The effects of temperature and relative humidity on mortality in Limothrips cerealium Haliday (Thysanoptera) overwintering in bark

Abstract

SUMMARYThe temperature, relative humidity and pressure changes in the bark of Austrian pine during the winter 1960/61 are described. The lowest temperatures occurred in December and January, and the highest mostly in March and April. There was a large diurnal temperature range of up to 34° C. on sunny days, which was usually greatest on the eastern side of the trunk. The relative humidity of air in bark crevices on the sunny side of the tree often fell below 30%: it increased to over 80% at night and in wet weather. In dull weather the temperature and relative humidity fluctuations were less. Changes of temperature and humidity produced contractions and expansions of up to 0.006 in. in the radial thickness of the bark.The susceptibility of L. cerealium to low temperatures was not affected by the acclimatization temperature within the range tested. Thrips were tolerant of continuous chilling and rapid fluctuations in temperature far more severe than conditions experienced in the field, providing that humidity was high. Prolonged artificial chilling caused muscular incoordination and eventually death. Activity ceased between 0° and 5° C. There was a suggestion, which was not satisfactorily proved, that cold hardiness increased slightly from October to March, but frost is unlikely to cause much mortality during the winter.Thrips died quickly in warm, dry air and such conditions in the bark in spring, when emergence is starting, are probably the most important cause of the small amount of mortality which occurs. In these conditions the bark contracts and thrips may be crushed or perhaps trapped and subsequently desiccated by the sun.