DISTURBANCE, COMPETITION, AND HERBIVORY EFFECTS ON RAGWORT SENECIO-JACOBAEA POPULATIONS

The balance of forces determining the successful control of ragwort Se necio jacobaea by introduced insects was investigated in a field exper iment by manipulating the time of disturbance, the level of interspeci fic plant competition, and the level of herbivory by the cinnabar moth Tyria jacobaeae and the ragwort flea beetle Longitarsus jacabaeae. We used a factorial design containing 0.25-m(2) plots arranged as 4 Bloc ks x 2 Disturbance Times (plots were tilled in Fall 1986 or Spring 198 7) x 3 Plant Competition levels (vegetation other than ragwort was Rem oved, Clipped, or Unaltered) x 2 Cinnabar Moth levels (Exposed, Protec ted) x 2 Flea Beetle levels (Exposed, Protected). The response of ragw ort was measured as colonization, survivorship, and reproduction of th e first ragwort generation, establishment of juveniles in the second g eneration, and changes in ragwort biomass from 1987 through 1990. We a lso 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 t he concentration of enemies and the concentration of ragwort among pat ches. We found that abundant buried seed and localized disturbances co mbined to activate incipient ragwort outbreaks, and that interspecific plant competition and herbivory by the ragwort flea beetle combined t o inhibit the increase and spread of incipient outbreaks. Time of dist urbance had little effect on the outcome of biological control. Under conditions in the Removed and Clipped treatments (where there was suff icient open space for germination and establishment), reduction in see d production in the first generation caused by cinnabar moth larvae le d to a reduction in plant numbers in the second generation, but caused only a weak effect on ragwort cover and no detectable effect on ragwo rt biomass over the longer term from 1986 through 1990. At the spatial scale examined, inhibition by the ragwort flea beetle and plant compe tition 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 stabl e pest-enemy equilibrium on a local spatial scale, (2) strongly endors e ''search and destroy'' and weakly endorse ''complementary enemies'' strategies suggested by Murdoch et al. (1985) as ways to improve contr ol, and (3) emphasize resource limitation in the pest at low density a s a key feature distinguishing biological control of weeds from biolog ical control of insects.