Mites
MITES FOUND ON AFRICAN VIOLETS
More than twenty-five different species of mites have been recorded from African violets. Fortunately, most of these are not injurious to the plant. Mites collected from violets fall into four dis tinct groups. These are scavengers, fungus feeders, predaceous mites, and plant feeders.
SCAVENGERS: The most common mites found on violets are scavengers. They feed on decaying organic matter. Actually, they infest the soil in which the violet is growing and not the plant itself. However, one may occasionally be found roaming around on the foliage. These species may be milk- white, yellowish, or brownish in color. Some are relatively large and easily seen with the naked eye. Magnification does help to identify them as being mites. When conditions are favorable, these mites may build up very large populations. They do not harm the plant, and no control is necessary. If pop ulations become large enough to be a nuisance, an approved acaricide applied to the soil surface should give adequate control.
FUNGUS FEEDERS: Several species of fungus feeding mites have been collected from violets. Mites which feed on fungi are occasionally found on the foliage but are most often found in or on the surface of the potting soil. Fungus feeding mites are usually not found in very large numbers. They do not harm the plant, and no control is necessary.
PREDATORS: Predaceous mites feed on many small organisms including insects such as thrips, soil mealybugs, and collembola. They also feed on other mites and are often found on plants infested with injurious species. Predaceous mites are more active than plant feeding species and may be detected running about the plant in search of prey. Predaceous mites are beneficial to the violet grow er, as many times they destroy harmful pests before populations become large enough to cause visible damage.
PLANT FEEDERS: Mites which actually feed on violet plants may cause severe damage or even kill the plants. These pests feed by inserting their mouthparts into the plant tissue and sucking out plant juices. Damage results from a loss of plant sap and from the injection of toxic chemicals into the plant. Several species of mites have been recorded as damaging African violets. These mites are extreme ly small and are seldom observed on plants before they cause visible damage. One who cultures vio lets should become familiar with the habits of dam aging species and the early symptoms that infestations produce in plants. This knowledge will enable growers to detect and control damaging species before significant damage can occur.
CYCLAMEN MITE, Steneotarsonemus pallidus'. The cyclamen mite is the most devastating mite attacking African violets. Its ability to increase numbers rapidly, its detrimental effects upon the plant, and the difficulty in controlling this mite make it the most feared of all violet pests. The cyclamen mite is found throughout the world. It was first reported in the U.S. in 1898 (in New York) and in Canada in 1908. At the present time, it is distributed throughout the U.S. The increase and spread of the cyclamen mite has been facilitated by the increased popularity of foliage plants and the resultant movement of plant materi als throughout the continent.
Description and Identification
The cyclamen mite is about 1/100 of an inch long when full grown. It is almost impossible to detect without the aid of magnification. Cyclamen mite eggs, although microscopic in size, are relatively large, being 1/2 the size of the adult itself. The eggs are oval in shape, glossy white, and are unmarked. Immature mites are milk-white in color and are shiny in appearance. The pupae are white in color and are inactive. Adult mites are oval in shape and are amber to tan in color. They are somewhat translucent, and they glisten as if wet or oily.
Life Cycle and Habits Cyclamen mites develop most rapidly at a temperature of 16 degrees C (60.8 degrees F) and at a humidity of 80-90 percent. Their eggs hatch in three to seven days, depending upon temperature. The immature or larval stage feeds on plant tissue for one to four days before molting to the pupal stage. Pupae develop and molt to the adult stage in two to seven days. A complete life cycle may be completed in one to three weeks. There are reports of the life cycle taking as long as seven weeks under colder conditions. Female cyclamen mites live for about one month. They lay from one to three eggs per day and may produce more than 100 eggs during their lifetime.
The improvement of the environment provided by a home or greenhouse enables cyclamen mites to multiply and develop year-round. Natural popu lations, exposed to the environment, overwinter as adults. The adults begin to lay eggs in the spring. A peak in population usually occurs in
June or July. In greenhouses very heavy populations often occur in the fall. Cyclamen mites avoid light. They seek out the protected area of the plant crown or folds in the leaf where it joins the petiole. Varieties with ruffled or quilted foliage provide more protection and may be susceptible to higher mite populations. Cyclamen mites have a very wide host plant range. They readily attack many species of plants used as shrubs or house plants. A number of the most common plants attacked by cyclamen mites are: African violet, Begonia, Chrysanthemum, Cyclamen, Dahlia, Delphinium, Geranium, Gerbera, Ivy, Larkspur, Petunia, Pikake, Snapdragon, Strawberry and Watercress.
Damage
African violets attacked by cyclamen mites exhibit specific characteristics. These characteris tics are generally evident long before the mite itself can be found. Damage occurs when mites insert their sucking type mouthparts into tissue and remove plant sap. During the feeding process, mites inject a toxic chemical into the plant. This chemical disrupts the normal growth pattern of the plant.
Symptoms of Infestation
Light infestations may cause blasting (the shed ding of small flower buds). Leaves may twist, curl, and become brittle. Flower buds may fail to open and, if they do open, they may be undersized, deformed, or have the colors streaked. Heavy infestations can result in the death of leaf and flower buds. Leaflets may appear excessively hairy and may soon die. The terminal of the plant may appear "fuzzy” and then may be killed (Fig. 8). If heavy infestations are neglected, they can kill the plant, or at least the center of the plant. After infestations have been controlled, plants may continue to exhibit certain symptoms for a considerable length of time (Fig. 9). These include the twisting and curling of leaves, undersized blooms, and the streaking of the colors of the flowers. These symptoms are an expression of one or both of two different phenomena. First, this may be the result of growth - distorting chemicals which have been injected into the plant by mites.
Second, in the case where the plant terminal has been destroyed, the symptoms result from a hor monal imbalance from the loss of auxin (a hor mone found in young growing tissue). The destruction of auxin results in the production of ethylene. Ethylene, in turn, causes epinasty or leaf curling. Plants will fully recover from cyclamen mite damage in time. After proper pruning of deformed growth, plants will appear perfectly normal.
Infestation and Spread
Considering the wide host range, it is impossible to isolate your plants and yourself from any possi ble contact with these mites for any length of time. More than likely, our plants are exposed to mites periodically. However, most of the time conditions are not favorable for the mites, and they either die off or they continue to exist in very low numbers until conditions are optimum. Then, there is a population explosion, and mites begin to produce symptoms on our plants which we can detect. How do our plants contract mites? Although mites are mobile, they do not move very far and are most often transferred from plant to plant by direct contact. Under heavy infestations mites can spread rather rapidly through a culture by simple random movement across benches and tables. As cyclamen mites are so small as to go undetected by the human eye, they can get a free ride from plant to plant, bench to bench, or greenhouse to greenhouse on human hands, tools, pots, other equipment, clothing, and sometimes on the fur of our pets.
Prevention and Cultural Control
Infestations may often be avoided by close inspection of all plants prior to purchase. The isola tion of all newly acquired plants for a period of time is an added precaution against spreading mites. The normal sanitary practices of culturing violets, such as the removal of spent and damaged plants, the use of clean equipment, and the frequent inspection of plants also aid in preventing the intro duction and spread of mites. Plants within a culture should be inspected fre quently for mites and symptoms of mite damage, especially suspect plants. Examine the blooms, buds, plant terminal (where petiole connects to crown), and the junction of the leaf blade with the petiole. These are favorite habitats for mites. Any plant with an abnormal growth pattern sucl twisting petioles or curling leaves should be gi special attention. It is not necessary to destroy a rare plant o refrain from purchasing a plant needed for y collection because it is suspected as having a n infestation. A grower should not panic when mi are found in a culture. Cyclamen mites can be c trolled. We must familiarize ourselves with proj control techniques and with the early symptoms mite infestations. Then we can obtain and maint; control of the pest. Heat may be used to control cyclamen mites, i stages can be killed with heat. For effective contn immerse infested plants with pot and soil, for f teen minutes in water held precisely at 110 degre< F (43 degrees C). A heat chamber saturated wit water vapor may also be used effectively. Expos infested plants in a heat chamber at 110 degrees (43 degrees C) for two hours.
Chemical Control
A number of acaricides are labeled by the EPA for the use of controlling mites on African violets Certain compounds require multiple applications a four to five day intervals to get control. Control may be achieved with only one application of cer tain other compounds. When using an acaricide, it should be applied as soon as an infestation is detected. Any delay increases the possibility of damage dramatically. Disbudding enhances control considerably, as mites often hide within a bud, and the acaricide does not come into contact with them. Due to the habits of mites, thorough coverage is necessary if control is to be achieved. Special attention should be given to the crown of the plant and to the junction of the leafblade and petiole. If a few mites are left untreated, resurgence of the population may occur.
BROAD MITE, Polyphagotarsonemus latus\ The second most important mite attacking African vio lets is the broad mite. This species is also called the yellow tea mite and the tropical mite. It is dis tributed throughout the tropics and is found in greenhouses throughout the temperate region. The broad mite readily attacks about fifty plant species including a number of agricultural crops, wild plants, and ornamentals. Ornamental hosts include gerberas, dahlias, zinnias, chrysanthe mums, as well as African violets.
Broad mites are colorless when young but become amber to dark green when grown. In greenhouses reproduction continues all year. Females lay four to five eggs each day. A genera tion may be completed in four to five days in the summer or in ten days in the winter. Damage from the broad mite is found almost exclusively on the underside of the older leaves. As this pest feeds, it injects toxic chemicals into the plant which cause leaves to become brittle and to roll or curl under at the edges. As damaged leaves continue to grow, they may split or crack, resulting in a ragged appearance. The lower surface of dam aged leaves may have a bronzed appearance. Damage to flowers results in distorted or discol ored petals. Heavy infestations stop growth and kill the plant. Broad mites are not as difficult to control as cyclamen mites. This species may be controlled with a heat treatment such as used for controlling the cyclamen mite.
PRIVET MITE, Brevipalpus obovatus: The privet mite belongs to a group of mites called the “false spider mites”. This mite is a pest of citrus and is found on more than 50 genera of ornamental plants. Privet mites are very small and difficult to see without the aid of magnification. They are flat, oval, and dark red in color. This mite is found on the underside of leaves and likes to gather along the leaf veins. When heavy populations occur, they can be found throughout the entire leaf. Damage from this mite may appear as small brown flecks on the foliage. Each fleck is actually a small group of dead cells killed by the toxic chemi cals injected into the leaf. The underside of infested leaves may appear bronzed while the upper leaf surface may show a reddish discoloration. The life cycle of this mite varies with tempera ture and may range from about eleven days at a temperature of 32 degrees C (89.6 degrees F) to twenty-six days at a temperature of 20 degrees C (68 degrees F).
OTHER MITES reported as damaging African vio lets include Brevipolpus phoenices, a species which is found on the underside of the foliage and causes chlorotic spotting of the foliage and stunting. The two-spotted spider mite, Tetranychus urticae, is the most common mite found on ornamentals and house plants. However, it does not attack violets frequently. These mites are red or greenish in color. Damage from this species appears as speck led or yellowish areas on the leaves.