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.