We evaluated the effects of snow cover and debris cover on overwintering success of Aphthona from 2001 to 2004 in southeast North Dakota. Chill degree-days, soil temperature, and duration of soil temperatures were monitored in field plots. Emergence of Aphthona from soil cores collected in October and held in the laboratory under simulated winter conditions did not differ across treatments. Field emergence of Aphthona was significantly reduced compared with emergence of Aphthona in the laboratory under simulated winter. Snow cover protected overwintering of Aphthona during 3 yr. Overwintering success was 77–94% lower in no-snow plots than in snow plots except in 2002. In 2001, when the soil temperature was as low as -4.4°C, 90% of the Aphthona beetles emerged from snow-covered plots. The ranges of winter temperature and winter period for overwintering Aphthona were determined as -5.0–4.5°C and 56–132 d, respectively, by using linear regression. A negative linear relationship between soil temperature and winter period may help predict the minimum needed overwintering period at a given winter temperature. Warmer temperatures during the winter of 2002 caused snow to melt in the snow plots, which led to excessive moisture that seemed to reduce overwintering success of Aphthona in those plots. Overall emergence was too low to conduct statistical procedures in 2003 and 2004 because of temperature extremes and warm periods during the winters that may have caused Aphthona to break diapause prematurely.
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