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United States Department of Agriculture

Agricultural Research Service

Title: Population Cycles of the Larkspur Mirid Heteroptera: Miridae

Authors
item Ralphs, Michael
item Jones, Walker

Submitted to: Journal of Entomological Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 10, 2000
Publication Date: October 1, 2000
Citation: Ralphs, M.H., Jones, W.A. 2000. Population cycles of the larkspur mirid heteroptera: miridae. Journal of Entomological Science.

Interpretive Summary: Tall Larkspur is a serious poisonous plant on mountain rangelands causing thousands of cattle deaths annually. The larkspur mirid (Hoplomachus affiguratus Uhler) is a native insect that is host specific to tall larkspur. It feeds on larkspur leaves and flowering heads, causing the heads to abort, and the leaves to become necrotic and then dessicate. It has been proposed as a biological tool to damage larkspur to prevent cattl from grazing it and becoming poisoned. Livestock will not eat larkspur that has been heavily damaged by the mirid. However, the mirid populations are cyclic. The objective of this study was to quantify their population cycle at 5 locations on 3 National Forests in 2 states, and determine if it is related to weather. The mirid population was high in Yampa, CO in 1989 and 1990, declined to where little damage occurred in 1992, then increased to the point where all larkspur plants were heavily damaged in 1996-1998. Mirid populations at Ferron, UT remained high from 1992-1995, declined in 1996, recovered somewhat in 1997, then crashed in 1998. Populations at 3 other Utah sites also declined greatly in 1998. The amount of larkspur damage was negatively correlated with the previous year's total precipitation and the previous September precipitation, and was positively correlated with July and August temperature the previous year. Mirid populations are cyclic and appear to be related to weather. 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.

Technical Abstract: Tall Larkspur (Delphinium barbeyi (L.) Huth) is a serious poisonous plant on mountain rangelands. The larkspur mirid (Hoplomachus affiguratus Uhler) is a native insect that is host specific to tall larkspur. It feeds on larkspur leaves and flowering heads, causing the heads to abort, and the leaves to become necrotic and then dessicate. The larkspur mirid has been proposed as a biological tool to damage larkspur to prevent cattle from grazing it and becoming poisoned. Livestock will not eat larkspur that has been heavily damaged by the mirid. The objective of this study was to monitor mirid populations over time and determine if its population cycles are related to weather. Mirids are difficult to count directly in a nondestructive manner. However, the extent of their damage to larkspur plants is clearly observable and can be readily estimated. We counted the proportion of aborted flowering heads, and visually estimated the percentage of leaf biomass damaged at the end of the growing season, as an indirect measure of the mirid population. Individual larkspur plants were marked and the extent of damage estimated a 5 locations on 3 National Forests in 2 states. In years when mirid populations were high, damage to larkspur leaves ranged from 50 to 100%, and greater than 75% of flowering heads aborted. The mirid population was high in Yampa CO in 1989 and 1990, declined to where little damage occurred in 1992, then increased to the point where all larkspur plants were heavily damaged in 1996-1998. Mirid populations at Ferron, UT remained high from 1992-1995, declined in 1996, recovered somewhat in 1997, then crashed in 1998. Populations at 3 other Utah sites also declined greatly in 1998. The amount of larkspur damage was negatively correlated with the previous year's total precipitation and

Last Modified: 11/1/2014
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