Spider mites need `soft' insecticides

Carrot insect controls include monitoring after application Spider mites are common pests on many woody and herbaceous plants. Usually spider mites are not a problem on carrots, however on occasion they may be cause for concern.

Spider mites are nearly invisible to the naked eye, but they can cause economic damage to many crops, including carrots, when present in large numbers.

Spider mites have piercing mouthparts that are used to pierce and rupture the epidermis of cells and suck up the contents of the cell. Over time the leaves develop a stippled appearance from the feeding and killing of individual leaf cells. In heavy infestations the leaves will eventually become bronzed, silvery, or yellow in appearance. Initially, the undersides of the leaves will have stands of silken threads across them.

As the infestation becomes more severe, the entire canopy becomes covered in very fine silk webbing. Examining the underside of the leaves will reveal very small moving dots, which are the spider mites themselves. In dusty conditions, the foliage becomes noticeably dirty as the dust clings to the web covered leaves. Eventually the plants lose vigor and the leaves begin to defoliate.

Spider mites are arachnids and are more closely related to spiders, ticks, daddy-long-legs, and scorpions than to insects. Web-spinning spider mites belong to the tetranychid family of the order Acarina. The most common spider mites are closely related and belong in the genus Tetranchus. There are dozens of species of spider mites that damage plants. Distinguishing the various species of spider mites is difficult, but fortunately is usually not necessary for control because their biology is virtually identical.

Tiny moving dots To the naked eye, spider mites appear only as tiny moving dots. With the aid of a hand lens however, some distinctive features can be seen. The adults have eight legs with an oval body. The body is translucent with many long bristles and various black spots on the back or sides. Two bright red eyespots are located in the front portion of the body. The color of the various spider mites can range from pale yellow to light green to brown to orange. The adult female mites are larger (0.25 mm) than the males with rounded abdomens while the males are smaller with a pointed abdomen.

Most spider mites reproduce best in warm weather, although some species of spider mites become inactive in warm weather. During warm weather, a generation may be completed in one week.

Mites have several natural enemies that help keep their population below damaging levels. Natural enemies of spider mites include several predatory mites (Metaseiulus occidentalis, Amblyseius spp., and Typhlodromus spp.). Six-spotted thrips, spider mite destroyer beetle, many fly species, minute pirate bug, big-eyed bug, and lacewings are all beneficial insects that prey on spider mites.

The use of broad spectrum insecticides may kill the beneficial insects, which then allow the mite population to increase to damaging levels. When insecticides are required, insecticides that are softer on the beneficial insects should be considered. Spider mite populations should be closely monitored after any insecticide application has been made to determine if an outbreak of mites may follow.

Sulfur dusts, insecticidal soaps, and oils are other mite suppressing materials that may be used on carrots. Outbreaks of spider mites usually occur in localized areas in a field, therefore spot treatments may be all that is necessary.

Outbreaks of spider mites are more likely to occur on water stressed plants. Dusty conditions also stimulate spider mite outbreaks. Properly irrigated fields and avoiding dusty conditions are important in management practices for spider mite control. Sprinkler irrigations have also been reported to suppress mite populations.

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