Spider Mites and Their Control

If you notice some strange little white spots on your foliage and the foliage gradually turning pale then brown, you may have mites.

Spider mites become problems in hot, dry weather.  The reason for this is that mites can complete their life cycle in 5-7 days in hot weather and dust on the foliage prevents predators from keeping them in control. Also, after the population gets large, mites spin a fine webbing on the undersides of leaves protecting them from washing or pesticides.

Mites are very small insect-like creatures that suck juices from the underside of plant leaves- resulting in a ‘stippling’ or ‘speckling’ pattern (see attached photo) in the initial stages. Later the entire leaf turns a pale bronze color then may die.  However, there are a number of natural predators that will keep mite populations in check. Also, mites tend to develop here and there- not uniformly over the entire crop. These are called ‘hot spots’ in mite terminology. Sometimes they don’t spread very far from the hot spot and sometimes they will be all over everything in a few days. 

Mites are common on tomatoes, peppers, eggplant, beans, marigolds, and many ornamental plants and weeds. Mites can blow into the area on wind currents or on mulches or compost so once they are present in the garden area they will probably find your plants.Be careful using traditional insecticides for mite control since that often controls the predators but not the mites. Mites are actually not insects and will not respond to many traditional insecticide materials.   

Applying a strong jet of water so as to ‘flip’ around the leaves has been shown to reduce mite populations and stimulate ‘antifeeding’ activity by the plant. However, if hot, dry weather persists mite populations may need to be treated with something to ‘smother and/or kill’ the mites.   There are no insecticides that will control mites well. Insecticidal soaps are probably the best choice.  Avoid any use of Sevin and most other chemical insecticides (they actually make mite populations much worse).  Malathion is partially effective- especially if you direct the spray in a fine mist to the undersides of leaves. 

Because of the rapid reproduction and since insecticides don’t control mite eggs, you will need to repeat spraying or washing every 3-4 days.  One of our gardeners (who is also a biologist and biochemist) has indicated good success in making insecticidal soap from Murphy’s Oil Soap (available at any grocery store) with water (about ¼ cup in a gallon of water).  Insecticidal soaps are different from ‘dishwashing soap’ so don’t use household detergents or soaps. You may burn or damage your plants. Submitted by Fairie Gardener.

Information for this article comes from  the Manhattan Community Garden newsletter.