Senecio Jacobaea in Northern California, an Enduring Success
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ENTOMOPHAGA 35 (I), 1990, 71-77 BIOLOGICAL CONTROL OF SENECIO JACOBAEA IN NORTHERN CALIFORNIA, AN ENDURING SUCCESS R. W. PEMBERTON (t) & C. E. TURNER Rangeland Insect Lab, USDA-ARS, Montana State University, Bozeman, Montana 59717 Biological Control of Weeds Western Regional Research Center USDA-ARS, Albany, California 94710 Seneciojacobaea, a poisonous weed from Eurasia, was brought under successful biological control in the Ft. Bragg, California area by 1976, through the combined action of the defoliating cinnabar moth (Tyriajacobaeae) and a root feeding flea beetle (Longitarsusjacobaeae). In 1987, 4 previously infested Ft. Bragg sites (3 sites where control had been documented and another unstudied site) were examined. Senecio jacobaea densities at these sites were 0.0, 0.0, 0.01 and 0.18 plants/m 2, indicating both continued and improved control of the weed. The flea beetle and the cinnabar moth both persist at the sites, despite very low numbers of S.jacobaea plants. The control of S. jacobaea has resulted in the return of near natural vegetation at the 2 coastal prairie sites and regained productivity at the 2 pasture sites. KEY-WORDS : Biological control, cinnabar moth, Longitarsusjacobaeae, poiso- nous plant, Seneciojacobaea, tansy ragwort, Tyria jacobaeae. INTRODUCTION THE WEED PROBLEM Tansy ragwort, (Senecio jacobaea L. : Asteraceae) is a biennial or short lived perennial herb that is native to Europe eastward to Siberia (Harper & Wood, 1957). The plant is an introduced weed in New Zealand, Australia, South Africa, Argentina, Brazil, Canada, and the United States (Holm et al., 1979). Tansy ragwort contains toxic pyrrolizidine alkaloids which cause liver damage and death to cattle and horses that ingest it (Kingsbury, 1964 ; Harris et aL, 1984). The weed can also displace more desirable forage plants and dominate pastures and other disturbed land it infests. In North America, S.jacobaea has been a problem in northeastern Canada and along the Pacific Coast in British Columbia, Washington, Oregon and Northern California (Frick & Holloway, 1964). Some tansy ragwort infestations in British Columbia appear to date from (1) Present address: Asian Parasite Laboratory, USDA Agriculture Research Service, Seoul, Korea c/o American Embassy APO San Francisco 96301 72 R. W. PEMBERTON& C. E. TURNER 1913 (Harris et al., 1971). The first Oregon record for the plant was at Portland in 1922 (Isaacson, 1978). Alice Eastwood made the I st California collection of tansy ragwort at Fort Bragg, Mendicino Co. in August 1912 (Alice Eastwood No. 1648 California Academy of Sciences, D. Bathe, pers. comm.). By the mid 1950s tansy ragwort had become an important weed of the Pacific Coast (Gilkey, 1957 ; Harris et al., 1971). Chemical and mechanical control of tansy ragwort have usually proven to be unecono- mical since most of the lands that the weed infests have relatively low productivity (Isaacson, 1973). Losses in Oregon were estimated to be at least $1.5 million and possibly as high as 10 million per year (Isaacson, 1978). BIOLOGICAL CONTROL HISTORY In the late 1950s a biological control program was begun against tansy ragwort by the USDA's Agriculture Research Service in Albany, Calif. (Holloway & Huffaker, 1960). Many European insect enemies of tansy ragwort were already known (Cameron, 1935 ; Harper & Wood, 1957), including the cinnabar moth (Tyriajacobaeae L., Arctidae) and a seedhead fly (Pegohylemyia seneciella Meade, Anthomyidae) which had already been (or were being) introduced to Australia and New Zealand (Julien, 1987). The I st insect to be released into North America for biological control of ragwort was the cinnabar moth, whose larvae feed on the foliage and flowers of tansy ragwort during the summer. In 1959 and 1961, 4,800 larvae, originating from French collections, were released at two Ft. Bragg, California sites (Frick & Holloway, 1964). These established in 1960 and 1962 (Frick & Holloway, 1964), and by 1963 the moth was abundant enough to defoliate almost I00 % of the ragwort at some sites (Hawkes & Johnson, 1978). This defoliation reduced the number of flowering stems but increased the number of rosette plants (Hawkes & Johnson, 1978). Since more control was needed, the seedhead fly (P. seneciella), whose larvae feed on the developing ragwort seeds, was released at Ft. Bragg in 1966. The fly, which was from France, established, but then was lost when the release site was destroyed (L. Andres, pers. comm.). A 3 rd insect Longitarsusjacobaeae (Waterhouse) (Chrysomelidae), was released in the Ft. Bragg area in 1968, 1969, and 1970 (Frick & Johnson, 1973). This flea beetle, which was collected from Italy, feeds on the root crowns, stems and leaf petioles of the plants during the larval stage in the late autumn, winter and spring (Hawkes & Johnson, 1978). By 1970, L. jacobaeae had established, and by 1972 the 955 beetles originally released had generated very large populations. At the Foresti ranch, where 511 beetles had been released, the weed was almost completely eliminated over an estimated 5 hectares by 1973, and the beetles could be found as far as 2 km from the site (Hawkes & Johnson, 1978). Measurements of tansy ragwort decline at 3 Fort Bragg sites continued until 1975 and 1976 when the densities had been reduced to 0.0, 0.6 and 0.2 plants/m 2 (Hawkes & Johnson, 1978). STUDY OBJECTIVES We undertook the study to learn : l) if tansy ragwort at Ft. Bragg was still being controlled by released agents, and if so, to what degree, 2) if the cinnabar moth and the ragwort flea beetle persisted in the area even at low plant densities, and 3) if control had been maintained, what vegetation had replaced tansy ragwort. MATERIALS AND METHODS Four Ft. Bragg sites were examined on September 8 and 9, 1987. Three of these sites (Todd Point, Foresti ranch and Smith ranch) were flea beetle release sites, where the BIOLOGICAL CONTROL OF SENECIO JACOBAEA 73 cinnabar moth had also been released or had colonized by the mid 1960s. The ragwort at the sites had been reduced to very low levels by the combined action of the flea beetle and moth (Hawkes & Johnson, 1978). The 4 th site, at MacKerricher State Park, had a small, very dense ragwort infestation in 1976, when both the flea beetle and the cinnabar moth had spread to the site (J. Drueeker & R. Hawkes, pers. comm.). Tansy ragwort density was measured by taking 200 one m 2 samples, at 5 meter intervals, along transects that zigzagged across the long and wide axes of the sites. The number of flowering and rosette form plants were recorded in each sample. When no ragwort was found in the samples, the site was thoroughly searched for plants. The number of ragwort plants at each site with flea beetles and cinnabar moths or their feeding damage were counted when plant densities were very low, or sampled (every 5 th plant was examined until 25 plants were checked) when plant densities were high. The Todd Point ragworts were not checked for insects. Qualitative listings of the dominant plants and their main plant associates were made at each site to get an idea of the replacement vegetation where ragwort had been controlled. RESULTS AND DISCUSSION Table 1 shows the densities of tansy ragwort we measured, and for comparison the ragwort densities measured from 1966 to 1976 by Hawkes & Johnson (1978). The Foresti ranch site is a pasture of moist prairie on river bottomland bordered by a Salix sp., located on the western and northern banks of Ten Mile River. The pasture has been lightly grazed (K. Olson, ranch owner, pers. comm.). No ragwort plants were found TABLE 1 Tansy ragwort plants/m 2 at 4 Ft. Bragg. California sites Autumn Foresti Todd Smith MacKerricher Sample Ranch Point Ranch State Park Dates 1966 -- 53.3 (l) _ _ (Tyria present) 1968 ( Tyria present ( Tyria -- Longitarsus present) released) 1969 15.3 1972 -- 71.1 (Longitarsus -- ( Longi tarsus released) released) 1973 -- 39.5 11.7 (i) __ 1974 -- 6.9 7.4 very dense (2). ( Longitarsus presen0 1975 0.0 0.6 0.5 -- 1976 (Spring) -- 0.6 0.2 (Tyria presenO (3) 1987 0.0 0.0 0.18 0.01 (t) pre 1987 densities from Hawkes 1978. (2) John Druecker pers. comm. 1988. (3) Robert Hawkes pers. comm. 1988. 74 R. W. PEMBERTON& C. E. TURNER in the 200 samples. Hawkes & Johnson (1978) also found no ragwort plants in 100 square meter samples. Twenty-one flowering ragwort plants but no rosettes were found within the ca. 4 hectare sample area. Adult L. jacobaeae >. 10/plant) were observed on 17 of these plants. Cinnabar moth feeding damage to the flowers was noted on one plant. The dominant plants were a Scirpus sp. and Holcus lanatus L. The primary associates were: Baccharis pilularis DeCandolle, Cirsium arvense (L.) Scopoli, C. vulgate (Savi) Tenore, Conium maculatum L., Digitalis purpurea L., Monardella villosa Bentham, Potentilla sp., Rubus sp., Rumex crispus L. and R. pulcher L, Tansy ragwort has been replaced by a mixture of native forbs, the introduced forage grass H. lanatus and exotic weeds. The pasture has regained its utility, although introduced thistles reduce its productivity. The Todd Point site is a coastal prairie on a bluff overlooking the ocean. The field was used to pasture sheep but no grazing has taken place since the early 1950s (J. Druecker, pets. comm.). No ragwort plants were found in the 200 samples and only 6 flowering plants were discovered in a search of the ca. 30 hectare field. Hawkes & Johnson (1978) recorded 0.6 plants/m 2 in 100 samples examined in 1976. There has been a significant improvement in the control, although the greatest reduction occurred from 1966 to 1974 when the ragwort population dropped from 53.3 to 6.9 plants/m 2.