A critique of silvicultural approaches to managing defoliating insects in North America

1 A variety of silvicultural techniques have been suggested for managing forest defoliating insects. The objectives focus on minimizing defoliation or minimizing damage from defoliation. 2 The theoretical foundations of many approaches have been built upon observation and correlation, and very little reliable empirical evidence exists to support the objectives of silvicultural manipulations. Existing experimental data have yielded inconsistent results. 3 We review the conceptual framework and underlying assumptions of the major silvicultural approaches recommended or in use in North America. 4 Well‐designed, long‐term studies are needed to clarify the effect of silviculture on defoliators and their effect on forests.     

Oviposition behavior in spruce budwormChoristoneura fumiferana (Clem.) (Lepidoptera: Tortricidae)

Epicuticular waxes of balsam fir, Abies balsamea,act as oviposition stimulants for fertilized females of the eastern spruce budworm, Choristoneura fumiferana(Clem.). Those same waxes do not stimulate unfertilized females, nor do the latter respond to nonchemical cues such as leaf texture, shape, etc. Ablation experiments suggest that the chemoreceptors responsible for the perception of waxes are located on the proboscis.     

Dendroentomological and forest management implications in the Interior Douglas-fir zone of British Columbia, Canada

Douglas-fir (Pseudotsuga menziesii var. glauca Mirb. Franco) forests in the Interior of British Columbia, Canada, show periodic defoliation due to western spruce budworm (WSB) (Choristoneura occidentalis Freeman) outbreaks. Tree defoliation causes a reduction in radial growth and is therefore visible in tree rings. In this paper, we identify WSB defoliation history, and critically examine the potential for using dendrochronological analysis by comparing tree-ring estimates with insect surveys. WSB defoliation history was investigated using cores from Douglas-fir growing in the Lac du Bois region of the Kamloops Forest District. Years with an abrupt decrease in radial growth were considered as negative pointer years that potentially reflected WSB outbreaks. The comparison with ponderosa pine (Pinus ponderosa Dougl., ex P. & C. Laws.) (a non-host species) permitted differentiation between growth reductions in Douglas-fir due to climatic effects and those due to defoliation by WSB. The dendrochronological data were matched with information reporting visible damage in Forest Insect Disease Survey (FIDS) and British Columbia Ministry of Forest records. Our objective-based method using ring-width measurements from host and non-host chronologies was compared with qualitative techniques based on the software program OUTBREAK. We were able to distinguish seven distinct outbreak events in 300 years of record.     

Disruption of male spruce budworm orientation to female moths by pheromone and pheromone analogues in a wind tunnel

Abstract. The ability of male spruce budworm moths, Choristoneura fumiferana (Clem.), to orientate to and locate virgin female moths was investigated in a laboratory wind tunnel. When presented with two caged ‘target female’ moths, more than 99% of the male moths locked on and flew directly to the females. When confined for 3 h before release, downwind of a three-dimensional array of thirty-five evenly distributed female moths (a density of 40/m³), over 90% of the males still successfully oriented to one or another of the females, although not always to the target females. With an array of thirty-five septa containing either E-11-tetradecenyl acetate on E-11-tetradecenol there was no visible change in behaviour; >90% of the males flew directly to the two target females, which indicates that neither the acetate nor the alcohol has potential for mating disruption. An array of thirty-five septa containing a 955 blend of E- and Z-11-tetradecenal, the main components of the spruce budworm sex pheromone, significantly reduced the numbers of male moths able to locate the target females, and effects were concentration-dependent. The highest concentration, which gave a release rate similar to that of a calling female, produced the same level of disruption as the thirty-five females, but sensory fatigue became a factor in addition to ‘false trail following’. Lower concentrations were significantly less effective, with no evidence of sensory fatigue. At all concentrations significant numbers of males flew to the septa (false trail following), but when male moths were left uncaged in the tunnel for a further 3h, most eventually located the females. This suggests that false trail following alone will not prevent mating. The level of disruption was not improved by the addition of 2% tetradecanal to the 955 blend. Similar levels of disruption to the 955 blend were obtained with an 80:20 blend. A 5050 blend still caused a significant amount of false trail following, but less than the other blends. The ability of male spruce budworm to orientate to off-blend trails was enhanced if the males were first exposed to the natural, female-produced pheromone.     

The influence of understory nectar resources on parasitism of the spruce budworm Choristoneura fumiferana in the field

1 Nectar provided by flowering plants has been suggested as an important factor enhancing parasitism rates of herbivorous insects. Artificial nectar has been shown to increase parasitoid longevity and fecundity in laboratory studies. 2 We studied the influence of understory nectar on parasitism of the spruce budworm Choristoneura fumiferana in a field experiment in which we either removed understory flowering herbs or sprayed a sucrose solution on the understory vegetation in 0.25 ha quadrats. 3 Cohorts of laboratory-reared second instars were transferred the field to evaluate parasitism in the quadrats. The larvae were harvested as fifth instars and reared for parasitoids. 4 Parasitoids of early instars caused greater mortality in the supplemental sucrose treatment than in the vegetation-removal treatment or the control. Parasitoids of older larvae did not respond to the treatments. Larger-scale treatments may be necessary to elicit a response by the larger parasitoids that attack older larvae.     

Flight speed of male spruce bud worm moths in a wind tunnel at different wind speeds and at different distances from a pheromone source

ABSTRACT. Flying male spruce budworm ( Choristoneura fumiferana [Clem.]) moths responding to virgin females and to synthetic pheromone in a wind tunnel maintained a constant rate of upwind progress when held by moving optomotor cues at a constant distance from the pheromone source. When allowed to progress upwind to the source, however, they slowed their upwind speed progressively as they approached it. They also adjusted their flight speed to maintain similar rates of upwind progress at different wind speeds.     

Influence of epicuticular waxes from white spruce and balsam fir on feeding behavior of the eastern spruce budworm

Fourth-instar larvae of the eastern spruce bud-worm Choristoneura fumiferanaClem. (Lepidoptera: Tortricidae) were shown in two-choice feeding tests to respond differently to epicuticular waxes from different white spruce and balsam fir trees. Larvae also showed different preferences for various wax fractions obtained by separation on silicic acid.Deceased.     

Characterization of DNA from three nuclear polyhedrosis viruses pathogenic for Choristoneura sp

Three nuclear polyhedrosis viruses isolated from larvae of the insect genus Choristoneura showed polyhedrins of 28–30,000 daltons, genome sizes of 78–82 × 10⁶ daltons, and guanine plus cytosine contents of 47.9–49.4%. It was demonstrated by comparison of restriction endonuclease fragment patterns that two of the viruses are closely related genetically.     

Toxicity of Bacillus thuringiensis entomocidal protein toward cultured insect tissue

The entomocidal protein from crystalline inclusion bodies of Bacillus thuringiensis can be bioassayed in vitro using cultured insect tissue. Larval cells of the spruce budworm, Choristoneura fumiferana, are damaged by enzyme-digested (activated) protein isolated from B. thuringiensis crystals. Measurement of toxicity is accomplished by detection of adenosine triphosphate (ATP) in treated cultures using firefly bioluminescence. The ATP content of toxin-treated tissue is inversely proportional to the amount of toxin added. Tissue cells from the spruce budworm exhibited maximum susceptibility to activated δ-endotoxin after 120 hr incubation. Probit analysis of tissue ATP response to toxin dose indicated 50% of the cells were damaged by 14.6 μg or less of toxin protein per 2 × 10⁵ insect tissue cells. Activated δ-endotoxin was entomocidal to insects as well, as detemined by mortality studies with second-instar larvae of the European corn borer. Electron microscopic observations of insect tissue treated with activated δ-endotoxin protein for 60 min revealed massive outer membrane disruption and subsequent loss of cytoplasmic constituents, accompanied by swelling of the nuclear membrane.     

A bioassay technique for Entomophthora sphaerosperma on the spruce budworm,Choristoneura fumiferana

A technique for conducting bioassays of Entomophthora sphaerosperma on sixth-instar larvae of the spruce budworm, Choristoneura fumiferana, was developed. Four assays were conducted by showering conidia on 10 larvae for each of 10 to 20 doses per assay. Dose was estimated by averaging estimates of the concentration of spores falling on water agar dishes before and after insect exposure. Maximum-likelihood probit analysis indicated significant regressions between log dose and probit mortality for all four assays. LC₅₀ values ranged from 11.21 to 18.77 spores/mm² with a weighted mean of 16.13 spores/mm². Slope estimates ranged from 0.92 to 1.87 with a weighted mean of 1.13. These low slope values may have been indicative of a highly variable test insect population, but also suggested a nontoxic infection process by the pathogen.     

Changes in the forest floor of Canadian southern boreal forest after disturbance

Abstract. The concentrations and contents of organic matter and nutrients in organic deposits on the forest floor were estimated along a 231-yr chronosequence following fire at the southern limit of the boreal forest in eastern Canada. The sampling design was stratified to take into account the variability related to the presence of the principal tree species as well as to the presence of large gaps created by a recent spruce budworm (Choristoneura fumiferana) outbreak. The forest floor showed a steady accumulation of organic matter and total nutrients with time-since-fire and a 50 % decrease in the concentrations of available P and K, but not N (as determined by aerobic incubation). The increase in forest-floor weight was accompanied by an increased storage of available N, Ca and Mg. The availability of N and Ca was more strongly affected by tree species and gaps than by time-since-fire. A high N-availability was observed under Betula papyrifera and in gaps, while high a Ca-availability was found near Populus tremuloides and Thuja occidentalis. In old sites, the forest floor of gaps, created by a recent spruce budworm outbreak, had a necromass similar to that of a young forest, but the low concentrations of available P and K of an old forest.     

RearingGlypta fumiferanae [Hym.: Ichneumonidae] on a multivoltine laboratory colony of the western spruce budworm (Choristoneura occidentilis) [Lep.: Tortricidae]

Methods were developed for rearingGlypta fumiferanae Viereck on a nondiapausing laboratory colony of the western spruce budworm,Choristoneura occidentalis Freeman. Both host and parasite are univoltine and undergo diapause in nature. In this study, the parasite's voltinism was synchronized with that of a nondiapausing host.G. fumiferanae mated readily in the laboratory, and 5 consecutive generations were reared with an average generation time of less than 8 weeks-much less than the 23 weeks needed for 1 generation to develop in the field. Developmental times are reported, and some aspects of behavior described.     

Oral exudate as a mediator of behavior in larval eastern and western spruce budworms (Lepidoptera: Tortricidae)

The production of oral exudate by larval eastern and western spruce budworms,Choristoneura fumiferana (Clem.) andChoristoneura occidentalis Free., respectively, was investigated in the laboratory. All larvae except those entering into a molt exhibited aggressive behavior and produced exudate in response to handling or intraspecific encounters. Larvae could be induced to produce exudate up to four times over 2–3 min and produced an average of 1.92±0.04 µl (X ± SE) per induction. Larvae on foliage spent much of their time maintaining their silken feeding tunnel, including spinning and combing silk and removing frass. Exposure to conspecific oral exudate deposited inside the tunnel, or released by agitated larvae inside the tunnel, increased the proportion of larvae that dispersed away from the tunnels and, apparently, increased the larval sensitivity to disturbances. The behavior induced by the oral exudate indicates that it acts as an epideictic (spacing) pheromone.