Spatial and seasonal patterns of phytophagous thrips in soybean fields with comments on sampling techniques.
Two species of phytophagous thrips, Sericothrips variabilis (Beach) and Frankliniella tritici (Fitch), colonised soyabean plants in Urbana in Illinois, Lexington in Kentucky, and Columbia in Missouri in 1976. The numbers of these thrips generally did not vary significantly among plots nor among plants within plots. However, both species were unevenly distributed on the plants. All stages of F. tritici were concentrated in terminal buds and blossoms. Adults of S. variabilis were found most commonly on the uppermost fully expanded trifoliate leaf or the trifoliate leaf immediately below it. Larvae of S. variabilis were generally concentrated on the third-sixth trifoliate leaves below the terminal. Both species occurred in soyabean fields throughout the growing season, but F. tritici was rare at Columbia. Populations of F. tritici peaked earlier than those of S. variabilis at Lexington and Urbana. Also, early season population densities of both species were generally higher and rates of population buildup and decline were more pronounced at the Kentucky site. Since soyabeans appear to be most vulnerable to attack by S. variabilis between stages V1 and V6, sampling should be concentrated during the vegetative stages of the crop for most purposes. Although selection of sampling methods for estimating population density of both species depends on the purpose for sampling, primary consideration should be given to the parts of the plants from which the sample units are to be taken.