Best defense against outdoor spider mites is a good healthy plant. You'll never be able to kill them all so you have to rely on other insects to keep their numbers low. Planting plants like carrots, cilantro, marigolds, chives, garlic, peppermint, and basil can help too. Aluminum foil as a mulch helps against flying insects. Copper tape deters crawling slugs and snails. Spraying your plants down with soapy water (dawn, ivory, dove, etc) can help keep plants clean (removes dust allowing leaves to collect maximum amount of sunlight), kills pests through suffocation, and makes it easier for plants to uptake nutrients.