Feeding Impacts of a Leafy Spurge (Euphorbia esula) Biological Control Agent on a Native Plant, Euphorbia robusta

The biological control agent Aphthona nigriscutis Foudras (Chrysomelidae) established in Fremont County, WY since 1992 on leafy spurge was released into a mixed stand a native plant Euphorbia robusta Engelm. During host range testing, E. robusta was a likely host for A. nigriscutis under laboratory conditions. In 1999, A. nigriscutis was observed feeding on both E. esula and 31 of 36 E. robusta plants present on about 2 ha (5 ac) where the visually estimated E. esula canopy cover was 50%. By August 2001, E. esula cover had declined to less than 5% and E. robusta plants had increased to 450 plants with 26 (5.8%) showing feeding damage. In 2006 Euphorbia esula ground cover was 2% and of 598 E. robusta plants originally marked, 391 could be located and four of these had damage consistent with A. nigriscutis feeding. For the 8-yr period, E. esula ground cover was inversely correlated to E. robusta density and positively correlated to A. nigriscutis feeding damage on E. robusta. This study shows that while also acceptable to A. nigriscutis in the field, feeding on E. robusta declined with declining densities of the target weed while E. robusta population densities increased. It seems that some risk in this regard is acceptable in light of the damage from the target weed and the generally high level of selectivity provided by biological control agents.

Genetic Variation in Aphthona nigriscutis Foudras (Coleoptera: Chrysomelidae) Populations Introduced into the United States to Biologically Control Leafy Spurge

Cellulose acetate electrophoresis was used to examine the levels and distribution of genetic diversity in Aphthona nigriscutis Foudras populations 5 to 9 yrs following their introduction to sites in North and South Dakota for the purpose of biologically controlling leafy spurge, Euphorbia esula L. Gene expression patterns of two enzymes, phosphoglucomutase (PGM) and malate dehydrogenase (MDH), were analyzed to determine the genetic variation and distance among and between populations in North America and Hungary where the species is indigenous. Dakota populations were variable and more similar to each other genetically than they were to the Hungarian population. North American populations were also genetically differentiated from each other as well as the Hungarian population according to geography and environment. Gene flow probably did not account for this genetic variability because natural dispersal of A. nigriscutis was limited and large distances existed between A. nigriscutis sites. Novel mutations nurtured by natural selection would, however, enhance the species' adaptability to geographic regions of the Dakotas as it served to distant populations genetically.

The Biology and Integrated Management of Leafy Spurge (Euphorbia esula) on North Dakota Rangeland

Leafy spurge, a long-lived perennial, grows in many habitats, from floodplains to grasslands and mountain slopes. The plant emerges in early spring and produces showy, yellow bracts that appear in late May. The true flowers emerge in mid-June. The plant spreads by both seeds and roots and contains a white sticky latex that deters grazing by many animals. Dicamba, 2,4-D, glyphosate, and picloram have commonly been used to control leafy spurge. Picloram plus 2,4-D is frequently used for leafy spurge control in North Dakota. Ten insect species for leafy spurge biocontrol have been released in North Dakota; the most successful have been the flea beetles,Aphthona nigriscutis, A. czwalinae, andA. lacertosa. The leafy spurge gall midge (Spurgia esulae) has been most successful near wooded areas. Herbicides combined with either the leafy spurge flea beetles or gall midge have controlled leafy spurge better than either method used alone. Grazing with sheep or goats is a cost-effective method for controlling leafy spurge top growth in large infestations. Grazing combined with fall-applied picloram plus 2,4-D reduced leafy spurge density more rapidly and maintained control longer than either method used alone. Several grass species are competitive with leafy spurge including ‘Rebound’ smooth brome, ‘Rodan’ western wheatgrass, ‘Pryor’ slender wheatgrass, and ‘Manska’ pubescent wheatgrass. Cultivating twice each fall after harvest for 3 yr in cropland completely controlled leafy spurge. A successful long-term management program should be designed for specific situations and should include combinations of herbicides, insects, grazing, and/or seeding competitive species.