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Damage by T. palmi is not unlike that caused by many other species of thrips; when populations are high, their feeding causes a silvery or bronzed appearance on the surface of the plant, especially on the midrib and veins of leaves and on the surface of fruit. Leaves and terminal shoots become stunted and fruit is scarred and deformed. Damaged leaves generally show a darkened, glossy, pearly appearance (Bournier, 1987). Johnson (1986) described heavy damage to watermelon foliage as bronzing and total destruction of the vine tips.
Bournier (1983, 1987) described damage to cultivated cotton caused by T. palmi and, among the symptoms, observed that the oldest tissue may thicken, warp and finally crackle. Damage to cotton seedlings by T. palmi has also been reported in Thailand by Wangboonkong (1981) if there are long periods of drought early in the season.
Apart from Karny (1925), who described T. palmi and observed that it infested both mature and seedling tobacco in Sumatra, one of the earliest reports of damage by T. palmi was Ananthakrishnan (1955). He described the damage to Sesamum plants in Madras, India, as malformation of the stamens, injury to the ovarian wall and the development of a dark pigment on the fruit wall, instead of the usual green colour.
Damage has been described by Nakazawa (1981) in Japan as yellowing of the leaves, topping, scratches on the fruits, malformation of the fruits, poor fruiting and death of the whole plant when populations are high. In Martinique, Denoyes et al. (1986) described the damage on the leaves of aubergine, cucumber, melon and other cucurbits. Pantoja et al. (1988) reported severe damage to cucurbits and solanaceous commercial plantings in 1986 in Puerto Rico, where adult and immature thrips fed gregariously on leaves, stems, flowers and developing fruits. Pepper plants became stunted with a bronzed appearance and aubergine plants showed premature fall of developing fruits and buds, and deformed fruits.
Kawai (1986b) studied the relationship between the density of T. palmi and the damage to Capsicum annuum and aubergine in Japan. He also studied the relationship between different densities of T. palmi and injury to cucumbers grown in a vinyl house (Kawai, 1986c). The growth of cucumber plants was retarded when thrip numbers were high. The tolerable pest densities were estimated at 5.3 adults per leaf for the total fruit yield and 4.4 adults per leaf for the yield of uninjured fruit (assuming an acceptable yield loss of 5% of the maximum yield).
Sakimura et al. (1986) observed that both adults and larvae of T. palmi feed gregariously on leaves, firstly along the midribs and veins. Stems are attacked, particularly at or near the growing tip, and are found amongst the petals and developing ovaries in flowers and on the surface of fruit. They leave numerous scars and deformities, and finally kill the entire plant.
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Walker (1992, 1994) has reviewed the pest status of T. palmi; much of the information given here is from these reviews, together with later information. The importance of the pest on vegetable crops in South-East Asia was emphasized by a workshop held in Bangkok, Thailand (Talekar, 1991) where seven of the eight papers presented listed T. palmi as causing concern for vegetable growers in their region. However, research on this species attacking vegetables in Taiwan was reduced when it was realised that it is effectively a 'resurgence pest', that is, populations tend to increase disproportionately following heavy use of insecticides. The reasons for this phenonmenon require further study.
The economic injury density in Japan has been estimated at 0.105 adults per flower or 4.4 adults per sticky trap per day on Capsicum annuum in vinyl houses (Morishita and Azuma, 1988), assuming an acceptable yield loss of 5% of the maximum yield. Kawai (1986b) also reported that economic injury thresholds were low in vinyl houses in Japan, assuming an acceptable yield loss of 5% of the maximum yield, with 0.08 and 4.4 adults per leaf for aubergine and cucumber, respectively, and 0.11 adults per flower for C. annuum. Morishita and Azuma (1989) considered counting injured fruits to be a better sampling method than counting insects on leaves.
In the Philippines, plantings of aubergine intended for seed production had to be abandoned due to severe T. palmi damage and even the application of insecticide as often as every 4 days failed to provide satisfactory control (Bernardo, 1991). Chang (1991) lists T. palmi as one of Taiwan's most important pest thrips; damage was first observed on cucurbits in 1979, but the species was incorrectly identified as T. flavus. T. palmi has also been identified as a important pest of potato in Taiwan by SEAMEO SEARCA (1991).
However, Bournier (1986) reported that T. palmi caused insignificant damage on cotton, tobacco and wild plants in Java, Sumatra and India. Miyazaki et al. (1984) also observed, during a survey of soyabean in Java, that T. palmi did not cause heavy damage except in one instance on aubergine.
Cooper (1991b) recorded infestations of 300-700 T. palmi per leaf on aubergine and cucumber, resulting in crop losses of 50-90% in Trinidad. He suggested that T. palmi may have been brought to Trinidad in the winds of a tropical depression during 1988, but it has also been postulated that it may have gained entry through plant material from another Caribbean island, for example Martinique, where it is reported as a serious pest. Pantoja et al. (1988) noted that the climatic conditions in Puerto Rico are favourable for the early development of large populations of T. palmi on commercial crops as well as on weeds. Guyot (1988) reported the disastrous economic effect that T. palmi had in Guadeloupe when aubergine exports fell from 5000 tonnes in 1985 to 1600 tonnes in 1986, and in Martinique where 37% of the vegetable crops of the two main co-operatives were attacked by T. palmi, including 90% of aubergine crops.
In Hawaii, Johnson et al. (1989) observed that, together with Aphis gossypii, T. palmi was the major foliar pest on Oahu (1984-85). Welter et al. (1989) studied mixed infestations of T. palmi and the western flower thrips, Frankliniella occidentalis, and noted significant reductions in total cucumber yield, mean fruit size and total fruit. The population trends of T. palmi on commercial watermelon plantings in Hawaii were surveyed by Johnson (1986). Peak infestation levels varied from 2.5 to 53.6 individuals per leaf and from 18 to 97% infested vine tips per planting.
Johnson (1986) pointed out that T. palmi could establish itself in the continental USA, given the extensive flow of air traffic between Hawaii and the mainland, especially California, but it was not until 1991 that T. palmi was found in the USA, not in California as predicted by Johnson but in Florida (FAO, 1991). Heavy infestations were detected on potato, aubergine, Capsicum, Phaseolus vulgaris, yellow squash and several weeds. The likely economic impact of this pest if it became established in greenhouses in the UK was considered to be very severe, with a benefit to cost ratio for one eradication campaign being as high as 110:1 (MacLeod et al., 2004).