Mites are not insects but microscopic arachnids. As such, they are more closely related to spiders and ticks than to insects, and, like spiders, most adult mites have 8 legs instead of 6; they do not possess wings. Various types of mites can be pests of protected crops.
Spider mites (Family: Tetranychidae) are sometimes referred to as ‘web-spinning mites’, as they can produce fine silken threads that can help them disperse from plant to plant on wind currents (and through accidental human contact). Spider mite infestations usually start on the underside of leaves, but when densities are high, extensive webbing can usually be seen covering both upper and lower surfaces of infested leaves, as well as the stems.
Spider mites feed in a similar way to thrips, piercing the surface cells of plant tissues (i.e., leaves, petals, etc.) and sucking out the cell contents. As with thrips feeding, this creates silvery ‘speckling’ on the damaged tissues, so in its early stages spider mite damage can resemble that caused by thrips. However, as mite densities increase, the entire leaf may be affected, eventually becoming dry, brittle and silvery-bronze or grey in color before dying and falling from the plant. Extensive leaf loss can reduce yields in some crops, and can affect the aesthetic value of ornamentals; if left unchecked, it can eventually result in the death of affected plants.
Spider mites can be found on a wide variety of plants, including many agricultural and horticultural crops. They thrive best in warm, dry, dusty conditions, and drought-stressed plants are particularly vulnerable to attack. High levels of nitrogenous fertilizer can also stimulate mite problems.
On a global scale, the two-spotted spider mite (Tetranychus urticae) is the commonest spider mite in protected cropping systems. It has been recorded on more than 200 different plant species, including tomatoes, strawberries, eggplant, cucumber, cannabis and numerous ornamentals (both woody and herbaceous). There are also other spider mite species that look similar, but which have more specialized host plant ranges.
Adult female two-spotted spider mites are approximately 0.4 mm long, with an oval body bearing 8 legs. They are typically a pale, translucent creamy color, with a large dark patch on each side of the body. However, body color is to some extent influenced by diet, and they may appear somewhat green, brown, or yellowish. Under outdoor (field) conditions, mated females (which are the overwintering stage) take on an orange or orange-red color in the autumn that persists until the following spring, when they revert to the more usual summer color(s). Adult males are smaller than females and have a body that tapers toward the rear; they are not known to overwinter successfully under outdoor conditions.
Spider mite eggs are typically laid in the webbing on the underside of infested leaves. They are spherical in shape, approx. 0.15 mm in diameter, and are initially translucent, becoming a more opaque cream color (like minute pearls) as they get closer to hatching. The immature stages (larva, protonymph, and deutonymph) resemble small adults, except that the newly hatched larvae have only six legs, and the usual dark patches on each side of the body may not be visible in individuals that have just molted.
The time taken to complete the life-cycle (from egg to adult) depends on temperature. When food supplies are adequate, it may take approximately 7-10 days at 84 °F (29 °C) and about 17-19 days at 68 °F (20 °C).
Spider mite adults and immatures
TARSONEMID MITES (Broad mites & cyclamen mites)
Globally, the family Tarsonemidae contains approximately 800 species. Of these, some are associated with fungi or bacteria, some feed on plants, and one (the tracheal mite) is a parasite of honeybees. Of the plant feeding species, only two are usually found as pests of greenhouse crops: the broad mite (Polyphagotarsonemus latus) and the cyclamen mite (Phytonemus pallidus). Both species attack a wide range of host plants.
Tarsonemid mites are very small, and difficult to see with the naked eye; usually, their presence is only noticed when feeding damage becomes apparent (described below). These small mites are readily moved around by air currents, by males carrying quiescent females to adjacent plants, and by workers handling infested plants and then clean ones. To limit their spread, consider discarding heavily infested plants and those immediately adjacent to them. Note, too, that broad mites have been known to attach themselves to the legs of adult whiteflies and can reach new plants in this way if the whiteflies are not controlled.
The predatory mites Neoseiulus cucumeris, Neoseiulus californicus and Amblyseius swirskii will attack broad mites (as well as spider mites and some other pests). However, they must be released proactively, before mite populations become well established. Commercial formulations of entomopathogenic fungi may also be useful.
Broad mite (Polyphagotarsonemus latus)
Description, recognition and life-cycle
Adult female broad mites are about 0.2 mm long, and adult males are about half that size. It is hard to see them clearly without a good dissecting microscope, and higher-powered, more specialized compound microscopes are needed to see finer details (which also involves slide-mounting the specimens).
The eggs are the easiest stage of broad mite to identify: they are approximately 0.1 mm long, rounded/oval in shape, and although translucent have a whitish appearance because their surface is covered with raised white patches: the overall pattern (when magnified) is somewhat similar to a traditional-style soccer ball.
The body of adult broad mites is broadly oval in shape and appears somewhat pointed at both ends (similar to a rugby ball or American football). The adults are typically whitish to pale amber or greenish in color, and a paler longitudinal stripe may be visible on adult females. Adults of both sexes have four pairs of legs, while the larva (the stage that hatches from the egg) has only 3 pairs. In adult females, the fourth pair of legs is thinner in diameter than the others and each terminates in a long, whip-like hair (seta); in males, the fourth pair of legs is well developed and slightly curved inwards as an adaptation for carrying immature females prior to mating (see below). Adult males are faster-moving and more active than the females.
The larval stage that hatches from the eggs is relatively slow moving, whitish, and ranges in size from 0.1 to 0.2 mm in length. After hatching, the larvae feed for 2–3 days and then develop into a quiescent nymphal stage. Males seek out quiescent female nymphs, pick them up with their hind legs and carry them (at right-angles to their own body) to new foliage. Mating occurs as soon as the adult females mature, and eggs are laid singly on developing leaves and buds (usually on the lower surface). The entire life-cycle (from egg to adult) takes 7–10 days at 50– 65 °F (10–18 °C) and 4–5 days at 70–80 °F (21–26 °C).
Host Plants and Damage
Broad mites have been recorded attacking a wide range of edible crops, although they seem to have a preference for members of the family Solanaceae (e.g. peppers, eggplants, tomatoes and potatoes); other edible host plants include cucumbers, strawberries, some types of beans, and various fruit crops. Amongst ornamentals, they have been found on African violets, ageratum, azaleas, begonias, chrysanthemums, cyclamen, dahlias, English ivy, gerberas, jasmine, impatiens, lantana, marigolds, New Guinea impatiens, peperomia, pittosporum, snapdragons, verbena, zinnias, and various others.
Broad mites are typically found in or near the growing points of host plants, where their phytotoxic salivary secretion causes distorted growth of the developing tissues. Young leaves fail to expand normally and instead tend to pucker and turn downward. Leaf margins can become brittle, and in some plants affected leaves appear tougher and darker than normal; they may also take on a purplish or bronze/coppery-red hue. The internodes on the stems tend to shorten, and affected plants may be stunted. Broad mite feeding on developing flower buds causes them to abort and drop off; lateral buds may also develop abnormally. Developing fruits may split or become deformed and/or russeted (often most severely around the stem end). In peppers, broad mite feeding can cause a characteristic twisting of stems and leaf petioles.
In some cases, broad mite damage may resemble physiological disorders, nutrient deficiencies or herbicide damage; in such cases, diagnosis can be complicated by the fact that the damage may only become apparent as the new leaves develop – which can be some time after the initial mite damage. Hence the growing points of the plant should be checked carefully for signs of live mites and eggs.
Cyclamen mite (Phytonemus pallidus)
Description, recognition and life-cycle
Cyclamen mites are similar in appearance to broad mites, but are slower-moving and slightly more elongated: adult females are approximately 0.25 mm long, while the male is about long 25% shorter. As in broad mites, the fourth pair of legs is modified in both sexes, being reduced to thread-like structures in adult females and somewhat curved in males for transporting quiescent females prior to their maturation (as described for broad mites, above).
Adult cyclamen mites are milky white to yellowish-brown or greenish, while the six-legged larval stage is whitish. The larvae are somewhat more elongated than the larvae of broad mites, and generally less active than the latter. The eggs are whitish/translucent, elongated-oval in shape (nearly twice as long as wide), and are often laid in clusters rather than singly. They can be readily distinguished from broad mite eggs by their smooth surface and the absence of raised white patches.
Host Plants and Damage
Cyclamen mite attacks many of the same host plants as broad mite, although, as its common name suggests, cyclamen are particularly susceptible to this species, as are New Guinea impatiens; strawberries are also commonly attacked.
Cyclamen mites feed in a similar way to broad mites and cause somewhat similar symptoms, including leaf distortion, twisting, discoloration, and/or curling. In some plants, affected leaves may become wrinkled, brittle, and roughened. Heavily infested plants may be severely stunted, with much smaller leaves than normal, and flower buds may abort or fail to open properly.
Cyclamen mite goes through the same life stages as described for broad mite. The time taken to complete a generation (from egg to adult) varies with temperature and humidity, but is typically about 1 to 3 weeks. At temperatures between 70 to 80 °F, cyclamen mites develop at less than half the rate of broad mites, and female fecundity is also lower (1–3 eggs per day compared to about 7 eggs per day for broad mites). At these temperatures, therefore, broad mite populations will increase much more rapidly than those of cyclamen mites.
In general, cyclamen mites prefer lower temperatures and higher humidities than do broad mites (approximately 60–72 °F, with relative humidities of 80–100%; survival is reduced when relative humidity is less than 70%. Perhaps for this reason, they tend to seek out shaded cracks and crevices in the plant canopy, often being found in the spaces between plant parts such as buds, floral structures, etc.
ERIOPHYID MITES (Russet Mites) (Family: Eriophyidae)
Eriophyid mites are extremely small (about 0.1 to 0.3 mm, with males typically being slightly smaller than females); because they are so hard to see, in most cases, their presence is only suspected once damage becomes noticeable.
These mites look very different from most other mites, having only 2 pairs of legs (rather than 4 pairs), with both pairs being located just behind the head. The latter is the broadest part of the mite, with the rest of the body being an elongated wedge-shape, giving these mites an appearance similar to miniature carrots when viewed under high magnification (as pictured). They are typically creamy-white to yellowish/tan in color.
Unlike spider mites, Eriophyids do not produce silken strands to aid dispersal, but are nevertheless capable of relatively long-distance movement by being carried on air currents. This is initiated by the mites attaching their rear-end to plant surfaces and raising themselves upright (perpendicular to the surface), greatly increasing the likelihood of escaping the boundary layer of air and being carried aloft by wind currents.
Most Eriophyid mites are associated with woody perennial hosts (trees and shrubs), but some species (including the two discussed below) use herbaceous hosts. Eriophyids, like most plant-feeding mites, pierce the cells of the host plant and suck out the cell contents; most species are highly host specific, and their host plants often react to compounds in their saliva by forming small, blister-like galls of characteristic shape, size, and color (usually on the leaves or developing flower buds). For gall-forming mites, these modified tissues form protective and highly nutritious feeding sites.
The so-called russet mites, however, live exposed on the plant surface, and instead of eliciting gall formation, cause distortion and discoloration of the leaves, stemsand (sometimes) fruits of their host plants. The two most damaging russet mites in indoor crops are the tomato russet mite (Aculops lycopersici) and the hemp russet mite Aculops cannabicola). Both species go through four life-stages: egg, 2 nymphal stages (sometimes referred to as larva and nymph), and adult.
Tomato russet mite (Aculops lycopersici)
As its common name suggests, this species can be a damaging pest of tomatoes; it will also feed on other members of the Family Solanaceae (e.g. pepper, eggplant, tomatillo and potato), but rarely causes severe damage in these crops.
Russet mite problems may be initiated through contaminated transplants, through accidental transfer by staff or equipment, or by wind-borne individuals entering through air vents, doorways, or similar openings. The presence of solanaceous weeds such as nightshades (Solanum species) around the growing facility increases the latter risk, particularly during warm, dry periods in mid- to late-summer, when external weeds start to dry out and die.
Damage to tomatoes is often first noticed around the base of the plant; if not controlled, the basal tissues deteriorate and the mites move upwards towards younger foliage, causing characteristic damage (described below) as they go. If damaged leaves are noticed, the greener foliage and stems immediately above them should also be examined for live mites, as they may already have moved upwards. If mite populations near the top of the plant become very dense and crowded, the mites may facilitate their own dispersal on wind currents, as described above.
Tomato russet mites feed and lay their eggs primarily on the under-surface of the leaves, on stems, and on leaf petioles. The two immature stages usually feed close to where they hatched, and tend to aggregate along the leaf margins, which may curl upwards (a symptom sometimes known as ‘taco leaf’). Affected stems become dark and discolored, and if developing fruits are attacked, the surface may become rough, dry and cracked. Infested leaves take on a bronze- or rust color before drying out and dropping from the plant; if leaf loss is severe, the plant may die.
Depending on temperature and humidity, the life cycle (egg to adult) can be completed in approximately 7 days, with adult females living about 3 weeks and producing 30–50 eggs in that time; survival is reduced by temperatures consistently higher than 30 °C.
If not detected and suppressed, russet mites have the potential to increase very rapidly. Close attention should be paid to seedlings and cuttings to ensure that all are mite-free prior to planting. Drought stress can also increase the susceptibility of tomato plants to A. lycopersici.
Some predatory mites (e.g. Amblyseius swirskii and A. andersoni) are known to feed on tomato russet mite, but must be introduced in a very proactive manner as soon as the pest is first detected. They are unlikely to be successful at suppressing populations that are already well established unless some remedial action is taken first.
Hemp russet mite (Aculops cannabicola)
Hemp russet mite is currently the only known Eriophyid mite associated with Cannabis sativa, and it is thought to be restricted to this host. It feeds primarily on the leaves, leaf petioles and growing points, and can cause severe stunting of young seedlings and cuttings.
On foliage, it is initially found mainly on the underside of the leaf, where It’s feeding causes curling of the leaf margins (upward and/or downward), followed by discoloration (russeting). When populations are very high, affected leaves may appear a coppery/reddish bronze and the mites may spread to the upper surface of the leaves, petioles, stems and any developing flower buds (of both sexes). It is also thought to feed on the glandular trichomes, with the consequent potential to reduce the yield and quality of resin production.
Hemp russet mite goes through the same developmental stages as tomato russet mite; the time taken to complete its life-cycle varies with temperature and humidity, but can take as little as 7 days under optimal conditions.
Once established, control can be difficult. In indoor facilities, prevention is key: new transplants and cuttings should be isolated from the main growing areas and closely inspected prior to plant to ensure that they are mite-free. Since Eriophyid mites can be carried on air currents, consider the air-flow through the facility, and try to arrange plantings such that air flows first to young plants rather than to older ones. Similarly, employees should avoid moving from areas with active mite populations to uninfested zones.
Biological control options for established russet mite populations are limited, although products based on the pathogenic fungus Beauveria bassiana may help. The predatory mite Amblysieus andersoni is known to feed on hemp russet mites, but must be introduced in a very proactive manner as soon as the pest is first detected. It is unlikely to be successful at suppressing populations that are already well established.
A close-up view of a dense population of hemp russet mite can be viewed here.
Tomato russet mite (Aculops lycopersici)