How to spot and get rid of this tiny pest
The red spider mite is a tiny sap-feeding pest, also known as the glasshouse red spider mite and the two-spotted spider mite. Early signs of infestation are mottled leaves and leaf drop, especially in plants under glass.
The mites love houseplants and greenhouse plants, both ornamentals and edibles. From bitter experience, I’ve had crops of aubergines and peppers ruined. They will also infest peaches and hybrids, vines, cucumbers and tomatoes. Ornamental plants targeted are Fuchsia, Pelargonium (geraniums), poinsettias, orchids and Impatiens (busy lizzies).
Red spider mites are so succesful because they can live on a huge range of hosts. They love warm, dry conditions, especially from March to October. However, infestations can happen at any time in a heated greenhouse. In a hot, dry summer, mites will attack plants outside..
What does a red spider mite infestation look like?
Beware, because they’re not easy to spot! You should look for:
- A pale mottling on the upper leaf surface. The undersides hide the mites, cast skins and eggshells. If you’re in doubt, use a magnifying glass.
- Red spider mites are actually yellow-green with two dark patches, completely dark, or reddish-orange in autumn.
- If you can see webs on plants, they’re under heavy attack. Leaves lose most of their colour and fall. Extreme infestations can even kill plants.
Controlling red spider mite
Check plants frequently from spring onwards. If you see mites, I’m only going to recommend organic methods, but they can be difficult to control. They can be difficult to control, as they breed quickly, and some are resistant to pesticides.
Avoid growing plants at high temperatures in dry, overcrowded places under glass. Keep up the humidity by spraying plants with water and damping down the floor during hot spells. This will help, but won’t end an outbreak.
Find badly affected plants and take them outdoors, especially in late summer. If you don’t, females will look for nooks and crannies to hide in over winter. Thoroughly clean your greenhouse with a suitable disinfectant, removing all debris before spring.
Biological controls
Phytoseiulus persimilis: About the same size as a red spider mite (0.5 mm), they have orange-red pear-shaped bodies and can be seen tapping the leaf with their front legs looking for prey. They feed on all life stages. Sold as nymphs and adults, simply place Phytoseiulus in sheltered positions on infested plants.
Amblyseius californicus: Not as efficient a predator as Phytoseiulus and less likely to control heavy infestations. Good as a preventative control. A predator of various species of red spider mite and is likely to survive winter when mite activity is low.
Predatory midge (Feltiella acarisuga): Small black fly native to the UK. The larvae eat red spider mite plus other mites and insects. Use in spring before heavy infestations, but only if red spider mite has overwintered. Less likely to work during the summer.
Atheta coriaria: A 3-4mm long dark brown rove beetle, predatory insect as an adult and larvae. Also controls fungus gnats and thrips.
Control with organic sprays
If you don’t want to use any sort of spray, then use the methods above. These sprays are labelled as organic, containing natural pyrethrum (Bug Clear Gun for Fruit & Veg, Neudorff Bug Free Bug and Larvae Killer), or fatty acids (Solabiol Bug Free). They need to be reapplied to keep numbers in check.
