ENVIRONMENTAL FACTORS CONTROLLING THE EVOCATION AND TERMINATION OF DIAPAUSE IN THE FRUIT TREE RED SPIDER MITE METATETRANYCHUS ULMI KOCH (ACARINA: TETRANYCHIDAE)
Authors: LEES A. D. 
Published: February 2008

Metatetranychus ulmi lays eggs of two types. The summer eggs, which are laid on the leaves of the host plant (e.g. apple), are of the non‐diapause type and develop without interruption. The winter eggs, which are deposited predominantly on the bark, enter diapause at the blastoderm stage of development. ‘Summer’ and ‘winter’ females lay eggs of only one type if exposed to constant environmental conditions. Diapause is facultative. Over seventy successive summer generations have been reared under diapause‐preventing conditions. Winter females appear in the first post‐diapause generation in response to stimuli which induce diapause.Three environmental agencies are capable of evoking diapause, namely photoperiod, temperature and nutrition. Mites feeding upon undamaged young or mature leaves obtain a plentiful food supply and the incidence of diapause is then determined solely by photoperiod and temperature. However, if the food supplies are restricted, winter females appear even when photoperiod and temperature are such as to prevent diapause. Such deficiencies, which are probably of a quantitative nature, are manifested when the diet consists of the cell contents either of senescing leaves or of ‘bronzed’ foliage previously damaged by the feeding punctures of large mite populations.With daily photoperiods lasting from 6 to 13 hr., and at medium temperatures (c. 15° C.), only winter females develop. The incidence of diapause falls to zero at 15–16 hr., and only summer females are produced in continuous illumination. About 40% of summer females appear in the absence of light. The mites respond to the absolute duration of the cycle of illumination and not to increasing or decreasing photoperiods.Provided the illumination exceeds a threshold of 1–2 f.c., the response is independent of light intensity.Radiation in the near ultra‐violet, blue and blue‐green regions of the spectrum is photoperiodically active. Maximum sensitivity occurs in the blue region. Wavelengths above 550 m μ, i.e. in the orange, red and infra‐red regions, are totally inactive even if the energy level is high.The mites are influenced directly by the photoperiod, not indirectly through the medium of the host plant.High temperatures (e.g. 25° C.) tend to prevent diapause even if the photoperiod is short. Low temperatures (e.g. 10° C.) induce some diapause even with a long photoperiod. Temperature activity seems to be mainly confined to the dark phase.Developing mites are indifferent to photoperiod, temperature and nutrition until the deutonymphal instar. This is also the period of greatest sensitivity, but egg‐laying females, if exposed to antagonistic conditions, can still be caused to ‘switch‐over’ to the alternative egg type. Eggs intermediate in character may be laid during the period of reversal.Winter eggs in diapause never hatch at 18 or 25° C. Diapause can be broken by chilling the eggs at 1, 5 or 9° C. for 150–200 days.In Tetranychus telarius photoperiod, temperature and nutrition play a similar role in controlling the onset of diapause; and females in diapause can be caused to feed and oviposit if chilled. The tropical species Metatetranychus bioculatus is without diapause and photoperiod has no influence on development.These experimental findings have been used in an interpretation of the life cycle and phenology of M. ulmi. In orchards where mite populations remain small throughout the season the first winter females appear at a time when food supplies are still plentiful. This is accounted for by the response to photoperiod.