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Pest Status
Background
Chapter 41
Senecio jacobaea L., Tansy Ragwort
(Compositae)
P. HARRIS, A.T.S. WILKINSON and J.H. MYERS
The status of tansy ragwort, Senecio jacobaea L., in Canada was described by Harris el
al. (1971). Since that time the distribution has changed slightly. A small stand of the
weed that had existed at Guelph, Ontario, for many years recently started to spread (J.
Alex, 1981, personal communication). A small infestation in the Gaspe Peninsula,
Quebec, recorded prior to 1900 could not be found by Watson & Muirhead (1979). The
weed has continued to spread in the lower Fraser Valley in British Columbia, and it
remains a serious socio-economic problem in the maritimes where there are many small
mixed farms with a few cattle. The small field size and lack of specialized equipment
makes chemical and mechanical control difficult and expensive. On the other hand, the
death of one or two cattle from ragwort poisoning is a serious loss to the family food and
income. There are no provincially compiled statistics of cattle deaths from ragwort as
diagnosis is difficult and many cattle are sent for early slaughter if ragwort poisoning is
suspected. However, in 1981, in Prince Edward Island the Montague Veterinary Clinic
reported 12-20 ragwort-related cattle deaths; the Kensington Clinic had 20 but many
more were suspected; the Provincial Veterinary Pathology Laboratory reported 10-12
but suspected more (L. Thompson, 1981, personal communication). Thus the total
cattle mortality from ragwort is similar to the 40-65 deaths estimated to have occurred
in 1968 (Harris et al. 1971).
The cinnabar moth, Tyria jacobaeae (L.), was established in the Canadian maritimes in
1964 and in British Columbia in 1965 for the biological control of tansy ragwort. Despite
difficulties in getting the moth established (Harris et al. 1975), it is now present throughout
the ragwort infested regions of Canada, including Newfoundland where no releases
were made (Larson & Jackson 1980).
The effect of the moth on the weed has varied widely. At Prince Charles, Prince
Edward Island, and Durham, Nova Scotia, the weed has practically disappeared from
the release sites although it has persisted in the disturbed ground along the roadsides
(Harris el al. 1978). At Durham it was largely replaced by goldenrod Solidago sp. At
Sussex, New Brunswick, the ragwort stand collapsed after defoliation by cinnabar
larvae while the Canada thistle, Cirsium arvense (L.) Scop., increased. Tansy ragwort
has since returned to the site and the density of cinnabar reached circa three larvae/stem
in 1980 with scattered but numerous larvae in 1981 (D. Finnamore, 1981, personal
communication). Near Nanaimo, British Columbia, over the past ten years an average
of 38% of the ragwort was defoliated with another 18% partially defoliated. This has had
little effect on the density of the flowering plants but they are smaller. In California,
Hawkes & Johnson (1978) found that, with cinnabar moth attack, the number and size
of the ragwort flowering stems decreased and the number of rosettes increased. Not all
the declines were the result of the cinnabar moth as Myers (1980) found that two of five
stands in Nova Scotia declined even though the cinnabar population was not large
enough to cause extensive defoliation.
Many papers have been written in Canada and elsewhere on the causes of the diverse
effects of the cinnabar moth on tansy ragwort. Harris et al. (1978a) related the collapse
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