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THE TWOSPOTTED SPIDER MITE
The twospotted spider mite, Tetranychus urticae Koch, is the major spider mite pest of ornamental plants and vegetable crops grown in greenhouses. Furthermore, this ubiquitous spider mite is a serious pest of numerous ornamental plants in home landscapes, and is of considerable importance as a pest of food and fiber crops throughout the world. The literature on spider mites in general, and the twospotted spider mite in particular, is voluminous; however, much of the pertinent information (including references) has been summarized by Huffaker et al., (1969, 1970), McMurtry et al., (1970), van de Vrie et al,. (1972), Jeppson et al., (1975) and Hussey and Huffaker (1976).
Contrary to popular belief, mites (including spider mites) are not insects. Although both insects and mites belong to the Phylum Arthropoda (because of their jointed appendages and exoskeletons), there are major differences between the groups. Adult mites characteristically possess 4 pairs of legs compared to 3 pairs in insects (larval mites do however possess 3 pairs of legs). Virtually all adult insects have 1 or 2 pairs of wings. However, neither of these features is found in adult mites. Whereas mites belong to the Class Arachnids which also contains the spiders, insects belong to the Class Insecta. All mites occupy the Subclass Acari (=Acarina).
The twospotted spider mite is a member of the family Tetranychidae which contains many harmful species of plant-feeding mites. There has been considerable confusion concerning the nomenclature (i.e., scientific name) of the twospotted spider mite. In the past, acarologists and applied entomologists commonly referred to the spider mites in question as Tetranychus bimaculatus Harvey or T. telarius (Linnaeus). Boudreaux (1956) examined the so-called "Twospotted spider mite complex" and demonstrated that more than one species was involved. In this case, the major species were the twospotted spider mite, Tetranychus urticae Koch and the carmine spider mite, T. cinnabarinus (Boisduval) (Boudreaux, 1956: Boudreaux and Dosse, 1963: Jeppson et al., 1975). Common names such as red mite, red spider mite, glasshouse spider mite, twospotted spider mite, and common spinning mite generally referred to the species complex; the same is true for the approximately 60 synonyms (i.e., scientific names) in the literature, the most common ones being T. telarius (Linn.) and T. bimaculatus Harvey (Boudreaux and Dosse, 1963; Jeppson et al., 1975).
In both male and female twospotted spider mites, development proceeds through the following stages: egg, larva, protonymph, deutonymph, and adult. The larval, protonymphal, and deutonymphal stages are further divided into feeding (active) and quiescent (resting) stages. The quiescent stages are referred to as nymphochrysalis (=protochrysalis), deutochrysalis, and teliochrysalis for larval, protonymphal, and deutonymphal stages, respectively. Thus, development of the twospotted spider mite can be summarized as follows: egg, larva (including nymphochrysalis), protonymph (including deutochrysalis), deutonymph (including teliochrysalis), and adult (Laing, 1969; van de Vrie et al., 1972).
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(Figure 1) Eggs (lower portion) and male (upper portion) of the twospotted spider mite.
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(Figure 2) Eggs and larva of the twospotted spider mite (on left) and egg of P. persimilis (right).
Females normally lay eggs (oviposit) on the undersides of leaves. According to Cagle (1949), the spherical egg is about 0.14 mm in diameter (Figure 1). The newly deposited egg is clear, but turns opaque and glassy as incubation progresses. Just before hatching, the egg is strawcolored and the carmine "eyespots" of the embryo become visible (Cagle, 1949). The larva (Figure 2) has 3 pairs of legs (hexapod). At the time of hatching, it is colorless, except for the carmine eye spots. After feeding, its color changes to pale green, brownish green or very dark green and two dark spots appear in the mid-portion of the body (Cagle, 1949). At the end of the feeding stage, the larva attaches to the substrate (i.e., leaf), becomes quiescent (nymphochrysalis), and is later transformed into a protonymph. The protonymph has 4 pairs of legs (octapod) and is somewhat larger than the larva. Its color is usually pale green to dark green and the two spots are larger and more pronounced than in the larva (Cagle, 1949). At the end of the feeding stage, the protonymph attaches to the substrate, enters the quiescent stage (deutochrysalis), and is later transformed into a deutonymph (Figure 3).
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(Figure 3) Quiescent nymphs of the twospotted spider mite.
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(Figure 4) Deutonymph (left) and mature female (right) of the twospotted spider mite.
The octapod deutonymph is generally larger than the protonymph (Figure 4), although similar in color pattern. At this stage, the males can usually be distinguished from the females because of the smaller size and wedge-shaped posterior of the former (Cagle, 1949; Laing, 1969). Following cessation of feeding, the deutonymph attaches to the substrate (Figure 5) and becomes quiescent (teliochrysalis). The octapod adult (Figure 6) eventually emerges from the teliochrysalis.
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(Figure 5) Male twospotted spider mite "guarding" quiescent female deutonymph.
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(Figure 6) Mature female/Twospotted spider mite.
Developmental time of the twospotted spider mite will generally vary with conditions such as temperature, humidity, host plant, leaf age, etc. However, temperature is the most important factor that influences the rate at which mites develop. The lower threshold for development is about 12°C (53.6°F), whereas maximum upper limit to the development is about 40°C (104°F) (Jeppson et al. 1975). Laing (1969) maintained the mites on strawberry leaflets at an average hourly temperature of 20.3°C (68.5°F) and relative humidity fluctuating from 55% to 98%. Under these conditions, the mites developed from egg to adult in an average of 16.5 days. Shih et al. (1976) cultured the mites on lima beans at 27°C (81°F) and 90(±)5% relative humidity. In this case, mites developed from egg to adult in an average of 7.6 days. Sabelis (1981) determined the developmental time required for an egg to develop to a female capable of laying eggs. In his studies, he reared the mites on detached rose leaves under two alternating day/night temperature regimes. The regimens studied were 25-35°C (77-95°F) and 10-20°C (50-68°F) for which he determined the developmental times to be 8.3 and 28.2 days, respectively. Additional aspects of developmental time are summarized in Table 1: in the next document, Phytoseiulus persimilis.
In a given colony of twospotted spider mites, both adult males and females can usually be found; however, females are normally about three times more abundant than males. Cagle (1949) provided an account of the characteristics of males and females. The male is much smaller and is considerably more active. The body is narrow and distinctly pointed posteriorly (Figure 5). The color of the male varies from pale to dark green, brownish, or at times, orange. The body of the female is oval-shaped and rounded posteriorly (Figure 6). Color of the females varies from light yellow or green to dark green, straw color, brown, black and various shades of orange. There are generally two large black spots one on either side of the body, hence the common name. However, there can be considerable variation in the expression of this particular character.
Generally, adult males can be found in close association with quiescent female deutonymphs (Figure 5). Evidence indicates that the quiescent female deutonymph releases a sex pheromone which attracts the male and keeps him in close proximity (Cone et al., 1971a, 1971b; Penman and Cone, 1972, 1974). The male usually remains in the immediate vicinity of the quiescent deutonymph and mates with the emergent female. When more than one male attempts to "guard" a developing female, fighting among the males often occurs; usually, larger males win these encounters (Potter et al., 1976a, 1976b). Such fights involve pushing and grappling with the forelegs, jousting with the mouth parts and entangling the opponent with silk.
The life span of the adult female is divided into the preovipositional period and the ovipositional period, the former being the time between emergence from the teliochrysalis to the deposition of the first egg. Apparently, the preovipositional period (9% of the time required to develop from egg to egg) can last less than 0.5 day and as long as 3 days depending on temperature. The period during which eggs are deposited (ovipositional period) can last from 10 days at 35°C (95°F) to 40 days at 15°C (59°F) (Sabelis 1981). An individual female can deposit over 100 eggs in her lifetime (Shih et al., 1976; Carey and Bradley 1982). The total number of eggs laid/female and the eggs laid/female/day can, however, vary with age, temperature, species of host plant, relative humidity, nutrition of host plant, exposure to pesticides, etc. (Watson, 1964; van de Vrie et al., 1972 Karban and Carey, 1984). Temperature and age of the female are especially important determinants of egg production (fecundity). However, Sabelis (1981) determined that fecundity was affected very little at temperatures between 20-35°C (68- 95°F). In his study, peak oviposition, (161 eggs/female) occurred at a temperature of 25°C (77°F), with the maximum rate (12 eggs/female/day at 25°C (77°F)) occurring 2 days after the first eggs are laid. The effect of temperature is particularly evident in greenhouses, where spider mite populations often develop rapidly soon after the onset of summer temperatures.
Sex determination in twospotted spider mites (as in many other spider mites) is arrhenotokous. That is, females develop from fertilized eggs and have the normal two sets of chromosomes (diploid); on the other hand, males develop from unfertilized eggs and have only one set of chromosomes (haploid). Unmated females give rise only to males; mated females can produce either female or male progeny. According to Helle (1967), a single mating will suffice to provide a female with enough sperm to produce diploid eggs for her entire ovipositional period.
The phenomenon of arrhenotoky is of importance not only from an academic standpoint, but also from a practical one. Because the male has only one set of chromosomes, new genetic features (arising from mutations) will be immediately expressed. Through natural selection, these characteristics can be added quickly to the population (Helle and Overmeer, 1973). Therefore, the potential for development of genetic resistance to insecticides and miticides in the twospotted spider mite is greatly enhanced by this method of reproduction. Because of the high reproductive rate and fast generation time and the intense selection pressure brought on by chemical control of this pest in the greenhouse, resistance may develop in a comparatively short time.
DISPERSAL AND DIAPAUSE
The dispersal ability of T. urticae in greenhouses is an important factor to consider in the control of this pest. Hussey and Parr (1963) indicated that twospotted spider mites dispersed in the following ways: migration of newly emerged (presumably mated) females to oviposition sites; dispersal from infested plants, simply by dropping off; and movements over soil surface in accordance with the plane of polarized light. There is direct evidence that the mites are able to suspend themselves on silken threads and thus be carried along by air currents. Mites can also be dispersed on the clothing of greenhouse personnel or through the movement of infested plant material. Despite the dispersal ability of the mites, it is not uncommon to find infestations in one portion of the greenhouse throughout the season, and perhaps even from season to season. Patchy infestations in the greenhouse are characteristic of twospotted spider mites. In greenhouses where ornamental plants are grown, patches are often found at the ends of benches near the walls and away from center aisles. The cause of these patches may be associated with poor spray coverage in these hard to reach areas.
Under certain conditions, twospotted spider mites can overwinter as diapausing (mated) females. The diapause is presumably induced by photoperiod (i.e., shortened day length), low temperatures, and unfavorable food supply (see Parr and Hussey, 1966, Jeppson et al., 1975). These diapausing females are yellowish-orange and hibernate in protected places (e.g., cracks, crevices) . They neither feed nor reproduce while in diapause. The diapause normally terminates in the spring when favorable environmental conditions return. In Florida, populations of twospotted spider mites cycle throughout the year, although sometimes at reduced rates of development during winter months. However, it is possible that a small portion of the population enters diapause during the winter months.
Twospotted spider mites feed on many species of plants and are a major pest of vegetables, ornamentals, fruit trees, hops, cotton, and strawberries (van de Vrie et al., 1972). At present, it is safe to assume that most of the major spider mite problems in greenhouses will involve twospotted spider mite.
The larva, protonymph, deutonymph, and adult feed mainly on the undersides of the leaves. When feeding, the body of the mite is tipped upward such that the 3rd and 4th pairs of legs are off the leaf surface and the mite is supported by the 1st and 2nd pairs of legs (Jeppson et al., 1975). Feeding is accomplished in the following manner: a pair of needle-like stylets penetrates the plants' parenchyma cells, the contents of which are then drawn into the body of the mite by a "pharyngeal pump". According to Laing (1969), protonymphs and deutonymphs spend about half their developmental times feeding and half in the resting or quiescent stage; the larvae spend slightly more time feeding than resting.
Damage to the plants is effected in several ways. First, feeding causes the destruction or disappearance of chloroplasts which then leads to basic physiological changes in the plant. Stomatal closure can be a primary host-plant response, and in such cases, uptake of CO2 decreases resulting in a marked reduction in transpiration and photosynthesis (Sances et al., 1979a, 1979b) . These effects can occur at spider mite densities that are too low to cause visible damage. Reduction of photosynthetic area by spider mite feeding is permanent and can only be compensated for by production of new foliage. Methods have been developed to quantify the amount of feeding and therefore damage for both cucumber and tomato (Hussey and Parr 1963, French et al. 1976, Anonymous 1976a, Anonymous 1976b) . Both methods utilize a leaf damage index (LDI) using the following 0-5 scale: 0 = no damage; 1 = incipient damage with one or two 1-5 mm diameter feeding patches or 5% of the leaf area damaged; 2 = more and larger patches than 1 and with 15% of the area affected, 3 = denser speckling with 30% of the area damaged; 4 = about 60% of the leaf area damaged; and 5 = over 80% of the leaf area damaged with the leaf being chlorotic (French et al. 1976). When the mean LDI reaches 2.0 on tomato, 33% of the leaf area is damaged and loss in yield can be expected (Anonymous 1976b). Loss in yield for cucumbers occurs when the mean LDI reaches 1.9 which corresponds to 30% of the leaf area damaged (Hussey and Parr 1963). Secondly, it is likely (but not firmly established) that the mites actually inject phytotoxic substances into the plant when feeding (see Avery and Briggs 1968; Jeppson et al. 1975; Liesering, 1960). Finally, the stippling or speckling of the upper leaf surface, plus the webbing produced by protonymphs, deutonymphs, and adults, leads to aesthetic injury, particularly in the case of ornamental plants.
The factors which determine the abundance or density of spider mites have been discussed in considerable detail by Huffaker et al. (1969, 1970), McMurtry et al. (1970), and van de Vrie et al. (1972). With respect of outbreaks of spider mites, particularly since World War II, there are two central "hypotheses" or tentative explanations to account for these events. The first is that the upsurge of spider mites is due to improved cultural practices, such as pruning, fertilization, and pesticide use. For example, outbreaks of spider mites can be induced by certain fertilization practices or by certain pesticides, regardless of natural enemies. Apparently, these cultural practices increase the nutritive value of the plant and thus enable greater reproductive activity on the part of the spider mites. The second explanation is simply that widespread use of broad-spectrum insecticides destroy or greatly hamper natural enemies of spider mites and thereby allow pest outbreaks to occur. There is reason to believe that both mechanisms can act in concert in inducing spider mite outbreaks.
Most of the pertinent information in the literature concerns the influence of pesticides on outbreaks of spider mites under field conditions. According to van de Vrie et al. (1972), increases in abundance of twospotted spider mite ave been observed following use of certain agricultural chemicals in many different crops. Although the causes of such increases in greenhouses have not been determined, it would be a sound practice to minimize the use of insecticides and miticides in the greenhouse since outbreaks of twospotted spider mites in the field are often correlated with pesticide usage.
Probably the most common scenario for outbreaks of twospotted spider mites in greenhouses is as follows: The spider mites are accidentally introduced into the greenhouse without any of their effective natural enemies; if host plants and physical factors (e.g., temperature) are sui, the population "explodes". Common sources of inoculum include infested plants carried into the greenhouse, spider mites (especially mated females) which cling to the clothing of greenhouse workers and weeds growing outside the greenhouse.
However, chemical controls used to control other pests (e.g., mealybugs or greenhouse whitefly) can destroy natural enemies which have been introduced into the greenhouses (see below) and thus, engender serious outbreaks of twospotted spider mites. Thus, insecticides, miticides, and fungicides should be used judiciously when natural enemies are present in order to minimize unnecessary problems with twospotted spider mites.
Many different natural enemies are associated with spider mites under field conditions. These enemies are either predators or pathogens; there are no known parasites (parasitoids) of spider mites (McMurtry et al., 1970).
In greenhouses, there are two categories of predacious species that feed on twospotted spider mites: those which occur naturally and those which are artificially introduced. The predacious phytoseiid mite Phytoseiulus persimilis Athias-Henriot, is the major species used to control twospotted spider mites in greenhouses. However, Metaseiulus occidentalis (Nesbitt), another predatory mite, has been evaluated for the control of mites on greenhouse grown roses with some success (Field and Hoy, 1984). Pathogens occur naturally under certain field conditions and appear to be an important regulator of spider mite populations. Hirsutella thompsonii Fisher has been proposed as a possible microbial control for twospotted spider mites in greenhouses but has only been effective in the laboratory (Gardner et al., 1982). For this reason, P. persimilis will be the only natural enemy treated in this section.