Red spider mite is a common bane of greenhouse gardeners, though in warm summer conditions it will happily colonise outdoor plants too.
It hides itself away on the undersides of leaves and feeds by sucking the sap of its host, causing upper leaf surfaces to turn pale and mottled. If this doesn’t give it away, the faint silk webbing certainly will – a telltale sign that an attack is getting out of hand. Mild attacks won’t cause long-term damage but a heavy infestation can severely weaken plants, causing leaves to drop and, in worst case scenarios, killing them off.
Despite its name, the red spider mite is yellow or pale green for much of the growing season, only turning red as autumn approaches. If you do happen to notice some of the barely visible mites, peer closely using a handheld magnifier. You will see they have two dark markings towards the front. This gives them their other name, the two-spotted spider mite.
When Spider Mites Attack
Spider mites love hot, dust-dry conditions where plants are already under stress. They start breeding from spring as soon as temperatures are consistently above 10°C (50°F). The first generations take 55 days to complete, speeding up to as little as 12 in the warm conditions they love.
Spider mites aren’t fussy, but prefer to feast on typical greenhouse crops including tomato, cucumber, peppers and eggplant. They’re also regular antagonists of fruits, most commonly strawberries but often grapevines, peaches and nectarines too. Other plants don’t escape their attention either, and they’ll attack a wide range of ornamental and house plants.
Once temperatures drop from late autumn the females retreat to somewhere sheltered to sit out the winter.
How to Prevent Spider Mites
Given their tendency to overwinter in nooks and crannies, the best way to prevent a fresh attack is to clear out the greenhouse as best you can during the winter and take the time to give it a thorough clean. As well as the framework, pay special attention to any staging, stacked pots, bundles of canes and so on. It’s good, hygienic practice to keep the greenhouse and the area surrounding it clear or weeds and general debris at all times.
Keep actively growing plants properly watered so they’re never struggling – spider mites seek out the weak! Ensure plenty of space between plants to improve airflow and the growing environment more generally.
In the heat of summer it’s worth making the effort to keep the greenhouse as cool as you can, and shading, ventilation and occasional damping down (wetting paths and areas of hard standing to increase humidity) will certainly help with this.
How to Control Spider Mites
If despite your best efforts spider mites do move in, there are a few techniques you can adopt to give them a hard time back. First, double-check your watering regime and increase it if plants are clearly water-stressed. Next, spray the leaves with a fine mist of water, taking especial care to spray the undersides and growing points where the mites lurk. Isolated infestations can simply be cut off and removed to the compost heap.
If the issue persists cover plants with lightweight cloth for a couple of days immediately after spraying. This temporary shade will also trap moisture around the leaves for longer and, hopefully, with a follow-up spray of water, send the mites packing.
Spider mites aren’t without their share of natural enemies. Avoid spraying with pesticides because mites are more likely to be brought under control by one of their predators. Leave greenhouse doors and vents open so they can get in and do their job.
To see off a greenhouse or polytunnel infestation once and for all, consider introducing a biological control sold for the job. The most common of these is Phytoseiulus persimilis, a type of predatory mite. Phytoseiulus is about the same size as the spider mite, but there the similarity ends. Phytoseiulus mites are far more active as they scour leaves in search of prey, tapping their front legs menacingly as they search.
Consider all of this a warning. Keep plants well-watered and not too hot, and there’s every chance the spider mite will pass you by.
Closeup photo of spider mite at top of page by ekamelev