Practical Boat Owner's experts answer a reader's question about spider mites

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Here’s a question from the October 2013 issue of PBO.

Invasion of the spider mites

QUESTION: I have a 1985 hurley 22 bilge-keeler which I keep on the Medway. The deck and cockpit finish is white, and this year these areas have been covered with a mass of tiny red mites, smaller than a pinhead. Hosing down the decks does not get rid of them, and using anything stronger than water will have to wait until the boat is lifted out. The marina manager has never encountered this before, but I have heard of a boat at Chatham with the same problem. Can you offer an explanation and treatment?
Mike Sheridan, by email

Environmental expert Sarah Brown replies:
I suspect you have an infestation of the sap-sucking red spider mite (Tetranychus urticae). These tiny animals come in many forms such as the fruit tree red spider mite, the citrus, box and conifer red spider mite and are actually arachnids, relatives of the spider.

They change colour through the year from yellowy/green in the summer to red in autumn and winter. They thrive in dry conditions, such as those found in an airy white cockpit in the summer sun. A single female can start reproducing at just 36 hours old and lay more than 100 eggs.

They may have been brought to your boat by other insects – possibly bees – which also suffer from infestations of mites, and are most likely to have come from a nearby source such as a stand of conifers or fruit trees. They are unlikely to find much to eat on your boat so I suspect the population will reduce naturally, though in the meantimethey are difficult to get rid of, as you have observed.

In the wild their numbers are controlled somewhat by predators such as Phytoseiulus persimilis, another mite, which eats the eggs – but that is not much use to you. Insecticides containing bifenthrin (from garden centres) will work, but I can make no guess about how it may affect your gel coat: some insecticides will stain.

A more eco-friendly solution is to spray the areas with a detergent solution and let it dry: 3 tablespoons of detergent to 1 gallon of water should work. Alternatively, use a mix of 1⁄2 a cup of starch or flour and 1⁄2 a cup of milk in 1 gallon of water, sprayed liberally every five days or so until you have got them all. The mites get caught in the mix and dry out, turn black and die.

They also hate damp conditions, so keeping the decks wet will help: dubious summer weather may be your best ally in this war with the dreaded reds! n environmental expert Sarah Brown runs cradle to Wave, a consultancy specialising in maritime sustainability issues.

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