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Discussion
Rhamnus cathartica L.. 187
The fruit is eaten by birds but usually only as a last resort. For example. robins will
take them sparingly when no other food is available in the early spring. It is claimed that
emodin in the green fruits prevents premature predation by birds (Trial & Dimond 1979).
Perhaps late retention of fruit well into the spring attests to this.
Several species of birds are cited as feeding on the fruit in Germany and England. and
mice have been seen to eat the seed from dried fallen fruit.
(d) Control
Buckthorn is so prevalent in Ontario. especially in the eastern counties. and the bush is
of such a large size that it is an almost impossible task to eradicate it by spraying. cutting,
or bulldozing. So very little, if anything, is being done to control it.
Herbicides give unsatisfactory results and 2,4,S-T which was recommended as a
treatment of freshly cut stumps is now outlawed in many states and provinces (Hanson
& Grau 1979). In any event, combinations of 2,4,-D and 2,4,S-T had little permanent
effect and treated bushes regrew from the base and many of the test buckthorn leafed out
one year after treatment (Switzer 1961). Ammate-X (ammonium sulfamate, duPont) is
recommended as either a foliar spray or a stump treatment in Wisconsin (Hanson &
Grau 1979) but early tests in Ontario showed that Ammate-X and fenuron (3-phenyl-l, 1dimethylurea)
had little effect in that most of the test plants over four feet high continued
to grow (Switzer et al. 1960, Switzer 1961).
Cutting buckthorn gives at best temporary control because the stumps send up
vigorous shoots in a relatively short time. However, cutting of individual bushes can
reduce the incidence of crown rust in nearby fields.
(e) Biological control
Because chemical control of buckthorn is at best temporary, biological control is an
attractive alternative and so at the request of Canada a survey for biological control
agents was made by tbe Commonwealth Institute of Biological Control in 1964
(Malicky et al. 1970). It was found that practically all the phytophagous insects specific
to or closely associated with Rhamnaceae are in the orders Lepidoptera and
Hemiptera. Many Coleoptera were found on Rhamnaceae but none was specific
(Malicky el al. 1970),
Investigations. particularly starvation tests suggest that the geometrids SCOlosia
vetulata Schiff., Triphosa dubitata L., and the Iycaenid Thecla spini L. may be
possible species for the biological control of R. cathartica in Canada. The geometrids
combine a high degree of host specificity with a potential to defoliate their host,
however. T. dubitata overwinters as an adult which may prove difficult in Canadian
situations. T. spin; may have too wide a host range to be an acceptable agent, but
there is some doubt as to the correctness of the host records (Malicky el al. 1970).
R. calharlica is only one of several species of Rhamnaceae capable of being an
alternate host to crown rust of oats (Dietz 1926). It is, however, the only one occurring
in any significant numbers in Canada (Peturson 19S4) .. Eradication of buckthorn would
not erase the problems of crown rust but would remove the site of the aecial or sexual
spores of the rust and thus the source of pathogenic variation through hybridization
and recombination of genetic material (Simons el al. 1979). Hybridization and recom-