Abstract
Strains of the large milkweed bug, Oncopeltus fasciatus, were selected through 17 generations in the laboratory for survival to treatments with DDT and Toxaphene which gave 50% kill or higher. The procedure employed in making the treatments is descr. In the 17th generation, 3.7 times as much DDT was required to kill 50% of the DDT-selected adults, and 1.8 times as much Toxaphene was required to give 50% kill of Toxaphene-selected adults, as compared to adults in the unselected strain. No significant differences were found in nos. of eggs produced between selected and unselected strains.

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