Cold Tolerance and Supercooling Capacity in Overwintering Adults of Elm Leaf Beetle Xanthogaleruca luteola (Coleoptera: Chrysomelidae)

Elm leaf beetle, Xanthogateruca luteola (Muller) is one of the key pests of elm trees all over the world, and survives winter in reproductive diapause in sheltered locations. Seasonal variation of whole body supercooling points (SCPs), LT50 (temperature at which 50% of the test individuals die) and survival rate after exposure to subzero temperatures were determined in field collected adults during October 2008 to May 2009 and October 2009 to May 2010. The SCP of adults decreased significantly from October (median = -13.8 degrees C) to January (median = -20.7 degrees C) in first year, relatively similar results was observed in the second year. The lowest LT observed in overwintering adults collected in January (-16.81 degrees C) in the first year and December (-15.59 degrees C) in the second year. Mortality at -15 degrees C for 24 h was >70% in early autumn in both years whereas it decreased to lower than 45% in early winter, the highest mortality (100%) was observed in adults collected in May in both years. Cold acclimated adults (30 d, 5 degrees C) in November 2008 exhibited significantly higher SCP (-12.21 +/- 0.64 degrees C) than nonacclimated adults (-15.57 +/- 1.35 degrees C). A 30-d exposure to 5 degrees C caused >20% mortality in November, while <9% mortality was observed in adults collected in December and January 2008. Overwintering adults died upon freezing and the lower lethal temperatures were within the range of SCP, indicating that X. luteola is a freeze intolerant insect.