Successful Biological Control of Ragwort, Senecio Jacobaea, by Introduced Insects in Oregon

Abstract

The purpose of our study was to estimate the variability in a biological control process on a regional scale, identify its causes, and quantitatively evaluate overall control success. We present evidence of the success of biological control of Senecio jacobaea (ragwort) in western Oregon following introduction of three natural enemies. First, observations from a single site showed that ragwort declined to <1% of its former abundance and has been replaced by a plant community composed predominantly of introduced perennial grasses. Second, a perturbation experiment showed that introduced insects, within one ragwort generation, can depress the density, biomass, and reproduction of ragwort to <1% of populations protected from natural enemies. Third, a 12-yr survey of 42 ragwort populations showed that strong and persistent depression of ragwort recurred at many sites and at different times. Three features of this case history may be useful in the development of ecological theory as an explanation and guide for biological control: (1) the impact of the natural enemies depends on the distribution of individual sizes and ages in the ragwort population; (2) the long-term dynamics of ragwort may be influenced by the presence of large persistent seed bank which is invulnerable to the natural enemies; and (3) the success of biological control of ragwort in western Oregon appears to be independent of variation in environmental conditions. Combining local, short-term experiments and regional long-term observations is a powerful method for demonstrating successful biological control.

Theoretical contributions to biological control success

“…Create an upsurge in ragwort abundance and natural enemies readily colonize and return the population to pre-perturbation levels in ragwort populations exposed to natural enemies in open cages. This outcome contrasts with persistence ragwort populations at high levels of abundance when protected from natural enemies by closed cages (James et al 1992;McEvoy et al 1991;. Stabilizing mechanisms, whatever they might be, are not found at local scales (e.g.…”

“…In the vernacular favored by ecologists, the outcome was robust, not highly 'context dependent.' A local pulse-perturbation experiment showed that introduced insects, within one ragwort generation, can depress the density, biomass, and reproduction of ragwort to \ 1% of populations protected from natural enemies (McEvoy et al 1991). Stability was not confirmed by modeling.…”

“…Early studies documented variability in the biological control of tansy ragwort on a regional scale, identified its causes, and quantitatively evaluated overall success in surveys of field populations in Western Oregon spanning 12 years and 42 sites (McEvoy et al 1991). The system of interactions led to strong, stable suppression of ragwort to \ 1% of ragwort's former regional abundance, and the speed of control (6-8 years) did not vary with precipitation, elevation, land use, or timing (year) of natural enemy release.…”

“…The stabilization of ragwort density is not strictly a deterministic process, but perhaps better described by stochastic boundedness (Murdoch et al 1985): the ceiling on fluctuations in weed density steadily declined after releasing biological control organisms (McEvoy et al 1991). Strong suppression of ragwort abundance triggered a successional process leading to replacement of ragwort by a plant community dominated by perennial grasses introduced long ago from Europe (McEvoy et al 1991), but the community changes also allowed recovery of populations of a North American native herb (the hairy stemmed checker mallow Sidalcia hirtipes C.L. Hitchc., Malvaceae) of conservation concern (Gruber and Whytemare 1997).…”

“…An invulnerable stage for ragwort exists, represented by a large, persistent seed bank buried in soil (McEvoy et al 1991). Factors regulating the number of individuals entering, remaining within, and leaving the seed bank of ragwort populations are fairly well known (McEvoy et al 1991; the consequences of the seed bank for ragwort population dynamics have been investigated using structured population models, under the simplifying but unsatisfactory assumption of a constant annual rate of recruitment of individuals from the seed bank (Dauer et al 2012). A recipe for neutralizing a seed bank is to minimize disturbance and maintain a competitive grass cover.…”