Effects of an early season stress on plants were evaluated by characterizing the response of soybean, Glycine max, to simulated seedcorn maggot (SCM), Delia platura (Diptera: Anthomyiidae), injury. Physiological responses, altered intraspecific competition, and stress interactions were examined in field experiments from 1982-1986 in Iowa. Two aspects of SCM injury, stand reduction and plumule destruction (causing formation of a Y-plant with two main stems), were considered. Phenological delay associated with plumule injury contributed to height differences between injured and uninjured plants, which, in turn, lead to substantial differences in many growth parameters. Plant density did not influence plot yields, but stands with increasing proportions of Y-plants had reduced yields in 1982-84 but not 1985-86. Total pods per plant was most influenced by density and proportion of Y-plants. Results from a replacement series of injured to uninjured plants demonstrated that uninjured plants were competitively superior to injured plants regarding growth and reproductive characteristics. A conceptual basis for categorizing stress interactions was developed. Interactions in weed incidence and weed effects on yield with SCM-injury were observed. Interactions were related to reduced competitive abilities of plants having abscissed plumules. Increases in potato leafhopper (PLH), Empoasca fabae (Homoptera: Cicadellidae), occured with increases in porportions of Y-plants. A phenological delay hypothesis proposes that plumule injury delays soybean development and delayed plants are more susceptible to PLH because of reduced leaf pubescence. Experiments supported the phenological delay hypothesis.