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Feeding by P. ulmi on leaves causes pale spotting, then as mite populations increase the leaves take on a characteristic 'bronzed' appearance. The undersurface of the leaf, in particular, becomes brown, and badly damaged leaves may fall early.
Other mites may cause similar symptoms. For example, the apple rust mite, Aculus schlechtendali, causes browning of the undersides of leaves.
Most of the natural enemies of P. ulmi have not been reared by biological control companies. However, predators have been introduced into orchards, sometimes in different geographical areas, by transporting shoots from other orchards, or by tying on bands of material, such as velvet, that have trapped overwintering phytoseiids. Typhlodromus pyri is the most important predator of P. ulmi. Strains of T. pyri resistant to organophosphorus insecticides have been introduced into orchards in the UK with success (Solomon and Fitzgerald, 1984) and also in the Czech Republic (Kocourek and Tlusta, 1997). Hardman et al. (1997) have used a strain of T. pyri resistant to organophosphorus and pyrethroid insecticides from New Zealand in apple orchards in Nova Scotia, Canada.
In some situations, a mixture of predator species may give better control than individual species used alone (Croft and MacRae, 1992).
Species of Amblyseius have been used for control in some areas, such as A. andersoni in southern Europe (Baudry et al., 1997) and A. fallacis in the USA (Croft, 1982).
Differences in susceptibility to P. ulmi between different selections or cultivars have been shown for apple (Goonewardene et al., 1976; Bielak and Dabrowski, 1985) and grapevine (Rilling, 1989). If resistance can be utilised in breeding programmes, it would contribute to mite management by slowing population increase, thereby improving the chances of predators gaining control.
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
Feeding by P. ulmi on leaves causes histological changes in the cells of the epidermis, spongy mesophyll and palisade layers. The chlorophyll content is decreased and photosynthesis, stomatal conductance and transpiration are reduced (Mobley and Marini, 1990; Rilling and During, 1990).
The result of this damage to leaf function is a reduction in dry matter accumulation by the tree (Beers and Hull, 1995). Fruit weight may be reduced, and there is often an effect on return bloom and fruit load in the following season on apple (Beers and Hull, 1990; Palevsky et al., 1996). Fruit colour and the concentration of soluble solids in the fruits of apple can also be reduced (Marini et al., 1994), as can the sugar content of grapes (Kast, 1992). Heavy mite infestations early in the year have the most effect in regard to yields in the same season.