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How to get rid of spider mite on greenhouse crops and houseplants

Summer’s well on the way and my greenhouse tomato and cucumber crops are looking promising, but this year I’m noticing the early onset of a greenhouse pest that has the potential to totally devastate everything it attacks: spider mite. These are very tiny pests that feed off the plant’s circulatory system, literally sucking the life out of them. Their ability to destroy large plants is out of all proportion to their size – they are smaller than grains of flour or dust particles, and are barely visible to the naked eye. I have seen very large specimen plants (such as an exotic banana plant) almost wiped out and left unchecked spider mites will happily destroy tomato plants and, in my case, threaten this year's cucumber crop too.

Healthy cucumber leaf [click to see]

Cucumber spider mite damage [click to see]

Foliage death caused by spider mite [click to see]Yellowing caused by spider mite sap damage. Fine webs will be visible in the area shown [click to see]Misting with water reveals the webs [click to see]

It's war! Spider mite can destroy a large plant [click to see]The first signs of spider mite are usually on foliage. It starts to go dappled and yellow as the chlorophyll is drained away by the sap-sucking pests, and it may result in totally desiccated patches of greenery as though the plant has been scorched.

Spider mites tend to gather more on the underside of leaves, between the veins. They also group on the base and stalk of the leaf, and if you look very closely you may see tiny, dust-like particles moving around. The presence of spider mite can often be revealed by spraying leaves with a fine mist of water: this will highlight very fine cobwebs that form on the ‘valley’ of leaves around the base and stalk. If you see these tell-tale signs, then it’s important to act straight away before they spread and destroy the whole plant in a few short weeks (which they will!), then spreading to other plants nearby.

Old-fashioned remedies include spraying with a water/ washing-up liquid (a drop or two only), or regular misting with water as spider mites thrive in hot, dusty conditions. At most, this might set them back a little, but it's not 100% effective and it’s necessary to use a more aggressive treatment to deal with the problem thoroughly.

I find many common amateur pesticide sprays have little effect these days. Compared to 20 or 30 years ago, pesticide resistance is now very high making pest control (aphids, whitefly and spider mites especially) far more difficult than it ever used to be.

Start by removing heavily infested foliage and destroy it. Two products to start with (remember, as we’re treating a food crop, sprays must be food-safe) are Bug Clear Ultra for flowers, fruit and veg, and Resolva Bug Killer. Be sure to spray the undersides of leaves and leave axils/ stems where spider mites flourish. There are steep restrictions on how often you can spray food (2-3 times maximum, say), and how long you have to wait before harvesting crops.

The problem of spider mite has plagued my greenhouse for a couple of years, and I have to say I’ve tried both of these products with limited results: in fact they are not specific for spider mite but can provide some useful control. Of the general garden-centre products, Scott's Bug Clear for Fruit & Veg is probably the best bet. It's a rape-seed oil based product that is 100% natural and is worth a try: mix it with water in a pressure sprayer.

Otherwise the next stage is to try specific products to kill spider mite. One suggestion is Spidermite Control (SMC), a natural, oily spray that claims to suffocates spider mites on contact (the same way Scott's Bug Clear for Fruit & Veg does). It is organic, natural, friendly to bees, safe for kids, wildlife. It also claims to beat whitefly and aphid.

An alternative is Safer’s Trounce, a Canadian product that's a contact insecticide with a natural chemical formula. It claims to be the only product with pyrethrins (insecticide) and fatty acids (breaks down pests on contact). A 500mL bottle of Trounce makes up to 10L, and a 1L RTU spray is also available. I would suggest this is likely to be a better all-rounder, especially for use on indoor plants.

A third way is to use a natural predator that dines on spider mites, and Phytoseiulus is sold in packs but they have been very heavily criticised for being very expensive and performing poorly.

I have wasted a small fortune on ineffective Ready To Use (RTU) sprays sold by garden centres. As SMC points out, one mature female spider mite can produce up to one million offspring in a month, so now it’s time to press the nuclear button – I’ve opted to try Trounce and I’ll keep you posted how well this spider mite pesticide does in my greenhouse, so be sure to check back.

At the end of the season I'll empty the greenhouse and fumigate it with a sulphur candle or two (if I can get any - they seem to be banned by the EU now), to kill off any overwintering pests. Below are links to the above products on Amazon, plus technical info links.

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