A single herbicide (Trimec® or Strike 3™) application in early season (March or April) was made to marginal areas around fields in 23-km² test sites of the Mississippi Delta in 1999, 2000, and 2001. The herbicide was used to kill broadleaf weeds in the marginal areas that served as hosts for tarnished plant bugs, Lygus lineolaris (Palisot de Beauvois). The herbicide treatment caused a significant reduction in wild host densities in the treated test sites in all 3 yr. Tarnished plant bug populations in treated test sites did not increase significantly in the treated marginal areas during April and May after treatment of the margins in the first 2 wk of March in 2000 and 2001. The herbicide application was made in the first 2 wk of April 1999, and plant bug populations increased in treated marginal areas in this year. The increase was thought to be caused by plant bugs moving to Italian ryegrass, Lolium multiflorum Lamarck, a previously unreported plant bug host, which was not affected by the herbicide. Laboratory tests showed that plant bugs would oviposit in flowering or nonflowering ryegrass when caged on ryegrass for a 6-d period. Newly emerged nymphs developed into adults (56%) when reared on floral spikelets of ryegrass, but no adults were obtained when they were reared on ryegrass stems and leaves. Rearing on floral spikelets beginning with third-instar nymphs resulted in 92% adults, whereas third-instar nymphs reared on stems and leaves produced no adults. These results showed that ryegrass could serve as a reproductive host for plant bugs when it flowered during late April and May. Application of the herbicide in March, when ryegrass was not in flower, resulted in no significant increases in plant bug populations on wild hosts (mainly ryegrass) during April and May in 2 yr of the field study.