Spider mites are tiny, sap-sucking pests, related to spiders. They may be only 0.5mm long but they can wreak havoc in a greenhouse or on house plants indoors. The most common spider mite in the UK is red spider mite, also know as two-spotted mite. For most of the year they are a pale green colour with two dark spots on their back, but in autumn and winter they turn orange-red and may be spotted in cracks in windowsills and in our homes, as they look for places to hibernate. Spider mites thrive in hot, dry conditions so life in the greenhouse or a centrally-heated home suits them perfectly. If the temperature stays above 12ºC, red spider mites can breed all year round.
Spider mite symptoms
Spider mites are easy to spot on green house plants and house plants. You will see fine webbing on the leaves and stems, while mottling is visible on the upper surface of leaves. Using a magnifying glass, look for mites and eggs on the undersides of leaves. The plants will lack vigour and may eventually die.
Find spider mite on
Spider mites are typically found on house plants and greenhouse plants, but you may also spot them outdoors on fuchsias, runner beans, busy lizzies and grape vines.
How to kill spider mite
The best way to tackle spider mites is to stop it reproducing. They thrive in hot, dry conditions, so improve air circulation and boost levels of humidity by misting plants with tepid water and standing bowls of water on the benches between plants. Damping down the floor of your greenhouse with water will also help increase humidity.
When daytime temperatures in the greenhouse hover steadily around 21ºC, you can try releasing the predatory mite, Phytoseiulus persimilis on to your plants. This is the most common biological pest control for red spider mite in greenhouses and the home. The predatory mites feed on spider mites at all stages of their lifecycle, including eggs. This quickly reduces the spider mite population and therefore plant damage.
If temperatures remain lower than 15ºC (the minimum temperature at which Phytoseiulus persimilis can survive), try using sprays containing fatty acids or plant oils, instead.