Freeze fitness in alpine Tiger moth caterpillars and their parasitoids |
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Authors: | T C Hawes D A Wharton |
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Institution: | (1) Department of Zoology, University of Otago, P.O. Box 56, Dunedin, New Zealand |
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Abstract: | The adaptive fitness of a freeze-tolerant insect may be mediated by both endogenous and exogenous interactions. The aim of
the study presented here was to characterize the freeze tolerance of alpine Tiger moth caterpillars (Metacrias huttoni) and highlight two poorly explored indices of the potential attrition of fitness: (1) downstream development and reproduction;
(2) parasitism. Caterpillars survived temperatures as low as −16°C and demonstrated >90% 72-h survival after exposures to
−10°C. Two-week acclimations at 5, 10, and 20°C had no effect on body water content, haemolymph osmolality or survival of
equilibrium freezing, but there was a significant elevation of the temperature of crystallization (T
c) in those caterpillars acclimated to 5°C. Cell viability of fat body tissue was resilient to freezing (−10 to −16°C), but
midgut and tracheal cells showed significant degradation. Pupation and eclosion were unaffected by freezing at −5 or −10°C.
Likewise, there were no significant differences in egg production or the proportion of eggs that hatched between control and
frozen insects. By contrast, the ability of tachinid larvae to survive freezing within their hosts means that parasitism plays
an important role in regulating population size. Mean parasitism of caterpillars by tachinids was 33.3 ± 7.2%. Pupation and
imago emergence of tachinids after host ‘endo-nucleation’ was >75%. Eclosed adult tachinids showed a non-significant increase
in the incidence of wing abnormalities in relation to low temperature exposure. |
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