DITERPENOID ALKALOID CONCENTRATION IN TALL LARKSPUR PLANTS DAMAGED BYLARKSPUR MIRID

Tall larkspur (Delphinium barbeyi) is a serious poisonous plant threat to cattle on mountain rangelands. The larkspur mirid [Hopplomachus af figuratus] has been proposed as a biological tool to damage tall larks pur in an effort to deter grazing by cattle and thus prevent poisoning . Preliminary data suggested that it may also reduce toxic alkaloid le vels. The objective of this study was to determine if damage caused by the larkspur mirid would reduce toxic alkaloid concentration. Larkspu r mirids were collected in the field in 1992 and placed on potted plan ts 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 me asured in two larkspur populations having established mirid population s and in two newly infested larkspur populations. In the 1996 field st udy, three widely separated larkspur populations infested with mirids were sampled. Mirid-damaged leaves were lower in toxic alkaloids in bo th years, but there were no differences in flowering heads. However, o nly at Yampa, Colorado, did mirids reduce toxic alkaloids to levels th at would not pose a threat to cattle. There was no difference in toxic or total alkaloid concentration between larkspur populations with lon g-term mirid infestations compared to newly infested plants. The plant -to-plant variability in alkaloid concentration was greater than diffe rences due to mirids.