The effects of the duration and degree of chilling, and the temperature of incubation, on hatching of winter eggs of Panonychus ulmi (Koch) were investigated. For chilling, o "C and 5 "C were more effective than -5" and 9 ', and the limits for the reaction were close to -10" and 1 5 ' . As the chilling period was increased from 60 to 200 days, the percentage hatch on incubation at 21' increased, and the mean incubation time and its variance decreased. Before the maximum effect of chilling was achieved, percentage hatch on incubation at 9" and 15' was higher than at 2 1 ' ; 27" was lethal to most winter eggs though not to summer eggs. After chilling, the later stages of diapause development could occur at temperatures from 0" to 21") i.e. above and below the threshold temperature for morphogenesis, 6-7" in both winter and summer eggs. Diapause development cannot, therefore, be a unitary process. The significance of the results is discussed in relation to forecasting the time of hatch in the field, and to the phenological aspects of hatching in the spring.
I N T R O D U C T I O NThere is considerable practical interest in the hatching of the winter eggs of the fruit-tree red spider mite (Panonychus ulmi (Koch)) in relation to chemical control. The extended hatching period, the variation from year to year in the date of peak hatch, and the variation of the latter from one orchard to another (Light, John, Gould & Coghill, 1968;Dicker, 1962) are all factors which render the optimum timing of acaricidd sprays difficult. Consequently there have been attempts to forecast the commencement of hatch, or of 50% hatch, on the basis of heat sums. Successful forecasting has been reported in Japan (Tsugawa, Yamada & Shirasaki, 1961)) but Light et al. (1968) in England were unsuccessful.Lees (1953) made a detailed study of the factors evoking diapause, and also showed that diapause could be terminated by chilling at I", 5" or 9" for 150-200 days. More study appeared necessary, however, as observations on winter eggs collected in March 1967 and stored at 3-5" showed that the mean incubation time and the spread of hatch at 21" decreased progressively from April to July, after which eggs became non-viable.Similar results appear to have been reported only by Tsugawa, Yamada, Shirasaki & Oyama (1966)) for a sequence of field-collected samples.Factors other than temperature have been reported to influence the percentage hatch. These include daylight as opposed to darkness, the spectral quality of light (Hueck, 1951; Becker, 1952) and relative humidity (Becker, 1952;Kuenen, 1946;Andersen, 1948)) which were reported to affect late embryonic development and I20 J. E. CRANHAM eclosion, rather than the termination of diapause. In the present paper only the influence of temperature during chilling and incubation will be considered; light during incubation was constant while relative humidity varied between 5 5 % and 80 %. The influence of temperature on hatching of the summer eggs was also studied, for comparative purposes.
METHODS
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