Marin IJ Articles
August 29, 2014
Just as we enjoy the dog days of summer, so do a scourge of pests, particularly spider mites. These little arachnids, related to spiders and ticks, look like tiny moving dots. They feast on your fruit trees, vines, berries, vegetables, roses and other ornamental plants particularly June through September. Also called webspinning mites, they are among the most ubiquitous of all pests in the garden.
These pesky creatures thrive in low rainfall areas of the west where warm temperatures above 70 degrees and dry plant foliage favor their development.
Low numbers of mites usually aren't a concern, but populations can build up to tremendous numbers in a short time if the conditions are right
Low numbers of mites usually aren't a concern, but populations can build up to tremendous numbers in a short time if the conditions are right and can be damaging, especially to annual plants. Hot weather, low humidity and little water encourage dust and dirt on leaf surfaces that is perfect for mite buildup — sounds just right given our current drought situation.
You're likely to see plant damage before seeing the pest. This mite sucks juices from plant cells; leaves display light-colored dots or flecking or turn a bronze or rusty color. On heavily infested plants, leaves can turn yellow and drop off. The presence of webbing on the under surface of the leaves is an easy way to distinguish them from other types of mites and insects such as aphids and thrips that can inhabit the undersides of leaves. Another test — shake a leaf over a white sheet of paper; if you see dark specks moving across the paper, it's most likely spider mites.
Spider mite damage to ornamental plants is usually an aesthetic concern, though they can kill plants if populations become high. Fruit tree yields for this season won't likely be impacted if the pests show up in late summer. Annual vegetable crops like squash and melons can drop lots of leaves and create the opportunity for sunburning. The mites attack the pods of sugar peas and beans resulting in direct damage to the vegetable.
The best protection against an invasion of spider mites is maintaining good cultural practices. Remove fallen leaves, branches or fruit that may serve as a host for them. Keep plants well watered and apply mulch to minimize plant stress. Forceful spraying of leaves, especially the undersides, is a good method for keeping these critters at a tolerable level. With limited water thanks to the drought, get creative! Invest in a 2- or 5-gallon pressurized sprayer. Collect that clean cold water that would otherwise go down the drain while you're waiting for the shower to warm up, fill up the sprayer, pressurize and blast the little creatures off your plants. Knocked to soil level, these pests aren't very successful at getting back on the plant. As with any application of water to foliage, do it early in the day so leaves can dry.
If you elect to use a pesticide, consider an insecticidal soap or horticultural oil, or a combination of the two. Be sure to cover the entire leaf surface, particularly the undersides of leaves. Repeated applications may be required. There are a number of plant extracts formulated to kill mites that include garlic extract, clove oil, mint oils, rosemary oil and cinnamon oil. These materials may injure some plants, so check labels and/or test them on a portion of the foliage several days before applying a full treatment. Apply soaps or oils on well-watered plants when temperatures are below 90 degrees.
Finally, don't use an insecticide to control mites. Since they are arachnids and not insects, the products will have little impact on the spider mites, may kill off natural enemies and actually stimulate mite reproduction!