The effect of weather on emergence and take‐off of overwintering Limothrips cerealium Haliday (Thysanoptera)

SUMMARY The temperature threshold for take‐off of Limothrips cerealium is 18·4° C., but a temperature‐dependent development period is necessary before thrips are ready to fly in spring. In April and May 1960 this ranged from 4 days at 25° C. to 42 days at 10° C. Thrips hibernating on the warm southern side of trees are therefore ready to fly sooner than those on the cooler northern side. Overwintering thrips were observed emerging from pine bark and their take‐off was recorded by a suction trap. Most thrips emerged when the sun shone on the bark and they usually took off a few minutes later, after combing and flexing their wings. Temperature was the most important single factor influencing take‐off: apparent temperature thresholds for take‐off ranged from 14° C., using air temperature, to 19° C., using bark temperature. These arbitrary values, which were only reflexions of the real temperature threshold, indicated the importance of standard procedures for measuring temperatures, and were a guide to expected behaviour in the field. Take‐off was not retarded by temperatures up to 35° C. Low light intensity reduced take‐off 1–2 hr. before sunset, but in spring the temperature often fell below the take‐off threshold before this. Relative humidity and wind speed were less important, and take‐off occurred at humidities from 27 to 53% and wind speeds from 2·5 to 4·5 m.p.h. Sultry weather was not a necessity for spring flight.