IntroductionOne of the difficulties frequently encountered by investigators in growing plants in greenhouses is the control of the common red spider, Tetraniychus telarius L. Corn plants are especially suspectible, and in a series of physiological studies at Wooster, Ohio, red spiders threatened to become a limiting factor. Insecticides suitable for use on corn plants were applied with knapsack, wheelbarrow, and high-pressure orchard sprayers, but did not give satisfactory control.The observations by HURD-KARRER and Poos (4) In the second preliminary experiment, 2 p.p.m. of selenium were added to the nutrient solution at 10-day intervals. When the mature plants were harvested, the plants in the control tank were practically dead as a resuLlt of spider injury, whereas the plants receiving selenium were nearly free of red spiders.There was no indication of toxicity to the plants in either test and the results were, therefore, encouraging. To test further the possibilitv of controlling red spiders by selenium-induced toxicity without disturbing normal plant development, a more elaborate series of experiments was undertaken and is reported in detail. EXPERIMENT I On October 25, 1937, a series of four tanks was planted to a single-cross hybrid. One tank was used as a control and received no selenium. Selenium was added to the other three tanks at the rate of 0.5, 1.0, and 2.0 p.p.M.3 Beginning two weeks after planting, five additions of selenium were made at approximately two-week intervals. In order to obtain a uniform distribution of red spiders, the plants were artificially infested December 13 and were harvested January 28. Twelve plants were composited for each sample for analysis. Observations made on the red spider infestations and on the condition of plants at time of harvest, together with the selenium content of the samples, are given in table I. 3 The sodium selenate used in these experiments was of technical grade, purchased from the Harshaw Chemical Co., Cleveland, Ohio. At the time of these first experiments we were not equipped to determine its purity, and it was accepted as 100 per cent. Later analysis showed only 85 per cent. selenate, so that these treatments were actually 15 per cent. less than the figures given.