Pest
Red spider mite
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Watch for | Watch for in April | Watch for in May | Watch for in June | Watch for in July | Watch for in August | Watch for in September | Watch for in October |
What it is
Red spider mite is not a spider but a mite less than 1 mm long, usually yellowish green with two dark spots during the growing season. It only turns orange-red in autumn when females prepare to overwinter. Under glass it can build up to huge numbers, and in hot summers it also attacks plants outdoors.
Symptoms
Upper leaf surfaces develop a fine, pale speckling or mottling, which spreads until leaves look dull, bronzed or yellow and eventually brown and fall. Turn a leaf over and use a hand lens to see the mites, their round eggs and pale cast skins. In heavy attacks, fine silk webbing covers shoot tips, with mites massing on it. Thrips cause similar silvering but leave tiny black specks of droppings.
Cause
The mites breed fastest in warm, dry air, and a generation can be completed in around a week at high summer temperatures. They spread on clothing, tools and new plants, and from weeds. In autumn, red females hide in cracks in greenhouse frames, canes and plant debris, ready to re-emerge in spring.
Control
Mist plants and damp down greenhouse floors on hot days, because mites dislike humidity. Remove the worst-affected leaves. The predatory mite Phytoseiulus persimilis is a very effective biological control under glass; introduce it as soon as you see the first mites and while temperatures are reliably warm. Avoid broad-spectrum sprays, which kill predators and often make mite problems worse. Soap or oil sprays are a last resort; follow the label.
Prevention
Clean the greenhouse thoroughly in winter, including frame crevices, canes and pots. Check new plants carefully before bringing them in. Water and ventilate well in summer, and do not let plants become drought stressed, as stressed plants are more vulnerable.
Plants often affected
Could it be something else?
These problems share some of the same symptoms. Compare them, or run the symptom checker with these signs ticked.
Months to keep an eye out: April, May, June, July, August, September and October.
Sources
Identification and life-cycle details were checked against the references below. Text and advice are our own. See our editorial policy.