Chromosomal proteins from the spruce budworm Choristoneura fumiferana

The electrophoretic properties of histones and non-histone chromosomal proteins (NHCP) were examined in second instar larvae of the spruce budworm, Choristoneura fumiferana. The five main histone fractions, typical of most organisms, were present. Subfractions were not observed for the very-lysine-rich F1 histone. By contrast to the histones, the NHCP displayed considerable heterogeneity with no fewer than 38 bands. The molecular weights of the NHCP ranged from 9000 to over 110,000 daltons.     

Risk of dispersal in western spruce budworm

1 Western spruce budworm Choristoneura occidentalis Free. larvae emerge in the spring before buds have expanded and spend a variable period of time foraging on branches and mining needles.
2 Losses of dispersing budworms during this needle-mining period are related directly to the severity of defoliation in previous years and inversely related to foliage biomass in the study plot and to temperature and rainfall during the needle-mining period.
3 Losses can be interpreted in terms of risk of dispersal, which is the product of the propensity of early-stage budworms to disperse in search of resources and the consequences of this behaviour for survival under variable ecological conditions.
4 A comparison of the species-specific nature of risk of dispersal in three conifer-feeding budworm systems of North America may elucidate the common nature but variable features of their respective population dynamics.     

Spatial scale and the detection of density dependence in spruce budworm outbreaks in eastern North America

Using two tests for direct density dependence and standard techniques of time series analysis, we identified density dependence in defoliation time series of the spruce budworm across its outbreak range in eastern North America over the years 1945–1988. We carried out analyses for the entire region and for grid cells of defoliation maps at five spatial scales created by aggregating the smallest grid cells. The rate of detection of direct density dependence, as assessed by two previously published methods, decreased with increasing spatial scale. Using both methods, density dependence was detected more frequently at the periphery of the outbreak range, where defoliation rate was lower. This result suggested that density-dependent regulation may be stronger in those areas. The first order autoregressive process was the basic model for defoliation dynamics overall and the most common model across spatial scales. Second-order processes were encountered much less frequently, and those commonly identified as resulting from delayed density dependence generally occurred across spatial scales at a rate expected by chance alone. Our results were similar to those of other published studies, which have found the detection of density dependence to decrease at larger spatial scales. The results also reinforced the importance of considering spatial scale when diagnosing population processes using time series of abundance for single species. Received: 26 December 1999 / Accepted: 17 March 2000     

Factors affecting oral regurgitation by larval spruce budworm

This study evaluated factors that influence the regurgitation behaviour of sixth instar spruce budworm, Choristoneura fumiferana Clemens (Lepidoptera: Tortricidae), reared on balsam fir, Abies balsamea (L.) Mill. (Pinaceae), under various experimental conditions in the laboratory. Upon physical disturbance, larvae discharged a median volume of regurgitant of 0.4 μl when fed and 1.6 μl when food‐deprived. Larvae deprived of food for 24 or 48 h disgorged more regurgitant than larvae feeding on balsam fir foliage, and the effect was consistent for laboratory‐reared and field‐collected larvae. The water content of the foliage fed upon by larvae had no immediate impact on the volume of regurgitant; following a 24‐h period of food deprivation, however, larvae that previously fed on fresh foliage discharged >2.5 times more regurgitant than larvae that previously fed on dry foliage. Self‐regulated regurgitation by larvae, measured using the number of regurgitant stains on filter paper, was >10 times higher when larvae had access to balsam fir foliage than when they were starved. The number of larvae confined inside the Petri dish (one or four individuals) had a relatively small effect on regurgitation. Larvae were deterred from feeding when balsam fir needles were entirely covered with regurgitant, but not when only a portion of the foliage was treated. These results suggest that the regurgitant does not serve as resource marking or spacing pheromone. The high level of regurgitation by larvae after contact with ants suggests that the regurgitant has evolved in part as a defence mechanism against natural enemies.     

Rate of pheromone release by individual spruce budworm moths

The rate and temporal pattern of pheromone emission by single and grouped female spruce budworm moths were measured by combining the trapping of pheromone on Porapak Q with a rapid luciferase bioassay developed for aldehyde pheromone. Pheromone release occurred mainly during the scotophase of a 12:12 L:D cycle in a series of bursts (up to 50 ng/hr) with considerable variability observed between insects. Analysis of over 30 individual females showed that 30% release no detectable pheromone, 60% release between 20 and 100 ng of pheromone and 10% release greater than 100 ng in a 24-hr period. Overall, a single female (1–3 days old) released an average of 60 ± 50 ng (± S.D.) of pheromone per night with a total of 260 ± 210 ng (± S.D.) being released over its life span (～7 days). Grouped females released lower quantities of pheromone. The amount of pheromone in the glands of female moths also displayed a rhythm with the levels beings higher later in the day than at the start of the photophase.     

Pattern analysis of eastern spruce budworm Choristoneura fumiferana dispersal

Dispersal has been proposed as an important mechanism in the broad‐scale synchronisation of insect outbreaks by linking spatially disjunct populations. Evidence suggests that dispersal is influenced by landscape structure, phenology, temperature, and air currents; however, the details remain unclear due to the difficulty of quantifying dispersal. In this study, we used data on the abundance and distribution of spruce budworm Choristoneura fumiferana larvae (potential dispersers) and adult male moths (dispersers) to make inference on the effects of air currents and host‐species abundance on dispersal. Hierarchical‐Bayesian and inverse modeling was used to explore 4 dispersal models: 1) isotropic dispersal; 2) directional‐dispersal; 3) directional‐and‐host‐species dispersal; and 4) host‐species dispersal. Despite their strong dependence on balsam fir Abies balsamea and spruce species Picea spp., the mapped basal area of these host species did not influence the pattern of dispersed moths. The model that best fit the data was the directional‐dispersal model, which showed that the prevailing dispersal direction was from the northwest (328°). We infer that the strong pattern of directional dispersal was due to a prevailing wind from the same direction. Our interpretation was corroborated by independent wind data during the period of active adult male budworm flight, particularly in the region with high larval abundance. Our results indicate that there was a relatively high probability of individuals flying at least 48 km with the wind where larvae abundance at source locations was also high. Such findings emphasize the importance of long‐distance dispersal on spatial distribution of adult male spruce budworms. Insight into the population‐level consequences of such dispersal patterns requires additional research.     

Ecological mechanisms of population change during outbreaks of the spruce budworm

Abstract.  1. Stage-specific survival and recruitment of spruce budworm were measured by frequent sampling of foliage in four outbreak populations over a 15-year period in Ontario and Quebec, Canada.
2. Patterns of change in population density during the outbreak collapse phase were closely linked to changes in survival of the late immature stages, and were determined largely by the impact of natural enemies.
3. Host-plant feedback also contributed significantly to survival patterns throughout the outbreak: annual defoliation influenced survival of fourth and fifth instars and fecundity while cumulative defoliation influenced survival of the very early larval stages (first and second) via impacts on stand condition.
4. Inclusion of this host-plant feedback reveals spruce budworm population dynamics as a function of density-related trophic interactions that vary in their order and strength of influence over time. This view re-introduces the importance of forest interactions as a component of dynamics of the spruce budworm.     

Host plant stimuli affecting oviposition behavior of the Eastern spruce budworm

The host selection of the female spruce budworm Choristoneura fumiferana (Clem.), Tortricidae, for oviposition is influenced both by the shape and the composition of the substrate. Needle extracts were bioassayed using artificial twigs. The active compounds are soluble in organic solvents and steam distillable. Synthetic D- and L- pinene occurring naturally in the extracts proved to be oviposition stimulants. The optical isomer L- pinene did not show any activity. Ablation experiments led to the conclusion that chemoreceptors for the host attractants and the stimulants are located on the antennae and probably also on the maxillary and labial palpi.

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Zusammenfassung Die Weibchen von Choristoneura fumiferana bevorzugen für die Eiablage ein nadelförmiges Substrat. Dabei spielen die mechanorezeptiven Sinneshaare auf den Ovipositor-Papillen vermutlich eine entscheidende Rolle. Petrolätherextrakte der Nadeloberflächen, Methanolextrakte und Wasserdampfdestillate ergaben eine erhöhte Eiablage auf damit behandelten künstlichen Nadelzweigen. Die aktiven Extraktfraktionen waren nur in organischen Lösungsmitteln löslich. Für die Attraktivität und stimulierende Wirkung der Extrakte müssen verschiedene Substanzen verantwortlich sein. - und -Pinen wurden mit Hilfe des Gaschromatographen in den aktiven Fraktionen nachgewiesen. Synthetisches D- und L- Pinen waren beide aktiv. Dagegen ergab L- Pinen bei den geprüften Konzentrationen keine erhöhte Eiablage. Mit Hilfe von Ablationsexperimenten (Amputation, Überziehen mit Paraffin) wurde versucht, die Chemorezeptoren für die attraktiven und stimulierenden Stoffe zu lokalisieren. Dabei ergab sich, dass die Geruchsrezeptoren auf den Antennen und vermutlich auch auf den Labial-und Maxillarpalpen für die Wirtswahl von Bedeutung zu sein scheinen.

     

Heritability of life‐history traits in the spruce budworm

The heritability of life‐history traits is of particular importance for insects that are very dependent on host conditions. Severe defoliation caused by the spruce budworm negatively impacts its food source, which in turn imposes environmental constraints on the insect. The heritability of those traits can help elucidate this species' evolutionary process. Heritability also helps identify which traits exhibit significant additive variance and can be key to understanding natural selection effects. Individuals were reared under laboratory conditions over three generations on an artificial diet. Heritability was estimated by parent–offspring regression. Fertility and fecundity demonstrated significant heritability followed by larval development, while pupal mass showed minimal heritable variation. These results suggest an important percent of additive variance in life‐history traits. This study contributes to our understanding of the relationship of this forest pest to its environmental conditions. This study also reveals an important genetic architectural structure of life‐history traits in the spruce budworm.     

Sampling procedures and adult sex ratios in spruce budworm

Unbiased estimates of sex ratios that reflect local abundance of adult insects are practically difficult to obtain because many gender‐specific behavioural adaptations differentially influence the catchability of males and females in commonly applied sampling procedures. Historic data on outbreak populations of spruce budworm, Choristoneura fumiferana Clemens (Lepidoptera: Tortricidae), the major pest of conifers in Nearctic boreal forests, include dozens of sex ratio observations for 10 different sampling procedures; these data illustrate the importance of understanding the reproductive ecology of adults to contextualize sex ratio assessments. Sex ratios of resident adults (assessed by rearing field‐collected pupae to adulthood or fogging host trees with insecticide) were not different from 1:1. Sex ratios of in‐flight adults collected using Malaise traps or light traps deployed in tree canopies were consistently male‐biased, which presumably reflects the higher level of flight activity for males relative to females. Sex ratios of moths captured outside the forest canopy (presumed migrants), in contrast, were consistently female‐biased, a trend which is expected because females seeking oviposition sites are more likely to undergo migration than are males. The sex ratio among adults that died from natural causes (collected on drop trays) was not distinguishable from 1:1. In pre‐outbreak (endemic) populations, sex ratios estimated by light trapping were much more strongly male‐biased than in outbreak populations. This surprising result should, however, be interpreted with caution because little is known of reproductive ecology in endemic budworm populations.     

Strains of protoplasts of Entomophthora egressa in spruce budworm larvae

Protoplasts of strain 458 (S458) and strain 521 (S521) of Entomophthora egressa had LD₅₀s of 620 and 8.8 cells/insect, respectively, for sixth-instar spruce budworm larvae. Both protoplast strains exhibited biphasic growth profiles with comparable growth rates in the larval hemocoel. The growth rates of the fungal strains increased as the total hemocyte counts declined. Hemocytopenia was greatest and most rapid in larvae containing S521 protoplasts. Protoplasts of S521 exposed to α-mannosidase and β-N-acetylglucosaminidase were as virulent as the control cells. β-Galactosidase reduced protoplast virulence.     

The role of sugars in western spruce budworm nutritional ecology

Abstract.

• 1 The western spruce budworm, Choristoneura occidentalis Freeman, and Douglas-fir, Pseudotsuga menziesii (Mirb.) Franco, have been used to test the hypothesis that variation in levels of foliar sugars form part of the basis for plant resistance to herbivore attack.
• 2 Budworm population growth was evaluated on artificial diets with 2–45% sucrose using a three generation bioassay. Diets with 1.2% and 3.9% N were tested to determine if responses to sugar were dependent on levels of protein. The 3.9% N diets were supplemented with a mineral salt mixture, so they had high levels of N and minerals.
• 3 The response of budworm population growth to sucrose concentrations ≤20% was convex at 1.2% N and flat for 3.9% N. Population growth on the 1.2% N diet, which had levels of N and minerals similar to host foliage, was good with only 2% sugar, but optimal at the 6% sucrose level; the number of F₁, F₂ and F₃ larvae produced declined substantially when sugar was increased to 11% and 20%. On the 3.9% N diets, population growth was equivalent for diets with 2% and 11% sucrose. Sucrose concentrations ≥29% were detrimental to the budworm at both N levels.
• 4 Sugar concentrations in Douglas-fir foliage varied between 5.7% and 18.4%. Thus, results from the 1.2% N experiment indicated that budworm performance was best on diets with sugar concentrations near the lower limit observed for host foliage. This implies that plants with higher foliar sugar may be inferior hosts for the budworm. Field observations supported this conclusion, as putatively resistant Douglas-fir trees had significantly higher levels of sugars in their foliage than nearby paired susceptible trees. Variation in foliar sugars among individual trees may be part of the mechanism in Douglas-fir resistance to C.occidentalis damage.

     

Misleading correlations: The case of the Canada warbler and spruce budworm

In a recently published article Sleep et al. ( 2009 ) suggested that 30-yr declines in Canada warbler (1975–2005) based on Breeding Bird Survey (BBS) data could be attributed to 30-yr declines in spruce budworm through a series of analyses based principally on correlations. We demonstrate that the relationship does not hold when a longer-term data set (1968–2008) is used for the analysis. Sleep et al. ( 2009 ) also demonstrated a positive relationship between correlations of Canada warbler abundance and time and spruce budworm defoliation and time using provincial data sets. We examined the underlying BBS data and found that there were insufficient observations of Canada warbler in the western provinces (Alberta and Manitoba) to support the conclusion because most Canada warbler observations occurred far (>100 km) from any budworm defoliation. As well, we used a density-dependent stochastic population growth model as proposed by Sleep et al. ( 2009 ) and found only 2 significant relationships (Nova Scotia, Alberta) between Canada warbler population growth rate and budworm defoliation at the provincial scale. We conclude that little analytical evidence exists to support the idea that Canada warbler decline is a function of spruce budworm decline. © 2011 The Wildlife Society.     

Sublethal effects of Bacillus thuringiensis on the spruce budworm, Choristoneura fumiferana

Exposing larvae of the spruce budworm, Choristoneura fumiferana (Clemens), to sublethal ( 50% lethal dose) levels of Bacillus thuringiensis subsp. kurstaki at various stages of their development significantly increased development time to the pupal stage and reduced pupal size and number of eggs laid per female, but did not affect the proportion of embryonated eggs. The changes in larval development time, pupal weight and fecundity depended on the larval stage that was treated. Exposure of fourth instars delayed larval development and reduced only male pupal weights with no effects on fecundity. Exposure of sixth instars delayed larval development to a lesser extent than exposure of fourth instars but had a pronounced effect on weight of both male and female pupae. The effect on pupal weight was sex dependent, as males tended to be more affected than females. The reduction in male pupal weight did not appear to influence fecundity, because the effect of exposure was explained by the change in female pupal weight. Effects on larval growth and pupal weight were proportional to the dose that was ingested during exposure, and were observed at doses as low as one-tenth of the LD₅₀. Ingestion of an LD₅₀ caused a 29 or 45% delay in development of, respectively, female or male larvae when exposed as fourth instars and a 30% reduction in female pupal weight when larvae were exposed as sixth instars.