Greenhouse and field studies were conducted to determine if oil-soluble dye could be used to mark Heliothis larvae that developed primarily on wild geranium, Geranium dissectum and G. carolinianum (L). Treatment of geranium with dye under greenhouse conditions marked 73% of the adults from larvae that developed on the treated plants. When a field of geranium was first infested with neonate tobacco budworms, H. virescens (F.), and then treated with dye, 28% of the emerging budworms caught by light traps in the immediate area were marked. Treating ca. 11% of the total wild geranium in an area produced marking in 6 % of the bollworms, H. zea (Boddie), and 8.3% of the budworms caught in pheromone traps in the area. When part of a test area was treated with a red dye and the remainder with a blue dye, red or blue dye was detected in trapped adults. However, when areas were treated either with blue dye or with red dye plus a nuclear polyhedrosis virus (NPV), only blue marking was detected, thus indicating control by the virus.