Managing weeds with insects and pathogens - Ecological Management of Agricultural Weeds - page 386

punicea surrounding citrus orchards in the Oliphants River valley reduced
populations of T. lativentre early in the growing season and almost eliminated
the weevil's ability to suppress bud, flower, pod, and seed production by the
tree. On farms in the same valley where insect pests of citrus were managed
through biological control, or where infestations of S. punicea were remote
from citrus orchards, T. lativentre populations were higher and S. punicea seed
production was substantially reduced by the weevil.
McEvoy, Cox & Coombs (1991) used observations of natural population
dynamics at a single site, data from a controlled field experiment involving
population manipulations,and results of a regional survey to examine biolog-
ical control of Senecio jacobaea by exotic insects in Oregon. The weed is a bien-
nial or short-lived perennial that displaces desirable forage and is toxic to
livestock. Its native range extends from Scandinavia south through Asia
Minor, and from Great Britain east to Siberia, but it has spread to New
Zealand, Australia, Argentina, and coastal areas of the USA and Canada.
Starting in the 1960s, three insects native to France and Italy were introduced
to control S. jacobaea in western Oregon: a moth, Tyria jacobaeae ; a fly,
Botanophila seneciella (formerly Hylemia seneciella ) ; and a flea beetle, Longitarsus
jacobaeae (McEvoy,Cox & Coombs,1991).Larvae of T. jacobaeae will feed on veg-
etative stages of the weed but prefer flowering individuals; complete defolia-
tion is common, but death is rare, and plants often develop secondary
flowering shoots after the primary shoot is destroyed. Larvae of B. seneciella
consume immature seeds and involucre bases.Adults of L. jacobaeae chew holes
in the leaves and larvae tunnel into leaf petioles and roots.
Population dynamics of Senecio jacobaea were monitored in an abandoned
pasture in western Oregon from 1981 through 1988 (McEvoy, Cox & Coombs,
1991). Tyria jacobaeae , Longitarsus jacobaeae , and Botanophila seneciella were all
present as the result of deliberate introduction at the site or migration from
other release points in the state, and each of the three insect species was
observed to feed on S. jacobaea plants at the study site in at least one year.
Density of the weed decreased dramatically between 1981 and 1988, from

90% of total plant standing crop to 1%. The reduction in weed biomass was

matched by an increase in biomass of four perennial grass species. Seed den-
sities of S. jacobaea in soil (to 10 cm depth) decreased in concert with the reduc-
tion in above-ground plant biomass, from 14500 seeds m 2 in 1982 to 4800
seeds m 2 in 1988.
Further insight into the impacts of herbivory on S. jacobaea came from a
field experiment in which high-density, mixed-age S. jacobaea populations
were created by sowing seeds directly and transplanting additional small
rosettes from a greenhouse (McEvoy, Cox & Coombs, 1991). Following an