Population Cycles of the Larkspur Mirid (Heteroptera: Miridae)

The larkspur mirid, Hoplomachus affiguratus Uhler, has been proposed as a biological control agent to damage tall larkspur, Delphinium barbeyi (L.) Huth, to prevent cattle from grazing it and becoming poisoned. The objective of this study was to monitor mirid populations and feeding damage to larkspur over time and determine if its population cycles are related to specific weather patterns. Individual larkspur plants were marked and the extent of damage estimated at 5 locations on 3 National Forests in Utah and Colorado. In years when mirid populations were high, damage to larkspur leaves ranged from 50 to 100% and greater than 75% of flowering heads aborted. In years and locations when its population was down, damage was minimal. The amount of larkspur damage was negatively correlated with the previous year's total precipitation and the previous September precipitation (r = −0.68 to −83) and was positively correlated with July and August temperature the previous year (r = 0.61 to 0.75). When mirid populations are high, damage levels appear to be sufficient to deter cattle grazing, but low levels of damage at the bottom of the cycle will likely not deter grazing.