With grants from Baltimore Gas and Electric Co. and PepsiCo, Hull hired five students last year who recruited hundreds of volunteers to begin clearing ivy from a portion of the arboretum. It's mostly been removed from the trees, but the ground-hugging vines have proved more difficult. Students trying to yank them up ran into flak from yellow jackets angry over having their subterranean nests disturbed. After a couple got stung, Hull said he decided to look for alternatives.