Panonychus ulmi Koch Fruit tree red spider mite

This widely distributed species is an important pest of fruit trees, especially apple, damson and plum. Infestations also occur on other hosts, including cherry, pear, various ornamentals and, occasionally, bush or cane fruits; spider mite damage on these hosts, however, is usually attributable to the two-spotted spider mite, Tetranychus urticae (p. 267). The mites feed mainly on the underside of leaves, withdrawing the cell contents to produce a fine speckling, visible from above. As pest numbers increase, affected leaves become generally dull and pallid, and finally silvery or bronzed. Severely damaged leaves are brittle and may drop prematurely. Heavy infestations reduce plant vigour and will affect fruit yields and fruit bud formation for the following year. Damage is usually most evident from July onwards, and attacks are especially severe in hot, dry summers.

BIOLOGY

Mite eggs overwinter on the bark of host trees, often clustered in dense red masses on the spurs and small branches. Less frequently, winter eggs may occur on the young shoots. Eggs hatch from late April or May to about mid-June. However, the precise timing of egg hatch varies considerably from orchard to orchard, as there are both early- and late-hatching 'strains' of the mite. There are several, usually about five, overlapping generations during the summer, and development from egg (through larval, protonymphal and deutonymphal stages) to adult takes about a month: individuals developing into males often omit the deutonymphal stage. Colonies develop on the underside of the expanded leaves, where white cast-off nymphal skins accumulate and the paler-coloured summer eggs are laid (cf. colonies of Bryobia spp. on fruit trees, p. 265 etseq.). The mites do not shelter under webbing; however, individuals may produce strands of silk which will then allow them to be carried by the wind to adjacent trees. Winter eggs are laid in September, in response to shorter days and declining temperatures, but may be deposited earlier if

Fig. 344 Female fruit tree red spider mite, Panonychus ulmi (x80).

tree conditions become unsuitable for further breeding; this often occurs when mite numbers are large and foliage becomes badly bronzed.

DESCRIPTION

Adult female 0.4 mm long, oval, strongly convex, dark red; hysterosomal setae relatively long and arising from conspicuous white tubercles; legs pale (Fig. 344). Adult male smaller than female and pear-shaped, tapering posteriorly; yellowish-green to bright red. Egg red and shiny; more or less spherical (c. 0.1mm in diameter) but slightly flattened and the top drawn into a fine stipe. Active immature stages pale yellowish-green to bright red.

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