance or performance of gall insects. Insects in the family Tenthredinidae were recently reviewed and thus omitted from this review. The abundance or performance of approximately half of 53 species examined was positively related to plant module size. However, negative and parabolic relationships were found for all major insect families with sample sizes ≥ 5 (i.e., Adelgidae/Aphididae, Cynipidae and Cecidomyiidae for abundance and Adelgidae/Aphididae for performance). This suggests that relationships between plant module size and the abundance or performance of non-tenthredinid gallers, although often positive, are best characterized as variable.

Sex biased intra‐tree oviposition site selection and larval foraging behavior of a specialist herbivore

Heterogeneity in the quality of oviposition and feeding sites within plants can significantly influence the distribution and abundance of herbivorous insects, but remains poorly understood. Field surveys and a manipulative study were conducted to evaluate the influence of variation within the crown of black spruce, Picea mariana (Mill.) B.S.P. (Pinaceae), on adult oviposition and larval feeding behavior of yellowheaded spruce sawfly, Pikonema alaskensis Rohwer (Hymenoptera: Tenthredinidae). Most eggs were laid in the mid to lower crown of 1.5–2 m tall trees. However, most of the few eggs that were laid in the upper crown (i.e., whorl 2) were female. Fourth and fifth instars dispersed acropetally, from the mid and lower to the upper crown, causing high defoliation in the upper crown. Late‐instar females were generally more abundant than males on the leader, the most apical shoot on a tree where eggs and early instars rarely occurred, strongly suggesting that more females than males disperse acropetally. This hypothesis was supported in a manipulative experiment, where only 15–20% of larvae in all‐male broods, but almost three‐quarters of larvae in mixed broods, dispersed to the upper crown. To our knowledge, this is only the second study to explicitly demonstrate preferential allocation of progeny sex through oviposition site selection by a herbivorous insect, and the first study to unambiguously demonstrate sex‐biased dispersal by the juveniles of an insect whose adult females can fly. This study emphasizes the important role of intra‐plant variation in shaping both oviposition site selection and the dispersal behavior of juvenile phytophagous insects within their hosts, and suggests that sex‐biased foraging behaviors may be necessary for some insects to accommodate the respective needs of immature females and males within heterogeneous host plants.

Foliage-age mixing within balsam fir increases the fitness of a generalist caterpillar.

1. Manipulative field studies were carried out to evaluate the foliage age preference–performance relationship for an extreme generalist herbivore, the whitemarked tussock moth (Orygia leucostigma Smith) (Lepidoptera: Lymantriidae), within balsam fir [Abies balsamea (L.) Mill].

Spatial heterogeneity within an evergreen conifer promotes foliage-age dietary mixing by a specialist herbivore

Dietary mixing of different food items enhances the performance of many animals; however, few studies have tested the mechanisms influencing mixing within individual plants by herbivores. We investigated the mechanisms promoting apparent foliage-age dietary mixing by a specialist sawfly, Pikonema alaskensis (Roh.) (Hymenoptera: Tenthredinidae), within juvenile black spruce (Picea mariana (Mill.) B.S.P.). Field defoliation surveys indicated that larvae generally prefer developing foliage, and most defoliation occurs in the upper crown of trees. However, when upper-crown developing foliage becomes scarce, many larvae consume mature foliage in the upper crown rather than dispersing to lower branches, where preferred developing foliage remains abundant. Manipulative field experiments were conducted to compare the performance of early and late-instar larvae fed mixed versus individual age classes (i.e. developing or mature) of foliage in the upper and lower crown. In contrast to all past studies, foliage-age mixing by P. alaskensis was not associated with nutrient balancing or toxin dilution. Instead, mixing was associated with an expansion of feeding breadth in larvae as they developed, coupled with high levels of variation in resource quality between upper and lower crown levels, which enabled older larvae consuming mature foliage in the upper crown to achieve similar survival as those consuming developing foliage in the lower crown. Our study emphasizes the importance of considering spatial heterogeneity in studies of foliage-age preferenceeperformance relationships for herbivores within trees and demonstrates how a herbivore can use dietary mixing to dramatically expand its resources while simultaneously accounting for heterogeneity within large, architecturally complex plants. Crown Copyright 2010. Published on behalf of The Association for the Study of Animal Behaviour by Elsevier Ltd. All rights reserved.

Intratree Variation in the Seasonal Distribution and Mortality of Spruce Budworm (Lepidoptera: Tortricidae) from the Peak to Collapse of an Outbreak

ABSTRACT A 5-yr field study was carried out to assess intratree variations in the distribution, abundance, and mortality of immature spruce budworm (Choristoneura fumiferana Clemens) (Lepidoptera: Tortricidae) from the peak to collapse of an outbreak. In most years, the highest density of overwintering second-instar (L2) larvae (per square meter of foliage) was located in the lower crown, whereas all subsequent stages (third- to sixth-instar larvae, pupae, and eggs) were at relatively higher densities in the upper crown. In contrast, overall abundance (per branch) throughout the season tended to be highest in the mid-upper to mid-lower crown. Mortality associated with 16 different parasitoid species varied significantly among years but varied among crown levels for only a few species. In particular, Apanteles fumiferanae (Viereck), Glypta fumiferanae (Viereck), Smidtia fumiferanae (Tothill), and Trichogramma minutum (Riley) all caused higher mortality in the upper crown of trees. Although infection associated with Nosema fumiferanae (Thomson) and mortality associated with fungal and viral pathogens often varied among crown levels, there was no clear or consistent trend from year to year. In general, trends in spruce budworm density and mortality within the crown were similar throughout all years of our study, despite significant variations in herbivore density, foliage availability, and parasitoid and pathogen impact. Our study indicates that intratree patterns of spruce budworm distribution and mortality are likely to remain consistent during an outbreak and further emphasizes the importance of intratree heterogeneity in shaping interactions within plant-herbivore-parasitoid communities.

Paradigms in Eastern Spruce Budworm (Lepidoptera: Tortricidae) Population Ecology: A Century of Debate

Abstract Three main hypotheses have been postulated over the past century to explain the outbreaking population dynamics of eastern spruce budworm, Choristoneura fumiferana (Clemens). The Silviculture Hypothesis first arose in the 1920s, with the idea that outbreaks were driven by forestry practices favoring susceptible softwood species. In the 1960s, it was proposed that populations were governed by Multiple Equilibria, with warm weather conditions releasing low-density populations from the regulatory control of natural enemies. Dispersal from outbreak foci, or “epicenters,” was seen as causing widespread outbreaks that eventually collapsed following resource depletion. However, in the 1980s, following the re-analysis of data from the 1940s outbreak in New Brunswick, this interpretation was challenged. The alternative Oscillatory Hypothesis proposed that budworm population dynamics were governed by a second-order density-dependent process, with oscillations being driven by natural enemy–victim interactions. Under this hypothesis, weather and resource availability contribute to secondary fluctuations around the main oscillation, and weather and moth dispersal serve to synchronize population cycles regionally. Intensive, independent population studies during the peak and declining phases of the 1980s outbreak supported the principal tenet of the Oscillatory Hypothesis, but concluded that host plant quality played a more important role than this hypothesis proposed. More recent research on the early phase of spruce budworm cycles suggests that mate-finding and natural-enemy-driven Allee effects in low-density populations might be overcome by immigration of moths, which can facilitate the onset of outbreaks. Even more recent research has supported components of all three hypotheses attempting to explain spruce budworm dynamics. In the midst of a new rising outbreak (2006-present), we discuss the evolution of debates surrounding these hypotheses from a historic perspective, examine gaps in current knowledge, and suggest avenues for future research (e.g., intensive studies on low-density populations) to better understand and manage spruce budworm populations.

Dietary mixing within the crown of a deciduous conifer enhances the fitness of a specialist sawfly

Dietary mixing is an important adaptation used by many animals to account for nutritional heterogeneity in their environment, but it remains largely unstudied for specialist insects within crowns of deciduous hosts. We carried out field and laboratory studies to explain dietary mixing by larvae of the larch sawfly, Pristiphora erichsonii (Hartig) (Hymenoptera: Tenthredinidae) of dimorphic foliage types (i.e. long shoots versus fascicles) within a deciduous conifer, Larix kaempferi (Lamb.) Carr. First-instar larvae fed mainly on long shoots, and late instars fed mainly on fascicles; however, during all instars, some individuals could be observed feeding on long shoots or fascicles. A mixed diet yielded higher overall survival due to benefits accrued during both early and late instars. Late instars had similar prepupal mass and developmental rate when provided a mixed or fascicle-only diet, but they suffered significant reductions in both performance parameters when forced to feed exclusively on long shoots. Overall fitness of females was 15–22% higher for larvae provided a mixed diet than for larvae provided one foliage type. Our results support the ‘ontogeny’ hypothesis, which attributes dietary mixing to changes in nutritional needs of larvae over the course of development, as well as the ‘complementary diet’ hypothesis, which contends that mixing of different foliage types is associated with nutrient balancing or toxin dilution. Our results suggest that architectural patterns of growth and nutritional quality within crowns of deciduous conifers exert selective pressure similar to that of evergreens on insect herbivores to diet-mix during juvenile development.

Evidence for a substantial host-use bottleneck following the invasion of an exotic, polyphagous weevil

1. The successful establishment of novel plant–insect interactions may depend on the availability of suitable hosts, which itself is influenced by the inherent flexibility of the herbivore for the native plants in its new range. The polyphagous beech leaf mining weevil, Orchestes fagi L., is a recent invader to eastern Canada, and while beech is a primary host, it remains unclear the extent to which it might also utilise co‐occurring secondary hosts, as has been observed in its native European range.

Temporal and spatial variations in microclimate influence the larval foraging behaviors and performance of a conifer-feeding sawfly.

ABSTRACT Herbivorous insects are often exposed to broad temporal and spatial variations in microclimate conditions within their host plants and have adapted a variety of behaviors, such as avoidance or basking, to either offset or benefit from such variation. Field experiments were carried out to investigate the influence of daily and intratree variations in microclimate on the behaviors (feeding, resting, dispersal, and hiding) and associated performance of late-instar larvae of the yellowheaded spruce sawfly, Pikonema alaskensis (Rohwer) (Hymenoptera: Tenthredinidae) within crowns of 1.25–1.5 m tall black spruce (Picea mariana [Miller] Britton Sterns Poggenburg); late instars feed on developing shoots of young spruce and are often exposed to microclimatic extremes with unknown effects on performance. Larvae fed diurnally from just after dawn (0800 h) until dusk (2000 h) and rested throughout the night, with brief periods of dispersal occurring in the morning and evening. Neither larval behavior nor abiotic conditions differed significantly between the upper and lower crowns of trees. Temperature, humidity, and solar insolation all explained >90% of variation in feeding; however, sunrise and sunset were the most likely cues influencing diurnal behavior. Most larvae (94%) fed on the bottom, shaded side of shoots, and field experiments indicated that this behavior is adaptive with respect to microclimate, probably reducing hygrothermal stress. Thus, behavioral adaptations by P. alaskensis to daily and within-shoot microclimatic variation may reduce the risk of hygrothermal stress during dispersal or feeding, while still allowing larvae to feed on the preferred and highly nutritious upper crown foliage of young spruce.

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

Field surveys were carried out to assess the effects of intra‐tree variation in developing shoot length within and among crown levels on the density and abundance of the balsam shoot‐boring sawfly, Pleroneura brunneicornis Rohwer (Hymenoptera: Xyelidae), in young balsam fir, Abies balsamea (L.) Mill. (Pinaceae). Overall, cardinal direction had no influence on shoot‐borer density or abundance; however, the highest percentage and abundance of bored shoots occurred on intermediate‐sized shoots within the crown (i.e., in the mid‐crown and on the distal‐lateral and medial‐lateral shoots). Comparatively, few shoot borers occurred in the upper or lower crown levels, or on the relatively large terminal shoots within branches. This distribution appears indicative of the higher suitability of intermediate‐sized shoots within hosts for either egg lay or larval performance. Results of this study are most consistent with predictions of the ‘optimal module size’ hypothesis, which posits that herbivore responses to plant module size should reflect the balance of tradeoffs between utilizing relatively large, nutritious shoots vs. small, more easily exploited shoots.