Michel Saint-Germain

Primary attraction and random landing in host-selection by wood-feeding insects: a matter of scale?

1 Most plant‐feeding insects show some degree of specialization and use a variety of cues to locate their host. Two main mechanisms of host location, primary attraction and random landing, have been investigated for such insects.

Landscape-Scale Habitat Selection Patterns of Monochamus scutellatus (Coleoptera: Cerambycidae) in a Recently Burned Black Spruce Forest

Abstract The host selection process of most phytophagous insects can be described as a sequence of behaviors leading from landscape-scale habitat location to host-plant scale, microsite selection. For the whitespotted sawyer, Monochamus scutellatus (Say), a fire-associated xylophagous cerambycid, host location and acceptance patterns have been relatively well described, whereas landscape-scale distribution patterns in recently disturbed areas have received virtually no attention. In a 5,097-ha recently burned black spruce forest of Quebec, Canada, we evaluated the variability of larval density of 569 trees in 114 plots, by using entry hole counts. This variability was then related to environmental variables ranging from tree- to landscape-scale. Both diameter at breast height (positive relationship) and fire severity (negative relationship) were significant at explaining larval density at tree scale. At larger scales, altitude had a negative effect on larval density, whereas plots having a higher percentage of unburned forest in a 500-m radius were more intensely colonized. The importance of the proximity of unburned stands could be linked to the feeding requirements of the adults, which should show preference for stands offering both egg-laying and feeding substrata, because several species of Monochamus have been shown to feed while being reproductively active. In our models, large-scale variables explained more variability in entry hole counts than did tree-scale variables. Thus, our results suggest that large-scale habitat location mechanisms may play an important role in the host selection process of the whitespotted sawyer. RÉSUMÉ (FRENCH). La sélection d’hôte des insectes phytophages comporte plusieurs étapes allant de la localisation d’un habitat à l’échelle du paysage jusqu’à la sélection d’un microsite de ponte à l’échelle de la plante hôte. La localisation et l’acceptation d’hôte ont été relativement bien décrites pour le longicorne noir Monochamus scutellatus (Say); cependant, la localisation d’habitat à grande échelle a retenu peu d’attention. Dans un feu ayant eu lieu en pessière noire au Canada, nous avons évalué la densité de larves dans 569 arbres de 114 parcelles d’échantillonnage par décompte de trous d’entrée. Ces densités ont été confrontées à des variables environnementales à échelles multiples. À l’échelle de l’arbre, la densité de larves était liée au diamètre de l’arbre et à la sévérité du feu. À plus grande échelle, l’altitude a eu un effet négatif sur la densité de larves, alors que les peuplements ayant davantage de forêt verte dans un rayon de 500 mètres ont été plus intensément colonisés. L’importance de la proximité de massifs verts peut être liée aux éxigences nutritionelles des adultes, qui devraient préférenciellement coloniser des peuplements comportant à la fois des sites d’oviposition et d’alimentation, puisque plusieurs espèces de Monochamus continuent de s’alimenter lorsque sexuellement actifs. Les variables à grande échelle ont expliqué plus de variabilité dans nos modèles que les variables à l’échelle de l’arbre. Il semble donc que les mécanismes de localisation d’habitat à grande échelle jouent un rôle important dans le processus de sélection d’hôte chez le longicorne noir.

Sampling Saproxylic Coleoptera: Scale Issues and the Importance of Behavior

Abstract Some currently used tree-scale sampling techniques targeting saproxylic insects capture individuals that are attracted to or landing on specific potential hosts. The success of such techniques is entirely dependent on strong primary attraction in targeted insects. However, up to this point, field experiments testing the primary attraction hypothesis have produced contradictory results. To test the efficiency of such techniques, and consequently, the strength of primary attraction for saproxylic Coleoptera, we sampled insects landing on contrasting snag types including new and old snags of five different tree species using sticky traps in a single mixed 135-yr-old boreal stand in Western Quebec, Canada. Ordination analyses showed homogenous assemblages among the different snag types and stovepipe controls, when considering either all species captured or only targeted functional groups, and very few species showed strong affinities to specific snag types. Species composition of assemblages was in several cases correlated with the species and status of trees neighboring the sampling units, which suggest that prelanding host selection mechanisms do not allow insects to single out a potential host while in flight. Our results suggest that primary attraction may play a role at larger spatial scales and help insects identify potential habitat patches, while selection of a single host at the local scale is done by trial-and-error through random landing. In such a context, future studies aiming at describing precise host-use patterns of saproxylic insects should rely on methods targeting larvae or emerging adults such as wood dissection and rearing.

Effect of fire severity on long-term occupancy of burned boreal conifer forests by saproxylic insects and wood-foraging birds

Fire severity can vary greatly within and among burns, even in the Canadian boreal forest where fire regimes consist mostly of stand-replacing fires. We investigated the effects of fire severity on the long-term occupancy of burns by (i) saproxylic insects and (ii) three wood-foraging birds. Based on observations made 6 to 11 years after fire in burned conifer forests that varied in fire severity in Quebec, Canada, our results indicate that low-severity portions of the burns likely provided snag conditions suitable for the long-term presence of deadwood-associated insects and birds. The black-backed woodpecker, a post-fire forest specialist, was still abundant 6 and 8 years after fire. This pattern was likely explained by the persistence of several saproxylic insect species that are associated with recently dead trees and by the positive effect of lower fire severity on the abundance of Arhopalus foveicollis, a cerambycid with a long life cycle in dead wood. The American three-toed woodpecker and the brown creeper, and their associated prey (Scolytinae beetles), were more abundant in burned stands of lower v. higher severity. We conclude that less severely burned snags and stands within high-severity burns may favour the long-term presence of trophic webs that involve saproxylic insects and wood-foraging birds in burned boreal forests.

Occurrence patterns of aspen‐feeding wood‐borers (Coleoptera: Cerambycidae) along the wood decay gradient: active selection for specific host types or neutral mechanisms?

Abstract 1. Determinants of host‐use patterns in plant‐feeding insects have been extensively studied, usually within the framework of optimality theory. Comparatively, factors driving host selection in saprophagous insects have received little attention.

Substrate selection by saprophagous wood-borer larvae within highly variable hosts

In insects completing their larval development within a single host, oviposition site is seen as a major determinant of offspring performance. However, in previous studies, the saprophagous wood‐borer Anthophylax attenuatus (Haldeman) (Coleoptera: Cerambycidae) showed no strong response to between‐host variations in nutritional factors influencing larval growth and survival. To explain such weak selection in adults, we hypothesized that substrate selection occurs at a smaller scale by larvae within hosts showing high variability in substrate quality. In this study, we described within‐host variability in wood density and determined whether wood‐boring larvae were found more often than expected in specific decay types. We characterized the variability of decay in 24 snags by producing wood density profiles for each. We then collected larvae from the same snags through wood dissection, and associated a wood density value to each by taking a wood sample around each larva found. We then compared ratios of available and used substrate types defined by wood density. We observed substantial within‐snag variation in wood density. Middle decay class (0.275–0.375 g cm−3) was significantly overused by larvae, whereas more decayed wood was clearly avoided. High within‐host variability in substrate quality and active or passive selection by larvae of specific substrate types suggest that selection pressures on adult behaviour could be lower than expected for a parasitic species, and might be linked with the weak selection observed at a larger scale by ovipositing adults.

Activity of flying beetles (Coleoptera) at two heights in canopy gaps and intact forests in a hardwood forest in Quebec

Abstract We studied the effects of forest height and forest gap on assemblages of flying beetles in an American beech (Fagus grandifolia Ehrh. (Fagaceae) — sugar maple (Acer saccharum Marsh. (Aceraceae)) forest in Quebec. From June until August of 2005, beetles were collected in Lindgren funnel traps placed in the canopy (20–25 m height) and upper understorey (3–5 m height) in proximity to five forest gaps (15–30 m in diameter) (at the edge of the forest opening or within the closed-canopy forest). We collected 1852 beetles representing 38 families and 172 species. Based on rarefaction curves, species richness was significantly higher in the canopy than in the upper understorey. Nonmetric multidimensional scaling ordination revealed a change in species composition in relation to vertical stratification but not to the forest gaps. Our findings confirmed the importance of the vertical forest gradient to overall diversity of forest coleopterans.

Saproxylic beetle tolerance to habitat fragmentation induced by salvage logging in a boreal mixed-cover burn

Saproxylic insect assemblages associated with burned forests are generally abundant and species rich, consisting of a mix of pyrophilous and secondary, opportunistic species depending on time elapsed since disturbance. Life‐history traits associated with each group suggest that they may respond differentially to habitat fragmentation caused by salvage logging, with pyrophilous species having a much higher dispersal potential. In a 2‐year‐old burn highly fragmented by pre‐ and post‐fire logging, we sampled saproxylic beetles in coniferous and broadleaf burned residual stands along a gradient of spatial context including intensity of fragmentation and isolation from source habitat using Lindgren multiple‐funnels traps. Beetle assemblages differed in composition between coniferous and broadleaf burned stands, with secondary users dominating the latter. Pyrophilous species increased in abundance with distance from the edge and avoided unburned patches within the fire. Secondary users did not respond negatively to fragmentation or isolation of burned habitats, with one exception, the alleculid Isomira quadristriata (Couper), being overall diverse and abundant throughout the study area regardless of salvage logging prevalence. No deleterious effects of isolation were thus detected in the occurrence patterns of secondary users, even up to 8 km from the edge. Our results suggest that older burns, especially those having some broadleaf cover, are intensively used by non‐pyrophilous saproxylic species usually associated with dead wood in green forests and may contribute to maintain broader saproxylic assemblages than originally thought, especially when considering the importance of dead wood volume pulses associated with fire in boreal forests.

Landing Patterns of Phloem- and Wood-feeding Coleoptera on Black Spruce of Different Physiological and Decay States

ABSTRACT We examined landing patterns of phloeophagous and xylophagous Coleoptera among trees and snags of different physiological and decay states in a pure open-canopy black spruce stand in boreal Canada to study prelanding host selection mechanisms in the absence of nonhost volatiles. Sticky traps were used to capture insects landing on high- and low-density natural snags (i.e., wood density), girdled trees, living trees, and stovepipe controls. Patterns were generally weak, with high within-group variability in species composition and landing rates. Within-group variability differed between groups, with highest variations in living trees and recent snags. Despite this evidence of frequent landing on suboptimal or inappropriate hosts, affinities were detected in most common taxa. Cerambycidae showed preferences for girdled trees. Common species of Scolytinae showed divergent preferences, because Crypturgus borealis Swaine and Dryocoetes autographus (Ratzeburg) were captured more often on high-density natural snags, Polygraphus rufipennis (Kirby) on girdled trees, and Orthotomicus latidens (LeConte) on living trees. These observed landing patterns are broadly consistent with current knowledge on the ecology of these species. Although preferences, and thus prelanding assessment of hosts based on volatiles, were detected in several species, the numerous landings observed on inappropriate hosts suggest that random landing at close range may be as common in pure stands as what was previously observed in mixed stands.