Spider Mites Love This Weather
While driving I passed a planting of roses that did not appear normal. Up close I saw that the plants were heavily infested with spider mites. The foliage and stems had a rusty red color and were covered with fine webs. The variety of roses appeared to be Knockouts, which are advertised as very resistant to insects.
This summer’s hot, dry weather is ideal for the growth and development of spider mites. The small spider-like creatures are about the size of the pointed end of a pin. They are often difficult to spot, but their webs are unmistakable.
To determine if your plants are infested with spider mites, hold a clean white sheet of paper under the stem of a plant. Sharply strike the stem of the suspected plant. If the plant has spider mites, they will appear as small dark dots moving on the paper.
If you don’t have a sheet of paper handy, prune off the end of a suspected branch and hold it toward the sun or a bright light. A gentle spray of water will make any webs very visible.
Horticultural oil, also known as summer oil, is one of the most effective sprays for eradicating and controlling spider mites. These highly refined oils are approved for use by organic gardeners. At this time of year, a two percent concentration is generally adequate and will not burn foliage or flower petals. However, to avoid potential foliar injury, apply the sprays only when temperatures are below 80 degrees.
Horticultural oil will not only kill the adults, it will also suffocate the eggs. Landscapes with heavy infestations of spider mites will require at least two treatments two to three weeks apart.
Alberta spruce and blue spruce are also very susceptible to spider-mite injury. Checking your plants with a white sheet of paper as described for roses is the best way of determining if you need to spray.
Here’s one caution: Spraying a blue spruce with oil will cause it to turn and remain green until next spring. If you do not wish to cause a color change, you’ll have to use a hard pesticide recommended for the control of spider mites.