The traditional ways to design biological control systems for plant invaders include (in order of decreasing emphasis) introducing, augmenting, or conserving natural enemies. Manipulating consumer–resource relationships in this way (1) emphasizes top‐down control of the invader by consumers rather than bottom‐up control of the invader by limiting resources, and (2) contributes to a rising number of control organisms introduced to North America that is creating complexity, redundancy, and risk. New concepts and methods have started to transform the way biological control organisms are found and developed by (1) combining herbivore and resource limitation of plant population growth and (2) using targeted life‐cycle disruption, which involves identifying plant life‐cycle transitions that are both amenable to manipulation and influential on population growth, and then targeting these for control. To illustrate these developments, we outline an experimental and computational approach for measuring how the processes of disturbance, colonization, and organism interactions (plant competition and herbivory) manifest their influence on weed life cycles and population growth of ragwort Senecio jacobaea, a biennial or short‐lived perennial herb. Manipulating these forces may lead to designs of biological control systems that are parsimonious, potent, and pose minimum risk to non‐target organisms.

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1% of its former abundance and has been replaced by a plant community composed predominantly of introduced perennial grasses.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.

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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 2 plots arranged as 4 Blocks x 2 Disturbance Times (plots were tilled in Fall 1986 or Spring 1987) x 3 Plant Competition levels (vegetation 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 germination 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.