Diapause Contents

Pupal diapause of Coloradia pandora: Thermal constraints on successful pupation

E. A. Gerson 1 , R. G. Kelsey 1 & D. W. Ross 2

1 U.S. Forest Serv., PNW Research Station, 3200 Jefferson Way, Corvallis, OR 97331, USA; 2 Dept. of Forest Science, Oregon State Univ., Corvallis, OR 97331, USA

The pandora moth, Coloradia pandora, is a native lepidopteran that periodically defoliates large areas of pine forest in the western U.S. In central Oregon, pupation occurs in loose soil over winter, and the pupal phase generally lasts 12 to 13 months of a 2-year life cycle. To determine the minimum cold storage time required to break diapause we held pupae at 5 o C for 8 to 24 weeks, then incubated them at 25 o C and measured the time until adults emerged. The minimum cold time required under these conditions was 12 weeks, but emergence rates were highest (87.5%) for 14 to 18 weeks at 5 o C. Cold storage in excess of 20 weeks resulted in emergence rates of less than 40%. In a separate experiment, 1000 pupae were maintained in field enclosures for 3 years while soil temperature was monitored. Seventy-two percent of these pupae emerged in Year 1. Prolonged (extended) diapause was observed in only 0.6% of the sample population. Soil temperatures fell below 5 o C for ? 21.7, 22.9, and 25.1 weeks over the three consecutive winters, and the minimum soil temperature was -2 o C. Based on results from the cold storage experiment, the 72% emergence rate for Year 1 in the field was higher than expected for ? 21.7 weeks of soil temperatures below 5 o C. However, the soil temperature profiles indicated that pupae used in the cold storage experiment probably were exposed to several weeks of cold soil in the field prior to collection for the lab study. In this case, emergence rates in the lab and field experiments correspond well. Soil temperature monitoring in the field revealed the duration of cold may be lethal to a large proportion of the pupal population in winters such as Year 3 with 25.1 weeks below 5 o C. From the bell curve for emergence rates established in the lab experiment, less than 40% of pupae would be expected to successfully complete diapause after a long winter (such as Year 3), compared to the 72% successful pupation rate observed for the shorter winter of Year 1. Together, the laboratory and field studies suggest duration of cold during the pupal phase is a variable, but potentially significant, mortality factor for pandora moths in central Oregon.

Index terms: pandora moth, Saturniidae, rearing, adult emergence, winter mortality.

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