Sometimes things are not always as they appear to be. This is certainly true of spider mites. Even though they may look like insects, they’re not. As such, whenever we talk about damage from insects on trees, shrubs and other ornamentals, we rarely consider spider mites. Knowing a little about spider mite biology and the best methods of control can be a great help in the quest to keep your customers’ trees healthy.
Mites versus insects
Spider mites are more closely related to spiders than they are to insects. These tiny arthropods have two body segments and as adults four pairs of legs. Insects, on the other hand, have three body segments and three pairs of legs. The first developmental stage of a spider mite is called the larva and has only three pairs of legs. Spider mites undergo gradual metamorphosis. The life cycle consists of the egg, larva, protonymph, deutonymph and adult stages.
Overwintering adults possess a characteristic reddish or orange color. In the spring, mites move from the crowns of the leaves of overwintering hosts and begin feeding and laying their eggs. Spider mite eggs are small, round and pearly white, and are usually deposited on the undersides of leaves. Eggs hatch in three to four days, and the six-legged larvae begin to feed. A generation generally takes seven to 10 days, with as many as seven to 10 generations per season. As the summer progresses, spider mites can become a problem on many landscape ornamentals such as spruce, arborvitae, hemlock, juniper, yew, some pines, holly, azalea, euonymous, oak and honeylocust as well as many bedding plants.
Spider mites use piercing, sucking mouthparts to extract cell sap from leaves or needles. They generally feed in colonies within fine webbing on lower surfaces. This webbing serves to transport mites from one leaf or needle to another, and to protect them from natural enemies. Mite damage is characterized by a general lack of plant vigor and a silvery or pale yellow discoloration of needles or leaves. Infestations are more common in areas associated with drought stress such as near buildings, especially on southern exposures, and along sidewalks, driveways and parking lots.
Controlling spider mites
Effective control of spider mites should begin with a thorough inspection of the plant to determine the location and extent of the infestation. For some trees, especially large ones, small pockets of mite colonies may have developed in several locations. In these situations, only the infested areas may need to be sprayed, rather than the entire tree.
Inspection for mite infestations is relatively simple, even though these critters are very small. The first step is to place a white or buff-colored piece of card stock under a branch with suspected infestation. Step two involves firmly but gently tapping the branch with your hand or small stick. Step three is to closely examine the card stock; if small greenish dots are seen moving around, it’s possibly spider mites. At this point, use your hand lens to distinguish between mites and other organisms (the moving dots could also be scale crawlers) that may be present on the tree. If you happen to be a fan of the television show “Pawn Stars,” you may prefer to use a large magnifying glass in a manner similar to Drew Max, the in-house forensic document examiner. A less precise, but equally effective version of step three involves dragging a finger across the card stock and if a smudge appears, then it’s probably wise to take a closer look.
Once you know what to look for, it’s important to set up a routine for inspecting landscape trees. Actually, better terms for this procedure are “monitoring” or “scouting,” which implies a regular set of individual inspections. As you interact with clients, impress upon them the need to establish a regular scouting program to spot small populations before they become full-blown infestations. In short, scouting should be frequent and thorough. Depending on your area of the country, scheduling a visit every two to four weeks would be appropriate. The second part of scouting, of course, is getting paid for it. Scouting is a valuable pest control procedure and requires a specialized skill set, and as such it should be invoiced as part of your total tree care package.
If upon inspection the spider mite infestation exceeds the damage threshold, an insecticide/miticide application(s) may be necessary. This is especially important for signature, sentimental or historic trees. As with most insect and mite pests, applications made early on when colonies are still small are generally more effective. Also, because most spider mite species feed on the undersides of leaves and many live within extensive webbing, getting the pest control product to the mites can be difficult. Applications made to upper surfaces only are generally not as effective. In situations where large trees are involved, or ones that are in close proximity to cars, buildings and human traffic, trunk injections may be an option. When choosing an injection technology and products, select an approach that causes as little wounding to the tree as possible, and has a demonstrated track record for even and thorough distribution throughout the tree canopy.
Products shown to be effective for spider mite control include abamectin, acephate, bifenthrin, bifenazate, dicofol, etoxazole and hexythiazox. Insecticidal soaps and horticultural oils can also be effective, but should be used with caution, especially during periods of drought or when tender new foliage is present. Especially with oils, avoid spray applications during periods of high daytime temperatures. Normally, horticultural oils would also work well on a surface-feeding pest like this, but they can cause damage to the waxes that create the desirable blue color on many spruces, so it’s not a recommended control measure. As with any pest control product, always read and carefully follow label instructions. Helpful information, such as the need for a surfactant or other adjuvants, pH adjustment, retreatment intervals and rate, will be provided therein.