Interplant movement of Heliothis virescens (Lepidoptera : Noctuidae) larvae in pure and mixed plantings of cotton with and without expression of the Cry1Ac delta-endotoxin protein of Bacillus thuringiensis Berliner

Laboratory and field studies were conducted during 1993 and 199a to quantif y interplant movement of Heliothis virescens (F.) larvae in pure and mixed plantings of cotton, Gossypium hirsutum L., with ('Event 531') and without ('Coker 312') the expression of CrylAc delta-endotoxin protein of Bacillus thuringiensis Berliner. Field studies were conducted with neonate, 4-, and 7-d-old larvae placed on 3-plant experimental units and observed at 24, 48, 72, and 96 h after inoculation of larvae. Combining larval movement across observations of neonates, 4-, and 7-d-old larvae, an estimated 52% of the larvae on pure plantings of Coker 312 had moved at least 1 plant by the cum ulative time required to reach the age of 10 d. More larvae placed on Event 531 cotton moved to an adjacent plant (13% of the neonates had moved at le ast 1 plant within 24 h) than those placed on Coker 312 (0% of the neonates had moved at least 1 plant within 24 h). When larvae were placed on Event 531 plants, an estimated 82% of the larvae had moved to an adjacent plant b y cumulative age of 10 d. Collectively, these data indicate that movement o f larvae from plant to plant increases with larval age and occurs more rapi dly for larvae placed on Event 531 cotton than on Coker 312. Previous studi es have suggested that resistance to B. thuringiensis could develop more ra pidly in insects exposed to seed mixtures of plants with and without endoto xin if larvae move between plants and if an external refuge exists. These d ata provide evidence of larval movement between plants in seed mixtures.