Tetranychus evansi Baker & Pritchard occurs on solanaceous plants, and causes serious damage to various crops in Africa and Europe. Tetranychus takafujii Ehara & Ohashi was found on Solanum nigrum L. in Japan in 2001 and described as a new species in 2002. However, because T. takafujii is very similar to T. evansi in morphology, doubt has arisen about the difference between these two species, although there is no report discussing their taxonomic status. Tetranychus takafujii is specialized on solanaceous plants and it is possible that this species gets out of hand after invasion of solanaceous agricultural crops. To evaluate its potential severity as a pest, it is necessary to assess its performance on a variety of food plants in a carefully controlled laboratory set-up. The present study determined the development on 21 crops and weeds, and the reproduction on Solanum nigrum of a Japanese T. takafujii population and six T. evansi populations from abroad. Four plants (potato, horsenettle, pear and cucumber) served as host plant for all seven populations and most larvae successfully reached adulthood. Life-history parameters, such as the rate of development and the intrinsic rate of natural increase (rm), became higher with temperature increasing from 15 to 35°C. The rm-value at 25°C was similar among all populations tested and it was close to the value obtained in T. takafujii [Kotsubo et al. (2004) J. Acarol. Soc. Jpn. 13: 71-76]. This indicates that T. takafujii has the potential to become a serious pest on solanaceous crops, just the same as T. evansi in Africa and Europe.

Tetranychus evansi Baker & Pritchard occurs on solanaceous plants, and causes serious damage to a variety of crops in Africa and Europe; it has not been reported from Japan. Tetranychus takafujii Ehara & Ohashi was found on Solanum nigrum L. in Japan in 2001 and described as a new species in 2002. It is considered to be a Solanaceae-specialist too and it is feared to become a severe pest on solanaceous crops, once it invades them. Because T. takafujii is morphologically very similar to T. evansi, doubt has arisen about the difference between these two species. Here we avoid the issue of taxonomy and focus on a solution for the potential problems that may arise when either of the species invades solanaceous crops. The present study determines the susceptibility of three T. takafujii populations from Japan and six T. evansi populations from various countries to 11 acaricides. The LC50-values of all nine populations tested were lower than the recommended concentration of every acaricide. These results suggest that mites from all nine populations can be controlled with acaricides when they occur on crops.

Population density of the citrus red mite, Panonychus citri (McGregor), in Japanese pear orchards remained low until mid-August, even after inoculation of pear leaves with a considerable number of adult female P. citri from May onwards. This raised the possibility that pear leaves contain a natural compound that suppresses an increase of P. citri populations. The rate of development from larva to adult was significantly lower on leaves collected in July than on leaves collected earlier or later, in several years. The population suppression was caused by molting inhibition and ovicidal activity, according to our close observation in the laboratory [Gotoh and Kubota (1997) Exp. Appl. Acarol. 21: 343-356]. To clarify whether a natural pear compound caused this molting inhibition, a methanol crude extract of pear leaves was isolated and added to a newly developed artificial diet, consisting of sodium caseinate, sucrose, levulose, glucose and inositol. The compound extracted from pear leaves resulted in the molting inhibition as observed on pear leaves. Based on infrared and NMR spectral analysis, the compound extracted from pear leaves closely resembled the synthetic acaricide hexythiazox. Furthermore, the LC50-values of the compound extracted from pear leaves for ovicidal activity of P. citri eggs and for inhibition of molting to protonymphs were similar to those of hexythiazox. These results strongly suggest that the molting deterrent extracted from pear leaves was in fact hexythiazox, an acaricide in use on pear trees, rather than a natural product. This suggestion becomes even stronger, considering that the molting inhibition was observed in a hexythiazox-spray year, but not in a non-spray year.