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

Published online by Cambridge University Press:  08 August 2012

Leah Flaherty ,
Jacques Régnière  and
Jon Sweeney
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Abstract

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.

Résumé

Le Tetropium fuscum (Fabricius) (Coleoptera: Cerambycidae) est un insecte xylophage paléarctique qui est établi dans les provinces de atlantiques du Canada depuis au moins 1990. Ni le nombre de stades larvaires ni les méthodes permettant de déterminer le stade des larves récoltées sur le terrain n'ont été documentés pour cette espèce. La largeur de la capsule céphalique de 949 larves a été mesurée afin de déterminer le nombre de stades larvaires chez cette espèce, d'estimer la moyenne et la variance des valeurs de largeur de capsule céphalique associées à chaque stade et de vérifier si la largeur de la capsule céphalique diffère selon le sexe. Les données relatives à la largeur de la capsule céphalique ont été analysées à l'aide de la méthode du maximum de vraisemblance de modèles de mélange gaussiens et de la règle de Brooks–Dyar. Nos résultats confirment l'existence de six stades larvaires et indiquent que la largeur de la capsule céphalique diffère selon le sexe chez les larves de cinquième et de sixième stades. Le risque de se méprendre sur l’âge et le sexe d'une larve variait entre 0,6%–12,8%, les risques d'erreurs les plus élevés étant associés à la détermination du sexe des larves de cinquième et de sixième stades.

Type
Original Article
Information
The Canadian Entomologist , Volume 144 , Issue 5 , October 2012 , pp. 720 - 726
Copyright
Copyright © Entomological Society of Canada 2012

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