Tall larkspur kills more cattle on western mountain rangelands than any other plant, disease or predator, yet larkspur is a dominant and beautiful wildflower, leading to social conflicts where herbicide is used on public anges. Alternative measures are being sought to replace herbicides for control. The larkspur mirid, Hoplomachus affiguratus (Uhler), is host-specific to tall larkspur, Delphinium spp. (Ranunculaceae). Previous research demonstrated that cattle and sheep will not graze mirid-damaged larkspur plants. The objective of this series of studies was to quantify the type and amount of damage the larkspur mirid can inflict on larkspur plants Damage was estimated in naturally-infested populations and in cage and greenhouse tests using different insect and plant phenologies and insect densities. Damage was confined to specific feeding sites; there were no systemic effects. Mirids preferred to feed on flower buds and young leaves. Flowering heads aborted in both greenhouse and field studies. Leaf damage was a function of mirid density and plant size. When mirids attacked younger, smaller biomass plants, more damage occurred. As insect density increased, leaf damage increased. Cattle did not graze aborted flower heads, but grazed most of the insect-free reproductive racemes that were allowed to flower. When large numbers of mirids infested plants in the spring, most of the plant material was severely damaged. There is potential for the larkspur mirid to be managed as a biological agent to damage larkspur plants to the point of avoidance by grazing cattle, thus reducing mortality due to acute poisonings.