Sulfur for Spider Mites

Spinning webs of destruction wherever they go, nearly invisible spider mites drain plant foliage of its cellular fluids. The eight-legged pests leave in their wake speckled, bronzed leaves that slowly turn yellow or red and eventually drop. Adding insult to injury, the bugs shroud their hosts in unsightly webs. Although sulfur has a long and successful record of combating spider mites, it has significant limitations.

Unlike most insecticides, sulfur kills the mites while doing little harm to the insects that prey on them. It also kills several plant-infecting fungi.

For some plants, using sulfur is worse than a spider-mite infestation. One example is apple trees (Malus domestica), grown in U.S. Department of Agriculture plant hardiness zones 3 through 10, depending on cultivar. While many apple varieties tolerate sulfur well, a few popular ones are extremely sensitive to it.

They include include 'Golden Delicious,' 'Red Jonathan' and 'McIntosh' (Malus 'Golden Delicious,' Malus' Red Jonathan' and Malus 'McIntosh'). The first is hardy in USDA zones 4 through 9 and the others in USDA zones 4 through 8.

Always check the label of sulfur products for a list of sensitive plants before using.

Don't use sulfur on drought-stressed plants. Avoid spraying when the temperature is above 90 degrees Fahrenheit or direct sun is on the plants. Wait at least a month before using sulfur on plants previously sprayed with horticultural oil; two months is better.

Sulfur dust irritates the skin, eyes and lungs. Before handling it, dress in a long-sleeved shirt, long pants, waterproof gloves, safety goggles and a respiratory mask.

Although sulfur can be applied dry or mixed with water to make a spray, the University of California Integrated Pest Management program recommends the dry dust for killing spider mites. Use it at the first sign of infestation.

Dust the plants in calm, dry weather when rain and wind won't remove it. Close the dust container tightly and shake it well. Sprinkle it over the infested plants to lightly coat all the leaves on both sides. Reapply it after rain or as needed to control later infestations.

The dust is more effective when sprinkled often and lightly rather than infrequently and heavily.

Dusting in early morning or late at night ensures that sun won't hit and possibly burn the plants.

Wettable sulfur dust powder

Measuring spoon

Clean bowl, 1-gallon 

Spatula

Handheld or power sprayer

Measure 3 tablespoons, or the label's recommended amount, of sulfur dust per 1 gallon of water and pour it into a clean bowl.

Work a small amount of water into the dust with a spatula. Continue adding water and mixing until you have a thin, smooth paste.

Dilute the paste with water until it's thin enough to pour into the spray bottle.

Fill the sprayer with enough water to dilute the spray to the correct strength.

Always follow the label's mixing and spraying instructions for any pesticide you use. A weak solution won't kill the mites, and an excessively strong one could endanger you and your plants.

Shake the solution repeatedly to keep the sulfur from settling. Coat the leaves until the spray drips from both their sides. Repeat the application every five days until the mites are gone.

Some manufacturers combine sulfur and insecticidal soap in ready-to-use sprays that control insects as well as spider mites. Although easier to use, they're much more expensive than sulfur dust.