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Factors affecting the changes in species composition between Tetranychus urticae and Panonychus ulmi (Acari: Tetranychidae) in Orchards: Hypothesis and case studies
The European red mite (ERM), Panonychus ulmi, and the two-spotted spider mite (TSSM), Tetranychus uritcae, are serious mite pests in pome fruit orchards worldwide. Abundance and species composition of spider mites in orchard ecosystem have been changed historically in many countries. To illustrate the cause for the historical changes in species composition between ERM and TSSM is an interesting topic. The abundances of the two mite populations have been largely changed in Korean apple orchards: ERM was dominant until 1970s, TSSM has been dominant from 1980s, and then ERM has increased abundantly in some orchards in recent years (Lee, 1990; Kim and Lee, 2005). In apple orchards in Japan, Amphitetranychus viennensis outbreaks occurred immediately after the use of organophosphates in the early 1950, and then, ERM became dominant species on apple until the 1960s followed by TSSM abundance from the late 1970s to the early 1980; Recently, ERM again increasing in IPMadopted apple orchards (Reviewed in Morimoto et al., 2006).
Kishimoto (2002) showed changes in the species composition of spider mites caused by different pesticide spray programs in Japanese pear orchards. Pesticides can become an important factor that affects mite species composition of mites through the process of resistance development and the removal of natural enemies. Pesticide resistance and the removal of natural enemies can explain the outbreak of spider mites in orchards, but do not fully explain the changes in species composition of mites. Morimoto et al. (2006) reported a possible exclusion of ERM by TSSM through interspecific interaction via their web: the complicated web created by TSSM increases the mortality of little webspinning species such as ERM. Morimoto et al. (2006) stated that the change in mite species composition can occur as a result of interspecific association between spider mites via their webs, without pesticide applications or the presence of natural enemies. However, their hypothesis does not explain the dominance of ERM in orchards under lower pesticide pressure. Also, Belczewski and Harmsen (1997) concluded that the application of additional or supplemental amounts of a Phylloplane fungus (Alternaria alternata) to apple leaves enhanced the population growth of TSSM compared to that of ERM. However, this does not explain the dominance of TSSM in orchards under heavy pesticide pressure where Phylloplane fungal population would be lowered by fungicides, and the dominance of ERM in unsprayed orchards where Phylloplane fungal population would be abundant because of no fungicide application.
The change in species composition among spider mites can not be projected with any single factor. It is required to examine all possible combinations of factors involving. Until now the change in abundance between ERM and TSSM has not been examined comprehensively. Therefore, we examine possible changes in population abundance between REM and TSSM with some assumptions. Also, case studies that deal with long-term and short-term changes between the two mites are presented.
