Managing weeds with insects and pathogens - Ecological Management of Agricultural Weeds - page 387

initial period in which plants in all plots were protected from herbivores with
cages, two treatments were established. Plants in “unprotected” plots had the
sides of their cages rolled up to expose them to herbivores; plants in “pro-
tected” plots remained caged and were treated with the insecticide carbofu-
ran. High densities of L. jacobaeae were observed in exposed plots. Because
densities of T. jacobaeae were lower than expected during the experiment,
plants were artificially defoliated to match the effect of grazing larvae.In plots
exposed to natural enemies (and their human mimics), S. jacobaea density,
biomass, and reproductive output all declined within 18 months to 1% of

populations protected from enemies.
Athird line of evidence demonstrating the suppressive effects of insect bio-
control agents on S. jacobaea came from a regional survey conducted by the
Oregon Department of Agriculture. Senecio jacobaea densities were found to
decline 93% across 42 sites in western Oregon in the six years following

introduction of L. jacobaeae (McEvoy, Cox & Coombs, 1991). Based on the com-
bination of observational and experimental studies, McEvoy, Cox & Coombs
(1991) concluded that introduced insect herbivores successfully controlled S.
jacobaea in western Oregon.
The majority of inoculative biological control agents used against weeds
have been insects. These have been dominated by Coleoptera (especially
Chrysomelidae and Curculionidae), Lepidoptera (especially Pyralidae),
Diptera (especially Tephritidae), and Hemiptera (especially Dacylopiidae and
Tingidae) (Julien, Kerr & Chan, 1984; Julien & Griffiths, 1998). The prepon-
derance of insect taxa as introduced weed biocontrol agents may be because
they are particularly well adapted to persist,disperse,and attack their hosts in
the rangeland and pasture environments typical of most weed biocontrol
efforts. Nonetheless, certain microbial taxa have proven effective as inocula-
tive agents for biological control of weeds in some environments. These taxa
typically have the ability to withstand dry periods and disperse relatively long
distances, often by wind (TeBeest, 1991; Watson, 1991). In Chile, for example,
the rust fungus Phragmidium violaceum was introduced from Germany to
control Rubus constrictus and R. ulmifolius on rangeland and pastures, and large
reductions in weed density resulted in several areas (Hasan & Ayres, 1990;
Julien & Griffiths, 1998). In Hawaii, importation and release of the fungus
Entyloma ageratinae in combination with release of an imported fly,
Procecidochares alanai , and a moth, Oidaematophorus beneficus , reduced densities
of the weed Ageratina riparia to

5% of preintroduction levels at many loca-
tions (TeBeest, 1996; Julien & Griffiths, 1998).
The rust fungus Puccinia chondrillina was introduced from Mediterranean
Europe into Australia in the early 1970s to control Chondrilla juncea in pastures