Deal with red spider mites
Glasshouse red spider mite (Tetranychus urticae) is a nearly invisible pest that attacks plants in greenhouses, conservatories (right), or even in the house. It can defoliate or seriously damage a plant in a matter of weeks. Leaves on affected plants display a fine, yellow mottling, before they dry up and drop off. When the infestation is advanced, you may see a very delicate webbing among the leaves. To view the mites, examine the undersides of the leaves with a magnifying glass in good light. Their oval bodies are less than 1mm long and have eight legs. Although they are called red spider mite, they are usually greenish, turning a rusty colour in autumn.
They thrive in hot, dry conditions, when their lifecycle speeds up at least fourfold. The simplest preventative is to maintain a moist atmosphere, by misting susceptible plants daily with water. Plants that are under stress through being potbound, under-watered or undernourished are vulnerable. Garlic and seaweed sprays help strengthen defences, especially if used early in the season, but they won't halt an infestation in full swing.
Soap-based sprays (available in garden centres) will deal with mild attacks. But for dense populations of mites, a more potent remedy is needed. If you use an insecticide, spray only in the evening, after bees have gone to bed. Organic gardeners can use pyrethrum as a last resort.
The predatory mite, Phytoseiulus persimilis, can be used as a biological control in glasshouses - although if the infestation is severe, an insecticide may have to be used first (observe the recommended period before introducing the predators). The bright-red, South American mite eats the eggs, nymphs (immature forms) and adults of red spider mite. (€20, including p & p, for enough for a small greenhouse, from Fruit Hill Farm, Bantry, Co, Cork. Telephone: 027-50710, web: www.fruithillfarm.com)