Studies on population dynamics of the scarlet mite, Brevipalpus phoenicis, a pest of tea in Indonesia

Tea is the national drink of Indonesia. The habitual consumption prevents intestinal infections; the production provides many Indonesians with a living. The production is affected by scarlet mites (Brevipalpus phoenicis GEIJSKES), an important pest of tropical and subtropical crops. It is one of the main pests of tea in Indonesia and inhabits virtually all tea bushes. The factors restricting the development of this mite on tea in West Java were studied by observations and experiments in the laboratory and the tea gardens of the Research Institute for Tea and Cinchona.

Scarlet mites multiply fast under favourable conditions. The intrinsic rate of increase (r m = 0.0610) however is far less than that of some spider mites. The scarlet mite populations develop continuously in the field, without synchronization. The mites stay on the undersurface of the tea maintenance leaves and are rather sedentary. A deteriorating leaf quality triggers off migration with a positive phototactic orientation, i.e. towards younger leaves of the bush.

Different tea clones and seedlings sustain significantly different mite densities as a consequence of differences in host plant resistance. Tea bushes are pruned once in four years in West Java. This strongly reduces the abundance of scarlet mites. The populations build up slowly and exponentially to a mean equilibrium level which is attained around two years after pruning. Most populations have a low and rather stable density during the second two years of the pruning cycle; some populations have high and fluctuating densities. Generally speaking, the populations especially increase during the dry seasons and decrease during the transitory periods. Maxima are reached usually at the end of the dry season. The fluctuations are not directly related to the average minimum or maximum temperature, average minimum relative humidity or the total rainfall. Application of copper fungicides (copperoxychloride) increases the average mite densities and especially the seasonal maxima. The numerical multiplication of field populations always remained much below that of populations in the laboratory under favourable growing conditions.

Many other arthropods beside scarlet mites inhabit tea leaves. Predators of scarlet mites were collected from tea estates in Java and Sumatra. The diversity of predatory mite species appeared to be particularly rich. A considerable number of species probably has not been described and is identified provisionally in this paper by a code name. The predatory behaviour of most species (Phytoseiidae and Stigmaeidae), the reproduction of three typical species and the capacity of three stigmaeid species to keep scarlet mites in check were confirmed in laboratory experiments.

A series of pesticides was screened in the laboratory for (undesired) toxicity towards scarlet mites, DDT was screened for toxicity towards predators. Most Phytoseiidae appeared to be susceptible, and most Stigmaeidae appeared to be tolerant to DDT. DDT, maneb and PH 60-42 were selected as exclusion pesticides with the intention of killing respectively: all the predators, the predatory mites and the predatory insects, without affecting the scarlet mites in a predator cheek experiment.

These exclusion pesticides were frequently applied in the field during the 16 months period of the predator check experiment. The effects deviated from the expectation in various respects but resulted in a sufficiently diversified predatory fauna to analyze the importance of several predator species as a factor restricting the development of scarlet mites. The effect of DDT was most unexpected. It killed the most common Phytoseiidae and permitted the Stigmaeidae and Amblyseius z to develop high densities. The density of scarlet mites decreased to a rather constant level below that of the (untreated) control, probably as a consequence of predation. The predators that made the most impact were the Stigmaeidae. They suppressed the level of abundance of scarlet mites in the DDT-treated and the untreated fields to 13% and 27% respectively of the abundance without these predators.

The more effective control by predators in the DDT-treated fields was interpreted by a selective killing with DDT of the less efficient predators (Ambly seius x and A. deleoni). The disappearance of these probably benefitted the other, more effective predators of scarlet mites (Stigmaeidae and Amblyseius z). The diversity of the ecosystem at the trophic level of the predators appeared not to be related to the effectiveness of the control of scarlet mites. Suggestions for control, especially the planting of resistant clones, conclude this paper.