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.

Native plant communities invaded by cheatgrass (Bromus tectorum L.) are at risk of unnatural high intensity fires and conversion to cheatgrass monocultures. Management strategies that reduce cheatgrass abundance may potentially allow native species to expand and minimize further cheatgrass invasion. We tested whether the selective herbicide imazapic is effective in reducing cheatgrass and ''releasing'' native species in a semiarid grassland and shrub steppe in north-central Oregon. The experiment consisted of a completely randomized design with two treatments (sprayed with 70 g ai ? ha 21 of imazapic and unsprayed) and three replicates of each treatment applied to either 2.5 or 4 ha plots. We repeated this experiment in three different sites dominated by the following native species: 1) bluebunch wheatgrass (Pseudoroegneria spicata [Pursh] A. Lö ve ssp. spicata) and needle and thread (Hesperostipa comata [Trin. & Rupr.] Barkworth), 2) needle and thread and Sandberg bluegrass (Poa secunda J. Presl), and 3) big sagebrush (Artemisia tridentata Nutt.). Nested frequency of all plant species in 1-m 2 quadrats was collected for 1 yr pretreatment and 4 yr posttreatment. In all three sites, cheatgrass frequencies were significantly lower in sprayed plots than unsprayed plots for 3-4 yr posttreatment (P , 0.1). Other annual plant species were also impacted by imazapic, but the effects were highly variable by species and site. Only two native perennial species, hoary tansyaster (Machaeranthera canescens [Pursh] Gray) and big sagebrush, increased in sprayed plots, and increases occurred only at two sites. These results suggest that a short-term reduction in cheatgrass alone is not an effective strategy for increasing the abundance of most native perennial plant species. Resumen Las comunidades de plantas nativas que están invadidas del pasto cheatgrass (Bromus tectorum L.) están en riesgo de fuego intencionales de alta intensidad y convertirse en monocultivos de éste pasto. Estrategias de manejo que reduzcan la abundancia del pasto cheatgrass podrían tener el potencial de permitir que especies nativas se expandan y minimicen la posible invasión por el cheatgrass. Probamos sí el herbicida Imazapicis que es efectivo en reducir el pasto cheatgrass y ''liberar'' especies nativas en pastizales semiáridos y matorral estepario en la parte centro-norte de Oregon. El experimento consistió en un diseñ o completamente al azar con dos tratamientos (asperjar con 70 g ia ? ha 21 de Imazapic y sin asperjar) y tres repeticiones por cada tratamiento aplicado ya sea a parcelas de 2.5 o 4 ha. Repetimos el experimento en tres diferentes sitios dominados por las siguientes especies nativas: 1) (Pseudoroegneria spicata [Pursh] A. Lö ve ssp. spicata) y (Hesperostipa comata [Trin. &Rupr.] Barkworth), 2) (Hesperostipa comata [Trin. &Rupr.] Barkworth) y (Poa secunda J. Presl), y 3) Artemisa (Artemisia tridentata Nutt.). Se recolectaron todas las especies en un cuadrante de 1 m 2 en frecuencia anidada por un añ o como pre-tratamiento y cuatro añ os co...

The impact of predation by the robber fly Proctacanthus milbertii Macquart on populations of adult grasshoppers from grasslands of the Nebraska sandhills was estimated. Densities of P. milbertii were estimated at 437 individuals per hectare (2 se=122). Overall densities of 23 species of grasshoppers were estimated to be 64,000 individuals per hectare with the most abundant species (Ageneotettix deorum) having a population size of approximately 15,000 individuals per hectare. Based on three estimates of predation level (ranging from 0.5 to 2 prey per day per robber fly), P. milbertii may take from 0.5% to 2% of the adult grasshoppers per day. Species of grasshoppers were taken by P. milbertii in about the same proportion in which they occurred at the study site and no size-selective component of predation was detectable.

We used mark‐recapture data to measure and model movement by the adult stage of the cinnabar moth Tyria jacobaeae (Lepidoptera, Arctiidae). Our objectives were: (1) to standardize the description of cinnabar moth movement and thereby provide a more reliable basis for comparison and extrapolation, and (2) to screen the potential influence of gender, physical condition, time, spatial location, and spatial heterogeneity in the environment on movement parameters. Recapture rates depended on wing wear (low 16%, high 11%) but were independent of behavior immediately after release (mobile 15%, immobile 14%), gender (male 13%, female 16%), or location in the study area (central 15%, peripheral 15%). Thus, mortality and/or emigration was greater for individuals with high wing wear compared to those with low wing wear. Moths were aggregated at the southwest margin of the study area both at initial capture and recapture. The direction of movement was biased in north and south directions, perhaps because forests on the east and west boundaries created a corridor channeling movement. The movement observed within a single generation did not conform to simple diffusion, which predicts that the mean square displacement (MSD) increases linearly with time. Instead, observed movement rates varied over time, and the relationship between MSD and time varied by gender, smoothly decelerating in males and erratically increasing in females. Observed movement (represented by the frequency distribution of displacements x for each time t with time measured at a daily resolution) was adequately approximated by a Weibull distribution, for which estimates of the shape and scale parameters were calculated. Male distributions were more frequently mounded, associated with higher values of the shape parameter α, and the scale of male distributions shifted sooner to longer distances, associated with higher values of the scale parameter β. The Weibull parameters varied by sex and time, suggesting that males were initially more mobile than females, but the magnitude of the difference varied over time. The speed and direction of individual movement were independent of the moth's spatial location within the field. We conclude that (1) movement speed depended on gender and time, while movement direction was related to spatial heterogeneity in the environment, and (2) much of the disagreement about movement rates among studies of the cinnabar moth can be related to researchers taking measurements and drawing inferences on different scales of observation, and therefore arriving at conflicting results.