SPIDER MITES By Dr. Richard Lindquist Department of Entomology, Ohio State University/OARDC Wooster, OH 44691 This information was originally presented in the May 1998 issue of OHIO FLORICULTURE. The following is an updated version of such article. Spider mites, especially two-spotted spider mites Tetranychus urticae (Figure 1), are among the most common pests of greenhouse crops. Other spider mite pests in greenhouses include the carmine spider mite, T. cinnabarinus and the Lewis spider mite, Eotetranychus lewisi. The Lewis spider mite (Figure 2) (Figure 3) (Figure 4)can cause problems on poinsettias. Why are spider mites such problems? There are many reasons, including small size (difficult to see), development mostly on leaf undersides (even harder to see, or hit with pesticide applications) rapid life cycle (populations build rapidly), wide host plant range (can be found nearly everywhere) and pesticide resistance (sometimes hard to kill). All of these things can contribute to their status as severe pests. The general biology of spider mites is similar, but the information below is based mostly on two-spotted spider mites. Spider mites feed by piercing plant tissue with their mouthparts and removing plant fluids. Each adult female can produce 100-200 eggs (varies with the host plant), which hatch in a few days into tiny larvae. Following a brief larval stage, several nymph stages occur before adults appear. The typical egg to adult cycle takes 7 to 14 days, but can vary considerably depending on temperatures. A diagram of the life cycle is shown in (Figure 5). All developmental stages occur on the plant. Hot and dry conditions favor spider mite development. As mentioned, most spider mites are found on the undersides of leaves. Feeding injury often gives the upper leaf surfaces a characteristic mottled or speckled appearance (Figure 3) - note that this is injury to poinsettias from Lewis spider mite feeding). Large numbers of mites produce visible webbing, which can completely cover leaves and flowers ( Figure 6 - two spotted spider mite & Figure 2 - Lewis spider mite). Spider mite populations often reach their peaks as crops mature. Because spider mites do not fly, detection is by looking at plants for signs of mites or mite feeding injury. Because even adult female two-spotted spider mites are about 1/50-inch long (and Lewis spider mites are about half that size), a hand lens is very useful and necessary. Spider mites can be controlled on greenhouse crops chemically or biologically. Conventional pesticides that usually provide good to excellent spider mite control include abamectin (Avid), bifenazate (Floramite), clofentezine (Ovation), dicofol (Kelthane) and pyridaben (Sanmite). There are mite populations that are resistant to Avid and Kelthane. Ovation acts primarily on mite eggs, so must be used before significant numbers appear.Pay attention to label directions on numbers of applications. For example, the Floramite label states that only one application should be made, followed by at least two other applications of products in different chemical classes, before once again using Floramite. The good news is that ALL of the other listed products registered for spider mite control are in different classes. Non-conventional products that can be effective against spider mites include cinnamaldehyde (Cinnamite), horticultural oil (Ultra-fine Spray Oil), insecticidal soap (M-Pede, Olympic Insecticidal Soap), andneem oil (Triact 70). The non-conventional products can be used as often as necessary without concern about rotating to different chemical classes. Within the next year or two there will be additional chemicals is still more classes that should be very useful in spider mite control. In addition to pesticides, there are some excellent biological controls available. One of the most widely used biological controls is a predatory mite, Phytoseiulus persimilis (Figure 7). Other predatory mites are also available that are more effective under certain specific environmental conditions (e.g high temperatures, low relative humidity) These predators can be obtainedfrom many commercial biological control suppliers. If done properly and under the right circumstances, biological control is as effective as chemical control.