TheHeliothisnuclear polyhedrosis virus was rapidly inactivated under field conditions on the upper leaf surface of cotton, soybean, and tomato. Inactivation of the virus was most rapid on cotton with little activity remaining after 24 h. While persistence on tomato was significantly better than on the other hosts, little virus activity remained after 96 h. Use of Shade™ and lignin sulfate as ultraviolet screens resulted in good virus persistence on all hosts through 48 h. Caging plants at night to prevent dew formation did not reduce virus inactivation. Persistence was good when plants were exposed from dusk to dawn but caged during the day to prevent sunlight inactivation. Undcr these conditions inactivation was greatest on cotton with 57% virus activity remaining after 96 h. Continuously caged plants of all hosts also had some loss of activity and while generally less than on plants exposed from dusk to dawn was not significantly different.