Diterpenoid Alkaloid Concentration in Tall Larkspur Plants Damaged by Larkspur Mirid

Abstract

Tall larkspur (Delphinium barbeyi) is a serious poisonous plant threat to cattle on mountain rangelands. The larkspur mirid [Hopplomachus affiguratus] has been proposed as a biological tool to damage tall larkspur in an effort to deter grazing by cattle and thus prevent poisoning. Preliminary data suggested that it may also reduce toxic alkaloid levels. The objective of this study was to determine if damage caused by the larkspur mirid would reduce toxic alkaloid concentration. Larkspur mirids were collected in the field in 1992 and placed on potted plants in the greenhouse. The resulting mirid-damaged leaves were lower in toxic alkaloids than leaves from uninfested plants. In the 1995 field study, toxic and total norditerpenoid alkaloid concentrations were measured in two larkspur populations having established mirid populations and in two newly infested larkspur populations. In the 1996 field study, three widely separated larkspur populations infested with mirids were sampled. Mirid-damaged leaves were lower in toxic alkaloids in both years, but there were no differences in flowering heads. However, only at Yampa, Colorado, did mirids reduce toxic alkaloids to levels that would not pose a threat to cattle. There was no difference in toxic or total alkaloid concentration between larkspur populations with long-term mirid infestations compared to newly infested plants. The plant-to-plant variability in alkaloid concentration was greater than differences due to mirids.