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Releases and Recoveries
HyJemya senecieJJa
Meade (Diptera:
Muscidae)
Tyriajacobaeae (L.)
(Lepidoptera:
Arctiidae)
(a) Ecology
Senecio j(lcoh(l('(l L. . 197
H. senecie/la larvae reduce seed production by feeding in ragwort flower heads. The
biology was described by Miller (1970). and an unsuccessful attempt to establish it in
British Columbia and Prince Edward Island in 1968 was reported by Harris el 01. (1971).
The fly has been established in New Zealand where it infested up to 77% of the heads at
peak flowering but the effect of the reduced seed production on ragwort density was not
determined. The fly was established in California and probably in Oregon and
Washington (Frick 19690) but the California site was subsequently destroyed (Julien
1981).
(b) Releases
A summary of releases in the current review period is shown in Table 50; none of them
became established. At least part of the problem was poor synchrony between emergence
of the flies and ragwort flowering: in 1970 the H. seneciella were received in May
and placed outside in a field cage in Prince Edward Island. The flies emerged between 8
and 17 June but the host plant did not flower until August. In 1971 emergence was
delayed until the second half of July but still did not result in establishment (L.
Thompson. 1981. personal communication). The results from British Columbia were
similar.
Establishment in the United States was obtained with much larger releases of circa
2000 flies. As egg viability was circa 25% (Frick 1969b) a larger release than that used in
Canada is necessary. The problem with the synchronism of fly emergence can probably
be overcome by releasing the mature larvae at the end of the summer; this could be done
with the Oregon or Washington stock providing that it is free of parasites. The value of
establishing the fly is less clear since most ragwort reproduction in pastures is by vegetative
propagation from existing plants. It is unlikely that the fly would survive in stands
defoliated by cinnabar larvae which consume the flowers preferentially.
Releases
Field collected larvae from Nova Scotia were released at two sites in New Brunswick in
1970 and at seven sites in Ontario in 1979 and 1981 (Table 50). These were the only
releases since those reported by Harris el 01. (1971). Establishment occurred at both
sites in New Brunswick resulting in widespread defoliation of tansy ragwort but on a
long-term basis this is a less than desirable level of control. The insect did not become
established in Ontario, possibly because of predation by ants (Alex, 1981, personal
communication). About 20% of the Ontario shipment died in transit indicating that the
popUlation was heavily stressed and probably diseased.