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Two-Spotted Spider Mites

May 20, 2013

Most commercial growers are already all too familiar with two-spotted spider mites (Tetranychus urticae). The two-spotted spider mite is one of the most challenging pests growers contend with.

Spider mites are not actually insects; they are related to spiders and ticks as they have eight legs, two body regions, and lack antennae.  The most distinguishing characteristic of the two-spotted spider mite is the two dark spots on each side of its body.  All life stages have eight legs except for the larval stage, which has only six.  The adult females and most immature stages are oval shaped, usually light yellow to green in color, and have two large dark green spots on either side of their body.  

Female spider mites lay tiny round white eggs on the underside of the leaves.  Depending on temperature, these eggs hatch into tiny 6-legged larvae in as little as three days which pass through several 8-legged nymphal stages before becoming adults.  The typical lifecycle from egg to adult takes 7 to 14 days, but can vary considerably with temperature.  

Two-spotted spider mites are difficult to detect because they are so small, measuring 1/50 inch or less than 1 millimeter in size.  In some instances, it requires the use of a 10 to 20X hand lens to be able to see and identify these mites.  When the mite populations are low, they are most commonly found under the lower surfaces of the leaves.  With severe infestations, they can be observed on all above ground plant parts which are often covered with the characteristic webbing of the mites, which is why they are referred to as spider mites. Early detection of two-spotted spider mites is essential and allows growers to treat isolated areas before they spread throughout an entire crop.  

The most common injury symptom caused by their piercing, rasping mouthparts is the presence of numerous small pinpoint sized spots, creating a mottled or speckled appearance on the upper leaf surface.  Heavy infestations may lead to severe chlorosis and leaf drop.

There are several biological control agents (BCAs) which are effective at controlling two-spotted spider mites.  Predatory mites (Amblyseius californicus, A. andersoni, and Phytoseiulus persimilis) are used the most frequently; however, the predatory midge (Feltiella acarigusa) is another viable option for controlling spider mites.  It is usually necessary to release BCAs before the spider mite population becomes high and will require multiple releases throughout the production cycle to provide suitable control.  When done properly, controlling spider mites biologically can be just as effective as using chemical strategies.

There are numerous miticides available for controlling spider mites. Ovicides (clofentezine and hexythiazox) primarily act on mite eggs and very young larvae and are best used when mite populations are low.  Several products containing the active ingredients abamectin, acephate, bifenazate, chlorfenapyr, etoxazole, fenpropathrin, fenpyroximate, and spiromesifen are effective at controlling various life stages of the two-spotted spider mite.  

As the crop canopy closes in, it becomes increasingly difficult for growers to control mites using miticides since they are located almost exclusively on the undersides of the leaves where it is hard for spray applications to make good contact with them.  Several miticides (abamectin, chlorfenapyr, or etoxazole) have translaminar activity. Translaminar products move through the leaf from the upper surface to the lower surface where the mite population feeds, increasing the efficacy of these applications.

To broaden the life stages controlled, many growers tank mix various miticides when making their applications.  It is particularly important to rotate the chemical families of the miticides being applied to prevent resistance to these chemistries.  

Regardless of the chemicals being applied, it is very important to ensure good coverage of the upper and lower leaf surfaces to obtain effective control of spider mites. In most instances, it will be necessary to make multiple spray applications to obtain adequate control of mite populations.

Several miticides have minimal impact on predators which allows growers to combine both biological and chemical control strategies.  Refer to each products label to determine which life stages they control and their impact on biological control agents.

Paul Pilon
Perennial Solutions Consulting
paul@perennialsolutions.com

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