In a 3-yr study, adult tarnished plant bugs,Lygus lineolaris(Palisot de Beauvois), were collected in sampling plots of mustard (Brassica juncea crispifolia(L.)) by sweep net. Sticky traps placed around these plots were used to sample flying bugs. Each year, four of eight mustard plots were treated regularly with insecticide. Thus, treated plots contained immigrant populations, and untreated plots contained immigrants and residents. A greater percentage of tarnished plant bugs in treated plots were female than in untreated plots. Immigrant adults, collected from treated plots, were younger than adults in untreated plots. Bugs caught on traps were younger than those collected from sweep net samples; on average, females were older than males. Within treatments, there was no difference in the vertical distribution of males and females on traps. The average age of females decreased with capture height. whereas male age was unaffected by capture height. The age and reproductive status of captured bugs indicate that young, parous females are most likely to colonize new host patches. Differences between the sexes in flight behavior and advantages of dispersal by parous females were discussed.