Red spider mites harm apple, pear, plum and other fruit trees. They injure foliage by feeding with piercing mouthparts and removing cell contents, including chlorophyll. Heavy mite feeding early in the season can reduce tree growth, yield, and also affect fruit bud formation for the following year.
There are four solutions: pesticides – usually kill the predators and leave the mites; oils and soaps – labour intensive; predator mites – very expensive; and finally, the earwig hotel – simple, cheap, permanent and requires next to no maintenance.
I recently spotted this solution on a permaculture farm in the Pyrenees. Simply stuff a terracotta plant pot with straw and hang it upside down either on or very close to the fruit tree. Within days, earwigs will start converging on the pot. The safe and dry nooks and crannies in the straw are perfect for them. Once they realise it's positioned near to a great foraging zone for the spider mites they love to eat, they'll make it a permanent abode.
On the fruit trees I inspected (apple, plum, almond, pear and peach) I spotted only a few mites. Elsewhere on the property, such as on brickwork of the nearby barn, they were very much in evidence.