Thrips palmi Karny (Thysanoptera: Thripidae)
Distribution. Until the mid-1970s the distribution of melon thrips was limited to Southeast Asia. In recent years it has spread throughout Asia, and to many Pacific Ocean islands, North Africa, Australia, Central and South America, and the Caribbean. In the United States it was first observed in Hawaii in 1982, Puerto Rico in 1986, and Florida in 1990. It has the potential to infest greenhouse crops widely, but under field conditions melon thrips likely will be limited to tropical areas.
Host Plants. Melon thrips is a polyphagous species, but is best known as a pest of Cucurbitaceae and Solanaceae. Among vegetables injured are bean, cabbage, cantaloupe, chili, Chinese cabbage, cowpea, cucumber, bean, eggplant, lettuce, melon, okra, onion, pea, pepper, potato, pumpkin, squash, and watermelon. Tomato is reported to be a host in the Caribbean, but not in the United States or Japan. Tsai et al. (1995) reported that cucurbits were more suitable than eggplant, whereas pepper was less suitable than eggplant. Other crops infested include avocado, carnation, chrysanthemum, citrus, cotton, hibiscus, mango, peach, plum, soybean, tobacco, and others. (See color figure 17.)
Natural Enemies. Natural enemies, particularly predators, are quite important in the ecology of melon thrips. There is strong indication that melon thrips abundance and damage are increased by application of some insecticides (Etienne et al., 1990). Among the most important predators observed in Hawaii were the predatory thrips Franklinothrips vespiformis (Crawford) (Thysanoptera: Aeolothripidae) and especially the minute pirate bug, Orius insidiosus (Say) (Hemi-ptera: Anthocoridae). Other predators in Hawaii were Curinus coeruleus (Mulsant) (Coleoptera: Coccinelli-dae), Rhinacoa forticornis Reuter (Hemiptera: Miridae), and Paratriphleps laevisculus Champion (Hemiptera: Anthocoridae). Other predators and parasitoids are known in Asia (Hirose, 1991; Hirose et al., 1993; Kajita, 1986). The parasitoid, Ceranisus menes Walker (Hyme-noptera: Eulophidae), shows particular benefit in many asian studies, and this wasp has been introduced to Horida (Castineiras et al., 1996a). Fungi known to affect melon thrips include Beauveria bassi-ana, Neozygites parvispora, Verticillium lecanii, and Hir-sutella sp. (Castineiras et al., 1996b).
Life Cycle and Description. A complete generation may be completed in about 20 days at 30?C, but it is lengthened to 80 days when the insects are cultured at 15?C. Melon thrips are able to multiply during any season that crops are cultivated but are favored by warm weather and suppressed by senescent crops. In southern Florida they were damaging on both autumn and spring vegetable crops (Seal and Baranowski, 1992; Frantz et al., 1995). In Hawaii, they also became numerous on vegetables during the summer growing season (Johnson, 1986). (See color figure 180.)
Egg. The eggs are deposited in leaf tissue, in a slit cut by the female. One end of the egg protrudes slightly. The egg is colorless to pale white, and bean-shaped in form. Duration of the egg stage is about 16 days at 15?C, 7.5 days at 26?C, and 4.3 days at 32?C.
Larva. The larvae resemble the adults in general body form though they lack wings and have a smaller body size. There are two instars during the larval period. Larvae feed gregariously, particularly along the leaf midrib and veins, and usually on older leaves. Larval development time is determined principally by the suitability of temperature, but host plant quality also has an influence. Larvae require about 14,5, and 4 days to complete their development at 15?, 26?, and 32? C, respectively. At the completion of the larval instars the insect usually descends to the soil or leaf litter, where it constructs a small earthen chamber for a pupation site.
Pupa. There are two instars during the "pupal" period. The prepupal instar is nearly inactive and pupal instar is inactive. Both instars are nonfeeding stages. The prepupae and pupae resemble the adults and larvae in form, except that they possess wing pads. The wing pads of the pupae are longer than that of the prepupae. The combined prepupal and pupal development time is about 12, 4, and 3 days at 15?, 26?, and 32?C, respectively.
Adult. Adults are pale yellow or whitish, and lack dark pigmentation but bear numerous dark setae on the body. A black line, resulting from the juncture of the wings, runs along the back of the body. The population is heavily weighted toward females. The slender fringed wings are pale. The hairs or fringe on the anterior edge of the wing are considerably shorter than those on the posterior edge. The thrips measure 0.8-1.0 mm in body length, with females averaging slightly larger than males. Unlike the larval stage, the adults tend to feed on young growth, and so are found on new leaves. Adult longevity is 10-30 days for females and 7-20 days for males. Development time varies with temperature, with mean values of about 20, 17, and 12 days at 15?, 26?, and 32?C, respectively. Females produce up to about 200 eggs, but average about 50 per female. Both mated and virgin females deposit eggs.
Careful examination is required to distinguish melon thrips from other common vegetable-infesting species. The Frankliniella species are easily separated, because their antennae consist of eight segments, whereas in Thrips species there are seven antennal segments. To distinguish melon thrips from onion thrips, Thrips tabaci Lindeman, it is helpful to examine the ocelli. There are three ocelli on the top of the head, in a triangular formation. A pair of setae are located near this triangular formation, but unlike the arrangement found in onion thrips, the setae do not originate within the triangle. Also, the ocelli bear red pigment in melon thrips, whereas they are grayish in onion thrips. In general, the basic body color of adult melon thrips is yellow, but in onion thrips it is yellowish gray to brown. The range of melon thrips in North America is quite restricted, so this also should aid in diagnosis.
The most complete summary of melon thrips biology and management was presented by Girling (1992). A detailed description of melon thrips is found in Bhatti (1980); Layland et al. (1994) also provided some diagnostic characters. Developmental biology was given by Tsai et al. (1995). Keys for identification of common thrips were presented by Palmer et al. (1989) and Oetting et al. (1993). Also, this species is included in a key to common vegetable-infesting thrips in Appendix A.
Melon thrips cause severe injury to infested plants. Leaves become yellow, white or brown, and then crinkle and die. Heavily infested fields sometimes acquire a bronze color. Damaged terminal growth may be discolored, stunted, and deformed. Densities from 1-10 per cucumber leaf have been considered to be the threshold for economic damage in some Japanese studies. However, studies in Hawaii suggested a damage threshold of 94 thrips per leaf early in the growth of the plant (Welter et al., 1990). Feeding usually occurs on foliage, but on pepper, a less suitable host, flowers are preferred to foliage. As the melon thrips prefer foliage, they are reported to be less damaging to cucumber fruit than western flower thrips, Frankliniella occidentalis (Pergande) (Rosenheim et al., 1990). Nevertheless, fruits may also be damaged; scars, deformities, and abortion are reported. In Hawaii, thrips were observed to attain higher densities on cucumber plants infected with watermelon mosaic virus, but it was not determined whether the plants were more attractive to adults, or more suitable for survival and reproduction (Culliney, 1990).
In addition to direct injury, melon thrips are capable of inflicting indirect injury by transmitting some strains of tomato spotted wilt virus and bud necrosis virus.
Sampling. Larvae and adults are collected from foliage. Adults tend to move toward young foliage, with nymphs tending to be clustered on foliage inhabited by adults several days earlier. Adults can also be sampled with sticky and water pan traps. Blue and white are attractive colors for thrips, and have been used to trap melon thrips (Layland et al., 1994; Kawai and Kitamura, 1987). However, yellow has also been suggested to be an attractive color (Culliney, 1990).
Insecticides. Foliar insecticides are frequently applied for thrips suppression, but at times it has been difficult to attain effective suppression. Various foliar and drench treatments, alone or combined with oil, have achieved some success (Seal and Baranowski, 1992; Seal et al., 1993; Seal, 1994) though it is usually inadvisable to apply insecticides if predators are present. The eggs, which occur in the foliar tissue, and the pupae, which reside in the soil, are relatively insensitive to insecticide application.
Cultural Techniques. Several cultural practices apparently affect melon thrips abundance, but few have been evaluated in the context of North American agriculture. Physical barriers such as fine mesh and row cover material can be used to restrict entry by thrips into greenhouses, and to reduce the rate of thrips settling on plants in the field (Kawai and Kita-mura, 1987).
Organic mulch is thought to interfere with the colonization of crops by winged thrips. Plastic mulch also is reported to limit population growth, but it is uncertain whether this is due to reduced rates of invasion or denial of suitable pupation sites. Crop stubble was not an effective deterrent (Litsinger and Ruhendi, 1984).
The effects of intercropping potato with onion on melon thrips populations was evaluated by Potts and Gunadi (1991). Although aphid and aphid-borne disease incidence were decreased in such potato plantings, the density of thrips on potatoes was increased. Thus, the benefits of such cropping practices are largely a function of which pests are likely to be most important in an area.
Heavy rainfall is thought to decrease thrips numbers (Etienne et al., 1990). However, there seems to be no evidence that overhead irrigation is an important factor in survival.
Biological Control. The predatory mite Neoseiulus cucumeris (Oudemans) has been investigated for suppression of melon thrips (Castineiras et al., 1997). The mite density is correlated with thrips density, but within-plant distribution differs among the two species, suggesting that though the mites may increase in numerical abundance they are unlikely to drive the thrips to extinction.
Host-Plant Resistance. Nuessly and Nagata (1995) reported that susceptibility to injury varied among pepper cultivars. They reported that though sweet and jalapeno types were sensitive to foliar injury, cubanelle and cayenne types produced acceptable size and quality fruit. This is the reverse of injury susceptibility to western flower thrips, so in areas with mixed thrips populations growers cannot rely solely on plant selection to avoid damage.