During the first years after appearance of the pear psylla, Psylla pyricola Förster, in the State of Washington (between 1939 and 1947), nicotine as the best available material was generally used in summer sprays for control. Subsequently, new organic insecticides, as they were found superior to nicotine, were recommended and widely used. As the psylla developed resistance to one after another of the insecticides in use, these were superseded by still newer insecticides. Parathion and related materials were highly effective for control from 1947 until about 1955 to 1957. Dieldrin came into wide use against psylla in 1957, but resistance to it and related materials developed rapidly (Harries and Burts 1959). In 1959, Guthion® (O,O-dimethyl 5-(4-oxo-1,2,3-benzotriazin-3(4H)-ylmethyl) phosphorodithioate) was recommended and remained highly effective for about 2 years. In 1961, 3 applications failed to effect control in an orchard at Gleed, Wash. When adult psyllids from this orchard were tested in the laboratory, their LD50 was 6.47 lb of 25% Guthion wettable powder per 100 gal of spray, as compared with an LD50 of 0.02 lb for adult psyllids from an unsprayed orchard. In August 1961, randomized field plot tests with Guthion in this orchard also failed to show satisfactory control.
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Harries et al. (1965) studied this question.