Tea, Camellia sinensis, is cultivated mainly at tropical and sub-tropical latitudes. Relative to the main tea-producing countries such as China and India, Japan is located at a more northerly latitude. Although it is the eighth largest tea-producing country in the world (10), most of Japan's harvested tea is not exported but consumed domestically as green tea (Fig. 1). As tea is cultivated intensively in Japan, its production has been heavily dependent on chemical fertilizers and pesticides: the annual rate of nitrogen application is 500-600 kg/ha in Shizuoka prefecture, for example, and conventional chemical pesticides are applied more than 10 times per year (18). However, several tea pests have now developed pesticide resistance due to such intensive use of chemical agents. Moreover, since green tea contains vitamins and functional compounds like cathechins (21), consumers prefer this type not only for its flavor but also for its health benefits, and liberal use of chemical pesticides is ill-suited to the demands of consumers. Given this background, new pest control strategies are needed for controlling tea pests.
Figure 1. Tea-producing prefectures in Japan. Solid circles on the map represent relative production from individual prefectures. The two highest-producing prefectures, Shizuoka and Kagoshima, account for almost 70% of Japan's annual tea production.
Tea is a perennial evergreen plant which seldom needs to be re-planted for several decades. The structure of cultivated tea trees is unique: the plucking surface has a dense covering of leaves, but the interior has very little foliage, providing both a habitat for various pests and a refuge for natural enemies (Fig 2A). Kawai (13) described the ecological characte-ristics of tea fields in Japan and identified the major insect pests as Kanzawa spider mite (Tetranychus kanzawai), tea leaf roller (Caloptilia theivora), oriental tea tortrix (Homona magnanima), smaller tea tortrix (Adoxophyes honmai), mulberry scale (Pseudau-lacaspis pentagona), black citrus aphid (Toxoptera aurantii), yellow tea thrips (Scirtothrips dorsalis), tea green leafhopper (Empoasca onukii), mugwort looper (Ascotis selenaria), and chafer (Heptophylla picea).
Figure 2. Tea tree under conventional chemical control in Kyoto (A). Closer look of plucking surface of tea tree showing dense tea leaves and interior with little foliage. Tea field treated with Adoxophyes honmai entomopoxvirus in Saitama (B). Numerous spider webs are shown on the plucking surface of the tea trees.
In response to the problems mentioned above, alter-natives to chemical pesticides have been developed to control tea pests. Kodomari (16) mentioned that the prominent biological control agents in Japanese tea fields were a pesticide-resistant predatory mite (Ambly-seius womersleyi) against the Kanzawa spider mite, as well as granuloviruses (Baculoviridae) and sex phero-mones against H. magnanima and A. honmai (Lepi-doptera, Tortricidae). In tea fields, where various pests still need to be controlled primarily by chemical pesticides, biological control agents that are sensitive to chemical pesticides should not be used. Baculovi-ruses, sex pheromones and resistant natural enemies are all suitable agents for use in field situations that require chemical control, such as tea. Furthermore, as well as an abundance of insect pests, tea has various indigenous natural enemies which play a role in suppressing pest populations (13). Biological control agents such as granuloviruses should therefore be applied in a way that does not disturb these natural enemies.
Biological Control of Tortricidae in Tea Fields in Japan Using Insect Viruses and Parasitoids
- Received Date: 31 January 2009
- Accepted Date: 19 May 2009
Abstract: Tea is a perennial and evergreen plant. Cultivated tea trees provide a habitat for insect pests and their natural enemies. In Japan, granuloviruses (GVs) have successfully controlled two of the most important pests of tea, Adoxophyes honmai and Homona magnanima (Tortricidae: Lepidoptera). The GVs are produced in vivo and a single application sustains pesticidal efficacy throughout a year, which encompasses 4 to 5 discrete generations of both species. A. honmai and H. magnanima also have various natural enemies, especially hymenopteran parasitoids. Such resident natural enemies also play a role in reducing the pest density in virus-controlled fields, but the effect of virus infection on parasitoids sharing the same host larva has not been well studied. Survival of one of the major parasitoids of A. honmai, Ascogaster reticulata (Braconidae: Hymenoptera), is reduced by virus infection of the host. Viruses, including GV and entomopoxvirus (EPV), and certain koinobiont endoparasitoids, including A. reticulata, are both known to regulate host endocrinology. However, the GV and EPV have distinct host regulation mechanisms, and consequently have different impacts on the survival of A. retuculata, when A. reticulata parasitizes a host that is infected with either GV or EPV. These additional effects on host regulation displayed by both viruses and parasitoids affect the outcome of virus-parasitoid interactions.