Full-text - Norsk entomologisk forening

Metatetranychus uimi 423vang were much higher than the average supercooling points ofM. ulmi winter eggs. It seems unlikely therefore that eggs fromUllensvang were killed by freezing. The mortality recorded musthave been caused by other factors. At As the eggs were exposedto long periods of temperatures below -20" C, and even -25" C,and it is possible that nucleation may take place under suchcondition. Some eggs may have died from long exposures to lowtemperatures in a supercooled state. At Nes the temperaturesdropped far below the average supercooling points of M. ulmiwinter eggs, and it seems reasonable to assume that most of themwere killed by freezing.ACKNOWLEDGMENTS. This work was carried out with financial supportfrom the Korwegian Research Council of Agriculture. Temperature dataare used with the permission of the Dept. of Physics, Agricultural Collegeof Norway, and of the Norwegian Institute of Meteorology. I am mostgrateful to Mr. T. Sekse and Mr. A. Hjeltnes for help concerning the collectionof winter eggs.SummaryFollowing the extremely cold winter of 1965166 mortalityrates of winter eggs of the Fruit tree red spider mite (Metatetranychusulmi (Koch)) were recorded. Of eggs collected at Ullensvangin western Norway 73.3% hatched under laboratory conditionswhile the hatching rates of eggs from As and Nes in easternNorway were 30.3% and 0.7% respectively. Lowest recordedtemperatures were -14.5" C at Ullensvang, -25.6" at As and-41.1" at Nes. Compared with their ability to supercool it seemslikely that most of the eggs from Nes were killed by freezing.The mortality recorded in eggs from As may partly be a resultof freezing, but also of other factors, like long exposures tolow temperatures in a supercooled state. Eggs from Ullensvangwere probably not killed by freezing, since temperatures recordedat this locality were much higher than the average supercoolingpoints of the eggs.ReferencesMACPHEE, A. W. 1961. Mortality of winter eggs of the European redmite Panonychus ulmi (Koch), at low temperatures, and its ecologicalsignificance. Can. J. Zool. 39: 229-243.SALT, R. W. 1961. Principles of insect cold-hardiness. A. Rev. Ent.6: 55 -74.SBMME, L. 1965. Changes in sorbitol content and supercooling pointsin werwintering eggs of the European red mite (Panonychus ulmi(Koch)). Can. J. Zool. 43: 881 -884.