Do visual cues associated with larger diameter trees influence host selection by Tetropium fuscum (Coleoptera: Cerambycidae)?

Tetropium fuscum (Fabricius) (Coleoptera: Cerambycidae) is an invasive phloeophagous beetle established in Atlantic Canada that infests stressed and moribund Picea Dietrich (Pinaceae) species. Successfully colonised trees tend to be large in diameter (>10 cm diameter at breast height), but whether diameter influences host selection, larval performance, or both, is unknown. We tested the hypothesis that T. fuscum host selection is influenced by visual cues associated with tree diameter by counting the number of adults landing on 29 Picea rubens Sargent ranging in diameter at breast height from 12.2 to 37.5 cm. All trees were wrapped with sticky bands and baited with aggregation pheromone and host volatiles to make them equally attractive with regard to olfactory cues. We found significant positive relationships between the mean number of T. fuscum per sticky band and tree diameter, and also between phloem thickness and tree diameter. We conclude that the positive association between host diameter and T. fuscum infestation is at least partially due to the positive influence of diameter on landing rate, and that this may benefit the beetle because larger diameter trees have more food for developing larvae. However, there was no effect of tree diameter on the mean number of adults per m2 of sticky band and thus no evidence that T. fuscum actively selects larger diameter hosts based on visual cues. The positive relationship between landing rate and host diameter may simply be due to greater chances of airborne beetles being passively intercepted on larger versus smaller trees.

Number of instars and sexual dimorphism of Tetropium fuscum (Coleoptera: Cerambycidae) larvae determined by maximum likelihood

Tetropium fuscum (Fabricius) (Coleoptera: Cerambycidae) is a Palaearctic wood borer that has been established in Atlantic Canada since at least 1990. Neither the number of instars nor methods for determining the instar of field-collected larvae have been documented for this species. Head-capsule width was measured for 949 T. fuscum larvae in order to determine the number of instars in this species, estimate the mean and variance of head-capsule widths associated with each instar, and identify whether head-capsule width is sexually dimorphic. Head-capsule width data were analysed using maximum likelihood analysis of mixture models and the Brooks–Dyar rule. Our results provide strong support for the existence of six larval instars, with sexually dimorphic head-capsule widths in instars five and six. The probability of misclassifying larvae into instar-specific and sex-specific categories ranged from 0.6% to 12.8%, with the highest probabilities occurring when assigning a sex to fifth-instar and sixth-instar larvae.