By: Jolene Adams, jolene_adams[at]comcast[dot]net
Two-spotted spider mites can be rusty green, greenish amber or yellow and they have eight legs. Overwintering females are red or orange. These mites have two (sometimes four) black spots on top. They are very small, but still visible. The best way to see them is with a small magnifying glass. The eggs vary from transparent and colorless to opaque straw yellow.
If spider mites infest your roses, you will begin to notice a dull appearance to the leaves. The undersides will feel “sandy” or rough. Fine webbing will appear on the undersides of leaves and in leaf axils. Leaves will begin to lose their color and become dry and lifeless. The leaves soon die and dry up.
BIOLOGY: Two-spotted spider mites are widely distributed in the United States and feed on over 180 host plants, including roses. Once a plant is infested, the mites spread onto nearby crops and ornamentals. Two-spotted spider mites pierce the epidermis of the host plant leaf with their sharp, slender mouthparts. When they extract the sap, the tissue of the leaf collapses in the area of the puncture. Soon a spot without green color forms at each feeding site. After a heavy attack, an entire plant may become yellowed, bronzed or killed completely. The mites may spin so much webbing over the plant that it becomes entirely covered.
Though insects and mites are in a group called the Arthropoda (meaning jointed foot), because jointed legs are common to both, spider mites are not actually insects. Being more closely related to spiders, they derive their name from the thin web that some species spin.
Two-spotted spider mites overwinter as adults in the soil or on hosts such as violets and hollyhocks. In mild winter weather, two-spotted spider mites continue to feed and lay eggs, although development in the winter is much slower than in the summer. In warm weather, six-legged larvae hatch from the eggs. They develop into eight-legged nymphs, which pass through two nymphal stages. After each larval and nymphal stage, there is a resting stage. The adults mate soon after emerging from the last resting stage, and in warm weather the females soon lay eggs. Each female may lay over 100 eggs in her life and up to 19 eggs per day. Development is most rapid during hot, dry weather. A single generation may require as many as 20 or as few as 5 days to reach adulthood and begin producing offspring.
CONTROL: Control of spider mites depends heavily upon an understanding of the biology of the mites. The mites are usually found on the underside of leaves. Thorough application of a miticide to the underside of the plant foliage is essential for good control. An alternative to chemical controls is a strong water spray applied to the undersides of the leaves every three days during hot weather. Miticide applications may be needed 7 to 10 days apart to kill mites that were in the egg and resting stages during the first application. In hot weather, an eye should be kept on the plants to check for reinfestation or for the offspring of mites missed on the first application.
If you choose to use a miticide, read the label carefully and apply as directed. Mites easily become resistant, so NEVER dilute the miticide — this will only ensure that future generations of mites can resist the miticides you use.
Some commonly used miticides are:
(The use of brand names and any mention or listing of commercial products does not imply endorsement by the American Rose Society, nor discrimination against similar products or services not mentioned. Individuals who use chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage and examine a current product label before applying any chemical.
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