Eotetranychus sexmaculatus (Riley, 1890) Ma

Vacante, V., 2010, REVIEW OF THE PHYTOPHAGOUS MITES COLLECTED ON CITRUS IN THE WORLD Vincenzo V, Acarologia 50 (2), pp. 221-241 : 231-233

Eotetranychus sexmaculatus is a polyphagous species and presents a wide geographical distribution (Australasian, Oriental, Nearctic, and Palearctic regions). Its dynamics of population is adversely influenced by dry weather conditions and they reach injurious levels only in the more humid coastal regions. Dry winds hinder the development of the mites populations, which increase gradually during winter and rapidly in spring and summer ( Jeppson et al., 1975; Jeppson, 1989). The six-spotted spider mite feeds on the undersides of leaves, seldom attacking fruits, except during very wide-spread infestations. Mite feeding on the lower leaf surfaces produces yellow depressions that are covered by webbing, and the areas on the upper surfaces that correspond to the locations of the mite colonies on the lower surfaces become raised and yellow or yellow-white, with a smooth, skinny surface. As the infestation increases the yellowish areas converge and the leaves become entirely yellow, distorted or misshapen, and drop prematurely (Mc- Gregor, 1956; Jeppson et al., 1975; Jeppson, 1989).

Tetranychus urticae is a very polyphagous species with worldwide distribution. Dry conditions facilitate its development ( Pralavorio and Almaguel Rojas, 1980) and optimum development occurs at 30°C and requires 7.3 days ( Sabelis, 1981). On citrus the damage of the two spotted spider mite is similar to that of the six-spotted spider mite, and the feeding activity on the undersides of young leaves produces chlorotic areas visible on the upper surface; severe damage may result in leaf drop ( Jeppson, 1989). In semitropical areas of the world T. urticae infests young leaves and green or mature fruits of all citrus species. The populations usually develop on a limited portion of the leaves. The leaves buckle at the site corresponding to the colonies and the upper surface becomes raised and turn a yellow-ochre colour.

On orange, lemon and other citrus fruits the feeding activity of the mite produces a blackish area around the navel end of fruit that grows when the pest populations infest the whole fruit ( Lewis et al., 1951; Dosse, 1964; Di Martino, 1985; Vacante, 2009). This species, like other spider mites, may cause worker allergy ( Burches et al., 1996).

Eotetranychus kankitus has been reported in the Palearctic and Oriental regions and in Japan, where its damage to citrus is similar to that of the sixspotted spider mite ( Ehara, 1964). Severe infestations of the citrus yellow mite on citrus trees causing leaves, flowers and fruit to drop prematurely and the withering of branches was reported by Chen (1999).

Eotetranychus lewisi has a wide geographical distribution (Palearctic, Afrotropical, Nearctic, Neotropical, and Oriental regions). The Lewis spi- der mite is injurious only to citrus fruit, its feeding resulting in a pale stippling of the rind. No damage is usually seen on the leaves but severe infestations cause silvering on lemon and either a silvering or russeting of oranges ( McGregor, 1956; Jeppson et al., 1975).

Eotetranychus yumensis has been reported in the Nearctic and Neotropical regions. Relatively high temperatures are necessary for its development, which occurs at 21-38°C, but at 43.5°C the eggs do not hatch. This adaptation probably confines the species to hot desert areas. The Yuma spi- der mite feeds on leaves, fruit and green twigs of citrus and produces a silvering of mature fruit ( Elmer, 1969; Jeppson et al., 1975).

The natural enemies of the Tetranychidae include viruses (non inclusion viruses), fungi (Ascomycota, Clavicipitaceae , Entomophthoraceae , Exobasidiomycetidae, Neozygitaceae ), insects ( Aeolothripidae , Anthocoridae , Chrysopidae , Coccinellidae , Coniopterygidae , Endomychidae , Reduviidae , Staphylinidae ), araneids ( Linyphiidae ), and mites ( Ascidae , Bdellidae , Cheyletidae , Cunaxidae , Phytoseiidae , Smaridiidae , Stigmaeidae , Tydeidae ) ( Vacante, 2010).

Despite the numerous beneficials known throughout the world, biological control alone is insufficient and chemical control is usually necessary. However, in some cases the spider mites are controlled by their predators, as for example P. citri in the Mediterranean region, where the pest is adequately controlled by the Phytoseiid mite Euseius stipulatus (Athias-Henriot) ( Vacante, 1986) or where Exobasidiomycetidae fungi Meira argovae Boekhout et al. , M. geulakonigii Boekhout et al. , and Acaromyces ingoldii Boekhout et al. , may control P. citri , E. orientalis , and T. urticae , in the laboratory ( Paz et al., 2007).

In general, the control of Tetranychids mites requires the use of acaricides and a correct management of horticultural practices. The choice of pesticides is important, and petroleum oils, despite being an old remedy, are to be preferred ( Vacante, 2010), as in the case of E. orientalis , where conditions clearly indicated the rational use of insecticides, including the appropriate distribution of petroleum oils and selective acaricides, which together help to limit the mite populations and indirectly promote a preliminary condition for IPM. Various citrus cultivars have sometimes been investigated in order to evaluate their resistance or tolerance to mite attacks ( Sadana and Kanta, 1972; Dhooria and Sandhu, 1973; Dhooria, 1982; Singh et al., 1983; Bhumannavar et al., 1988). On the whole, IPM represents the best solution for the control of P. citri , T. urticae and other species on citrus. This strategy demands the use of selective acaricides and insectides, and particularly of petroleum oils ( Rosen, 1986; Vacante, 1986; McCoy, 1996; Childers et al., 2007).

Regrettably, there have been no significant innovations in the control of pests of this family and most efforts of researchers were focused on rationalizing the use of chemical means and on IPM. Of particular importance were the efforts to adopt sampling methods and threshold values, as for E. kankitus in China ( Wang, 1985), T. urticae in Spain ( Martinez Ferrer et al., 2006) and P. citri in several countries ( Jones and Parrella, 1984; Zalom et al., 1986; Rodriguez and Ramos, 1998; Hare and Phillips, 1992; Song et al., 2003). However the monitoring of the populations is difficult, due to variables such as climate, mite distribution and chemical control, all affecting the economic threshold (ET).