Spider mites are tiny organisms closely related to spiders. They afflict many diverse plant species, including landscape shrubs, especially roses, arborvitae, juniper and azalea, by sucking minute amounts of sap from the leaves. Their feeding activity causes small dots on the leaves and, as feeding intensifies, leaves turn yellow and fall from the plant. On heavily infested shrubs, you may observe white webbing on the leaves. Controlling spider mites involves cultural and preventive measures with chemical controls available to manage severe infestations.
Before You Spray
Some methods to prevent and control spider mites do not require the use of chemicals. You can often remove spider mites from your shrubs and break their webs by spraying the shrub with a sharp jet of water, aiming particularly for the undersides of the leaves where mites like to congregate. You can hose down plants as often as necessary to bring mite populations under control. Mites also like dry, dusty conditions, so keep your plants properly watered and periodically spray down any nearby dusty surfaces that can lead to conditions conducive to mite problems. Keep your plants properly watered and fertilized, as healthy shrubs can better resist mite infestations and won't succumb as quickly to mite-related damage.
Other insects and mites in your landscape can help you to naturally control spider mite populations. Ladybugs, pirate bugs, lacewings and thrips naturally feed on spider mites. Predator mites will also feed on spider mites, and you can purchase predator mites from specialty garden suppliers and release them on your plants. Able to consume up to 20 spider mites in one day, predator mites can quickly bring a problem under control.
Dormant oils sprayed before new growth begins in the spring can kill overwintering spider mites that affect evergreen shrubs. Manufacturers have formulated the dormant oils so that you can apply them again during the growing season if mite problems persist.
If mite infestations refuse to reduce with the use of nonchemical control options, you may need to use a pesticide to save your shrub. Because beneficial insects play an important role in controlling pest populations in your landscape, you should always begin with control measures that won't harm beneficials. Insecticidal soaps and horticultural oils offer two natural options that will kill mites without harming your beneficial allies. Apply these every week to 10 days, coating the mites with the spray, as neither soap nor oils have residual effects. If you need to use conventional pesticides, alternate the use of two different products. Because mites have short life cycles, they can quickly develop resistance to chemical controls.