SPIDER MITES

a, Spider mites (natural size) on under side of leaf showing typical type of injury; b. adult and young (40 times natural size); c, leaf showing both spider mite injury and potash deficiency.

SPIDER MITES

Spider mites are so small that they can hardly be seen without a magnifying glass. At least seven species are known to attack cotton. They may be greenish or yellowish in color, but the females are usually reddish to carmine and the smaller males reddish yellow. Spider mites multiply rapidly. There may be as many as 17 generations a year. Hot, dry weather is most favorable for rapid multiplication. A heavy rain often checks an outbreak. They are found throughout the Cotton Belt. They feed on almost 200 kinds of plants, including many garden and field crops, ornamentals and weeds. In the South they pass the winter on leaves that remain green, such as wild blackberry, Jerusalem-oak, wild vetch, and violet. They move to cotton early in the summer. When cotton is no longer suitable for food they return to weeds or other plants. They crawl on the ground and are carried by wind or rainwater.

Spider mites live on the under side of the leaves, where they lay their eggs and spin delicate webs. They suck the sap from the leaves causing the under surfaces to become thickly dotted with whitish feeding punctures. Spider mite injury, often called rust, is first indicated when blood-red spots appear on the upper surface of the leaves.

The entire leaf then reddens or turns rusty brown (as in the case of potash deficiency), curls, and drops from the plant. The loss of leaves causes shedding of small bolls and may prevent the lint from developing properly in large bolls.

Control: The spread of spider mites to cotton may be prevented by destroying weeds around the fields and by controlling the pest on dooryard plants. An infestation can often be stamped out by pulling out and destroying the first few cotton plants that become infested. Dusting cotton with finely ground sulfur at the rate of 10 to 25 pounds per acre is the most practical direct-control measure. A second application a week later is necessary to kill the spider mites that hatch after the first application. The under side of the leaves should be covered thoroughly with the dust.

In areas where spider mites are a pest, dust mixtures of organic insecticides used against cotton insects should contain at least 40 percent of sulfur, or 1 percent parathion, or some other organic phosphorus compound, to prevent spider mite increase.

TEPP at the rate per acre of 0.5 pint of 40 percent concentrate in a spray, or its equivalent, effectively controls heavy populations of spider mites.