Spider Mites on Walnut in Northern California

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Abstract
The problem of spider mites on walnut has become more acute since the advent of DDT and other newer organic insecticides. With DDT the increase in the spider mite population mounts as the dosage is increased. The principal species of spider mites involved are the Pacific spider mite (Tetranychus pacificus McG.), two-spotted spider mite (T. telarins (L.)), and the European red mite, Panonychus ulmi (Koch). Induced increases in spider mites involve other factors ill addition to interference of insecticides with natural enemies. These factors may exert their influence either upon the spider mite or through the host. Populations of the Pacific spider mite and the two-spotted spider mite, but not the European red mite, have been suppressed where scbradan has been used to control the walnut aphid. With the European red mite schradan induces an increase in the population. In the hot interior San .Joaquin Valley, which has a climate less favorable for European red mite than the more coastal regions, natural enemies usually control the pest before it has had opportunity to do serious damage. In the cooler regions natural enemies may fail to he effective and acaracides may be required. Walnut can tolerate a relatively large spider mite population. This tolerance allows sufficient time to determine whether natural enemies will have an opportunity to control the pest. Natural enemies frequently reduce the spider mite to a sub economic level even though spider-mite-inducing materials such as DDT or Sevin®(l-naphthyl-N-methyl carbamate) are used. Control of spider mites has been complicated by the problem of resistance. A more effective acaricide may become a spider mite- inducing agent once the pest develops resistance to it. or the newer accuracies tested the most effective were Tedion® (2,4,4',5-tetrachlorodiphenyl sulfone, 80% technical) and Kelthane® (1, 1-bis(p-chlorophenyl) -2,2,2-trichloroethnnol).

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