Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)




During 1982 and 1983, selective soil and foliar insecticides were applied to soybean, Glycine max (L.) Merrill, planted on narrow (51 cm) and wide (76 cm) row spacings to manipulate soybean insects. Individual species and complexes of arthropod species were manipulated in plots to determine the effects of subterranean and above-ground insects on soybean growth and yield potential. Selective insecticides used in a selective manner proved to be an effective tool for studying insect pest complexes. Sub-threshold levels of individual pest species, in combination, had a significant negative impact on yield. In addition, in 1982, the plots in which soybean looper, Pseudoplusia includens (Walker), populations approached the economic threshold, exhibited yield reductions comparable to those plots containing sub-threshold levels of several pests. Threecornered alfalfa hopper, Spissistulus festinus (Say), populations of one-half and 1.8 times the economic threshold in 1982 and 1983, respectively, were associated with the lowest yields compared to all other pest species and complexes. Yields due to row spacing were not consistent, with higher yields on 51 cm row spacing in 1982 and 76 cm row spacing in 1983. Significant interactions in yield were found between row spacing and the insect complexes maintained, suggesting that present economic thresholds established on conventional row spacings may need to be revised for narrow row spacings. No yield differences were found due to soil insecticide treatments. Although responses to row spacing were detected for several insect species, the differences were of such a small magnitude that further study is warranted before definitive conclusions can be drawn.