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Overwintering adaptations in Arctic sawflies (Hymenoptera: Tenthredinidae) and their parasitoids: cold tolerance1

Published online by Cambridge University Press:  02 April 2012

Leland M. Humble
Affiliation:
Natural Resources Canada, Canadian Forest Service, 506 West Burnside Road, Victoria, British Columbia, Canada V8Z 1M5 (e-mail: [email protected])

Abstract

Although the extreme winter conditions of Arctic habitats are considered to be important determinants of the faunal composition of Arctic regions, the overwintering biology of most Arctic insects is unknown. A significant proportion of the insect fauna at high latitudes are hymenopterans, yet little information is available on their overwintering strategies. In this study I examined the overwintering strategies of willow gall-forming and catkin-feeding nematine sawflies and their parasitoids. All sawfly species overwintered as prepupae, were freezing-tolerant, and survived exposure to −50 °C. Freezing at high subzero temperatures was initiated by ice nucleators associated with the posterior hind gut wall. Heterogeneity of overwintering habitats with respect to temperature was not a determinant of the overwintering success of these Arctic sawflies. Divergent overwintering mechanisms were evident in the sawfly parasitoid taxa. Endoparasitoid larvae, like their sawfly hosts, were freezing-tolerant. Freezing of immature, feeding endoparasitoid larvae occurred at the freezing point of the host prepupa and was a consequence of the inoculation of the endoparasitoids' fluid compartments by ice crystals growing in the host hemolymph. Peculiarities in the structure of the endoparasitoid larval gut suggest that the site of nucleation is across the gut wall. Outside their hosts, however, endoparasitoid larvae can supercool extensively, and their tolerance of extremely low temperatures is similar to that of their hosts. Overwintering strategies adopted by the ectoparasitoids were also diverse, with both freezing-tolerant and freezing-intolerant species present in the parasitoid community. Freezing-intolerant species could not survive winter temperatures in the field in the absence of an insulating layer of snow.

Résumé

Même si les conditions extrêmes de l'hiver dans les habitats arctiques sont considérées être des facteurs déterminants importants de la composition faunique de ces régions, la biologie de la plupart des insectes arctiques pendant l'hiver reste inconnue. Une proportion importante de la faune entomologique des hautes latitudes est constituée d'hyménoptères; on connaît néanmoins peu de choses sur leurs stratégies pour survivre à l'hiver. La présente étude examine les stratégies d'hiver des mouches-à-scie de la sous-famille des nématinés qui produisent des galles sur les saules et qui se nourrissent des chatons, ainsi que celles de leurs parasitoïdes. Toutes les espèces de mouches-à-scie passent l'hiver à l'état de prénymphes, elles sont résistantes au froid et elles survivent à des expositions à −50 °C. Le gel à des températures élevées sous le point de congélation est initié par des noyaux de formation de glace associés à la paroi du tube digestif postérieur. L'hétérogénéité des habitats utilisés pour passer l'hiver en fonction de la température n'est pas un facteur déterminant de la survie à l'hiver chez ces mouches-à-scie arctiques. Il y a une variété de mécanismes pour survivre à l'hiver chez les taxons parasitoïdes des mouches-à-scie. Les larves d'endoparasitoïdes sont, comme leurs hôtes, tolérantes au gel. Le gel des larves immatures d'endoparasitoïdes qui s'alimentent se produit au point de congélation des prénymphes qui sont leurs hôtes et est causé par l'inoculation dans les compartiments de fluides de l'endoparasitoîde de cristaux qui se développent dans l'hémolymphe de l'hôte. Des caractéristiques particulières de la structure du tube digestif de la larve endoparasitoïde laissent croire que le site de nucléation est situé au-delà de la paroi du tube digestif. À l'extérieur de leur hôte, cependant, les larves endoparasitoïdes peuvent subir une importante surfusion et leur tolérance aux températures extrêmement basses est semblable à celle de leurs hôtes. Les stratégies adoptées par les ectoparasitoïdes sont aussi diverses et la communauté de parasitoïdes comprend à la fois des espèces tolérantes et intolérantes au gel. Les espèces intolérantes ne peuvent survivre aux températures hivernales en nature en l'absence d'une couche isolante de neige.

[Traduit par la Rédaction]

Type
Articles
Copyright
Copyright © Entomological Society of Canada 2006

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