Physiological basis for flexible voltinism in the spruce beetle (Coleoptera: Scolytidae)

Abstract

AbstractThe spruce beetle, Dendroctonus rufipennis (Kirby), has described life cycles of 1–3 years. Although temperature has been shown to be strongly associated with flexible voltinism in the spruce beetle, the physiological basis for this phenomenon is not clear. Two competing hypotheses were tested under laboratory conditions. First, we tested the hypothesis that larval diapause, induced by cool temperatures during or before instar III, initiates prolonged life cycles while larvae not diapausing complete development to adults before the first winter. We compared development times at constant temperature (12 °C) and field-simulated thermoperiod treatments against development times in a reference (21 °C) treatment for which there is no indication of diapause induction. The constant temperature treatment was not significantly different than the reference treatment, although there were a few outliers. The thermoperiod treatment was significantly longer than the reference treatment, but only by a few days. These results provide little support for the hypothesis of larval diapause induction during or before instar III. Second, we investigated the hypothesis of life-cycle regulation through life stage specific developmental temperature thresholds, particularly, a relatively high threshold for pupation that might prevent development beyond the prepupal life stage under cool conditions. We found little evidence of distinct differences in low-temperature thresholds between life stages. Instar-IV larvae held at ≤ 15 °C, however, did not pupate for 125–300 days, a developmental arrest that suggests diapause. Based on all present and previous investigations, the induction-sensitive phase appears to be late in the instar-IV or early in the prepupal stages. For semivoltine spruce beetles, this life stage occurs late in the growing season, after most temperature-dependent development has been completed. It is our conclusion that spruce beetle voltinism is primarily under direct temperature control and that prepupal diapause is the default overwintering strategy for individuals not completing development to maturity by fall.