Disturbance, Competition, and Herbivory Effects on Ragwort Senecio Jacobaea Populations

Ecological Monographs, 63(1), 1993, pp. 55-75 © 1993 by the Ecological Society of America DISTURBANCE, COMPETITION, AND HERBIVORY EFFECTS ON RAGWORT SENECIO JACOBAEA POPULATIONS PETER B. MCEVOY, NATHAN T. RUDD, CAROLINE S. Cox, AND MANUELA HUSO Department of Entomology, Oregon State University, Cordley Hall 2046, Corvallis, Oregon 97331-2907 USA Abstract. The balance of forces determining the successful control of ragwort Senecio jacobaea by introduced insects was investigated in a field experiment by manipulating the time of disturbance, the level of interspecific plant competition, and the level of herbivory by the cinnabar moth Tyria jacobaeae and the ragwort flea beetle Longitarsus jacobaeae. We used a factorial design containing 0.25-m = plots arranged as 4 Blocks x 2 Disturbance Times (plots were tilled in Fall 1986 or Spring 1987) x 3 Plant Competition levels (veg- etation other than ragwort was Removed, Clipped, or Unaltered) X 2 Cinnabar Moth levels (Exposed, Protected) x 2 Flea Beetle levels (Exposed, Protected). The response of ragwort was measured as colonization, survivorship, and reproduction of the first ragwort generation, establishment of juveniles in the second generation, and changes in ragwort biomass from 1987 through 1990. We also made annual measurements from 1987 through 1990 of the allocation of space (the limiting resource in the Unaltered competition treatment) among the categories ragwort, other species, litter, and open space. Natural enemy responses were characterized by relating variation in the concentration of enemies and the concentration of ragwort among patches. We found that abundant buried seed and localized disturbances combined to activate incipient ragwort outbreaks, and that interspecific plant competition and herbivory by the ragwort flea beetle combined to inhibit the increase and spread of incipient outbreaks. Time of disturbance had little effect on the outcome of biological control. Under conditions in the Removed and Clipped treatments (where there was sufficient open space for ger- mination and establishment), reduction in seed production in the first generation caused by cinnabar moth larvae led to a reduction in plant numbers in the second generation, but caused only a weak effect on ragwort cover and no detectable effect on ragwort biomass over the longer term from 1986 through 1990. At the spatial scale examined, inhibition by the ragwort flea beetle and plant competition took the extreme form of elimination of all ragwort individuals except the pool of seed buried in the soil. Our findings lead us to (1) reject the view that successful biological control leads to a stable pest—enemy equilibrium on a local spatial scale, (2) strongly endorse "search and destroy" and weakly endorse "complementary enemies" strategies suggested by Murdoch et al. (1985) as ways to improve control, and (3) emphasize resource limitation in the pest at low density as a key feature distinguishing biological control of weeds from biological control of insects. Key words: biological control; colonization; competition; disturbance; field experiment; insect herbivory; life cycle; Longitarsus jacobaeae; population dynamics; Senecio jacobaea; succession; Tyria jacobaeae. INTRODUCTION control. Do herbivores impose a low, stable pest—en- Classical biological weed control is achieved by her- emy equilibrium at a local spatial scale, or does local bivore introductions that reduce and maintain pest extinction occur? Is success more likely from a single populations below an economic threshold resulting in "best" herbivore species or from the cumulative effects the replacement of the weed by more desirable vege- of multiple herbivore species? Is plant competition tation. Some spectacular and widely cited cases are necessary to augment the impact of natural enemies control of Opuntia in Australia (Dodd 1940) and on the dynamics of weed populations? Klamath weed Hypericum in California (Huffaker and The control of ragwort Senecio jacobaea L. by cin- Kennett 1959). Despite interest aroused by these suc- nabar moth Tyria jacobaeae (L.) and ragwort flea beetle cesses, there has been comparatively little effort in en- Longitarsus jacobaeae (Waterhouse) provides an ideal suing years to test assumptions and predictions of eco- system for investigating questions about successful bi- logical theories offered as explanations of biological ological weed control. Ragwort is representative of a large class of weed problems in which an exotic plant, toxic to livestock and capable of displacing desirable Manuscript received 3 February 1992; accepted 6 April forage, infests large areas of nonirrigated pasture and 1992; final version received 27 April 1992. range. Introduced natural enemies are well established 56 PETER B. McEVOY ET AL. Ecological Monographs Vol. 63, No. I and have exerted strong and steady depression of rag- negative effects of the enemy on the pest are sufficiently wort over much of the Pacific Northwest under very strong to depress pest density to low levels, and suffi- diverse conditions (Hawkcs and Johnson 1978, McEvoy ciently aggregated to stabilize the interaction over the 1985, 1989, Pemberton and Turner 1990, McEvoy et longer term, thereby preventing wide amplitude fluc- al. 1991). The population ecology of ragwort has been tuations, outbreaks, or local extinctions. The focus of studied for half a century in native populations in Eu- theoretical research, applied mainly to control of in- rope (Cameron 1935, Harper and Wood 1957, Demp- sects rather than weeds, has been to determine what ster 1971, 1975, 1982, van der Meijden 1971, 1979, degree of aggregation will yield stability, the relation- Forbes 1977, Dempster and Lakhani 1979, van der ship between aggregation and organism density, and Meijden and van der Waals-Kooi 1979, Lakhani and the various forms of heterogeneity (individual, spatial, Dempster 1981, Crawley 1983, Crawley and Nacha- and temporal) that yield the desired level of aggregation pong 1984, 1985, Crawley and Gillman 1989) and in (Hassell and May 1973, 1974, May 1978, Hassell 1984, introduced populations in North America (Harris et Kareiva 1990, Pacala et al. 1990). Several mechanisms al. 1971, Isaacson 1971, 1978, Myers 1976, 1980, My- that aggregate risk have been found to confer stability ers and Campbell 1976, Harris et al. 1978, McEvoy on simple mathematical models, and theorists have and Cox 1987), Australia (Bornemissza 1966, Schmidl suggested that screening candidate biological control 1972a, b) and New Zealand (Poole and Cairns 1940, systems for such stabilizing mechanisms could im- Wardle 1987), and therefore many of the ecological prove the prospects for successful biological control processes affecting ragworts distribution and abun- (Hassel 1978, 1985). There are now both theoretical dance have been recorded. and empirical grounds for doubting the wisdom of this strategy (Murdoch et al. 1985, Murdoch 1990). Mur- Three issues motivating this study doch et al. (1985) claim that many successful control Local equilibrium vs. local extinction. -Nearly 40 yr systems are characterized by local extinctions and rein- ago, referring to successful control of the Klamath weed vasions, and many such systems appear to lack the by the defoliating Chrysolina beetle, Huffaker (1953) mechanisms that yield stability in models. More un- wrote: "A weed under biological control by its density- settling, there is theoretical and experimental evidence dependent. controlling insect would of course undergo that some stabilizing mechanisms operating on a local the oscillations characteristic of this type of control. scale actually interfere with control and lead to higher The success of this project will be ascertained not on pest densities (Murdoch and Stewart-Oaten 1989, the basis of the number of acres completely freed of Murdoch 1992). the weed but in terms of the average level of weed Alternative mathematical theories have addressed abundance over a period of years as it is maintained the question of how local populations that individually in equilibrium with the insects feeding on it, as con- exhibit unstable dynamics may collectively persist at trasted with its proved capabilities in the absence of a larger spatial scale (Reeve 1988, 1990, Taylor 1990). the beetles. It is expected that if the program is suc- In theory this requires sufficient asynchrony and suf- cessful, the stands of weed will be greatly reduced in ficient migration among local populations to prevent size and will constantly shift from place to place, as in elimination on more global scales. The challenge is to its turn is discovered and gradually weakened or de- establish what the sufficient conditions are. By exam- stroyed. New weed patches will of course be coming ining biological control systems in this way, we are into being all the while." Since successful control of likely to find new attributes whose importance is ob- Klamath weed was primarily responsible for fostering scured by the classical approach. the establishment and expansion of the subdiscipline Field experiments can help discriminate between of biological control of weeds in North America, it is these alternative theories by answering an important significant that early practitioners characterized the bi- question: To what extent does stability on a local spa- ological control process as leading to a general condi- tial scale contribute to persistence of biological control tion of local instability and stable average concentra- in the field

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