Resistance of tetranychid mites.
The intensive use of organic pesticides in orchards in the Crimea (USSR) has resulted not only in an increase of tetranychid mites but in a change in the species concerned in the infestation. The use of DDT in the 1950s caused heavy infestation by Bryobia rubrioculus (Scheuten) (redikorzevi Rekk). In the 1960s, when DDT was replaced by organophosphates, this species was largely replaced by Tetranychus viennensis Zacher, and in the 1970s, when dicofol (Kelthane) was used with organophosphates, Panonychus ulmi (Koch) became the dominant species. These changes resulted not only from the elimination of various natural enemies by the chemicals, but also from the selection of resistant forms from heterogeneous populations, and it is pointed out that the greater the degree of heterogeneity present, the greater will be the chance of development of resistant strains. At present, neither organophosphates nor dicofol are effective against the mites. Resistance to dimethoate (Rogor) has increased by a factor of 208-266, and that to dicofol by one of 400-500. The methods used to determine the LC50 are described (a dipping method usually being used), and a formula for determining the results is provided. Existing treatments are evidently useless against P. ulmi, and new compounds were tested towards the end of 1976, after the failure of organophosphates and dimethoate. The poorest effect was shown by tetradifon (Akaritox) and some others, including Mitran (a mixture of chlorfenethol and chlorfensulphide). Milbex (a similar mixture) and Neoron (bromopropylate) proved effective against P. ulmi and afforded protection for 20 and 40 days, respectively. Good results were given also by cyhexatin (Plictran), a mixture of dicofol and dinocap (Akartan), propargite (Omite) and other chemicals, including dinobuton (Acrex). In tests against T. viennensis on apple, a single application of Neoron or Omite reduced infestation by 95% until the end of the season, but tetradifon (Akaritox) seemed less effective.