Two sides of a coin: host-plant synchrony fitness trade-offs in the population dynamics of the western spruce budworm.

Conifer-feeding budworms emerge from overwintering sites as small larvae in early spring, several days before budburst, and mine old needles. These early-emerging larvae suffer considerable mortality during this foraging period as they disperse in search of available, current-year buds. Once buds flush, surviving budworms construct feeding shelters and must complete maturation before fresh host foliage senesces and lignifies later in the summer. Late-developing larvae suffer greater mortality and survivors have lower fecundity when feeding on older foliage. Thus, there is a seasonal trade-off in fitness associated with host synchrony: early-emerging budworms have a greater risk of mortality during spring dispersal but gain better access to the most nutritious foliage, while, on the other hand, late-emerging larvae incur a lower risk during the initial foraging period but must contend with rapidly diminishing resource quality at the end of the feeding period. We investigate the balance that results from these early-season and late-season synchrony fitness trade-offs using the concept of the phenological window. Parameters associated with the variation in the phenological window are used to estimate generational fitness as a function of host-plant synchrony. Because defoliation modifies these relationships, it is also included in the analysis. We show that fitness trade-offs characterizing the phenological window result in a robust synchrony relationship between budworm and host plant over a wide geographic range in southern British Columbia, Canada.

Molecular characterisation of a cypovirus isolated from the western spruce budworm Choristoneura occidentalis.

A novel cypovirus, assigned CoCPV, was isolated from natural populations of the western spruce budworm, Choristoneura occidentalis. The complete nucleotide sequences of genomic segments S2-S5 and S7-S10 were determined. Each segment contained a single open reading frame. Conserved motifs 5' (AGUUU......UUUGUGC) 3' were found at the ends of each segment. Analysis of S2, which encoded a putative RNA-dependent RNA polymerase protein, confirmed CoCPV belonged to the genus Cypovirus within the family Reoviridae. Further phylogenetic analysis using S10 (the polyhedrin gene) aligned this virus with species type-16, closely related to a cypovirus isolated from C. fumiferana.

Seasonal changes in foliar terpenes indicate suitability of Douglas-fir buds for western spruce budworm.

The terpene composition of current-year buds of Douglas-fir, Pseudotsuga menziesii (Mirb.) Franco, was analyzed from before budburst to after buds were fully flushed. Terpene composition was measured at weekly intervals for several seasons at eight different locations in the southern interior of British Columbia, Canada. Discriminant functions relating terpene composition to suitability of buds for newly emerged western spruce budworm, Choristoneura occidentalis Freeman, were developed based on terpene profiles of the buds and bioassays measuring the suitability to budworms of a sister group of buds. Changes in percent composition of bud terpenes before and during budburst were closely associated with changes in the suitability of the buds to utilization by budworms at both the tree and site by date levels. Use of a degree-day scale for bud suitability removed much of the year-to-year variation, but remaining differences among sites suggest additional sources of variation influencing the insect host plant relationship. The success of correctly classifying bud suitability using terpene profiles demonstrates the value of foliar terpenes as indicators of seasonal changes in suitability of Douglas-fir foliage during the critical spring emergence period of western spruce budworm. This indicator could be used to screen individual trees susceptible to budworm damage and identify sites at high risk of damaging defoliation.