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Releases and Recoveries
Rhinocynus conicus
(Froel.) (Coleoptera:
Curculionidae) (a) Ecology
Trichosirocalus
horridus (panz.)
(Coleoptera:
Curculionidae)
Carduus IIutans L., 117
The biology and host specificity of R. conicus has been treated in detail by Zw61fer &
Harris (in press). The weevil is native to Europe, western Asia, and North Africa
between latitudes 30 0
N and SooN. Its main host is C. nutans but there has been race
formation on other thistles in the genera Carduus, Cirsium, Silybum, and Onopordum.
Teneral R. conicus emerge in July to early August in southern Ontario and August in
Saskatchewan. The weevil has a second generation if the day length is over 16 hrs.
Individuals released in a cage in southern Ontario on the 27 June when the day length was
15 hr 48 min did not breed, but those released in mid-July in Saskatchewan with a day of
16 hr 5 min did so. Laing & Heels (1978) reported a second generation from weevils
released in southern Ontario in August, but these were mated in the laboratory. presumably
under long day conditions. Normally, in Canada, there is a single generation emerging
after mid-summer and then hibernating in the soil litter. They appear again in the spring
to feed on the leaves of bolting thistles and oviposit on the involucral bracts of the flower
buds. Oviposition starts slightly before the buds are available in the spring with some
wastage of eggs on the leaves enclosing the terminal flower bud and the first heads receive
an abundance of eggs. Rees (1977) counted over 500 eggs on a single head.
The eggs are covered with a cap of chewed thistle and the larva bores directly into the
flower bud from under the cap. It mines the receptacle and sometimes the peduncle as
well as feeding on the young ovules. Typically the receptacle mine fills with callus on
which the larva feeds (Shorthouse, 1982, personal communication). Thus, like a gallformer,
the larva stimulates production of its food supply. The mature larva forms a
hard pupal chamber in the thistle head which remains after emergence so the number
that developed in a head can be counted.
(b) Releases
Most of the R. conicus stock established in Canada was collected from C. nwans growing in
the French Rhine Valley around Mulhouse. As the initial results with this stock on C.
acanthoides were disappointing, additional weevils were imported from the smallheaded
thistle C. personata (L.) Jacquin as well as stocks from Cirsium spp. Day length
at the release date probably prevented many of the colonies from breeding until the year
following release (Table 23). With hindsight, it would have been better to have held them
in laboratory storage until the following spring or mated them under long day conditions
in the laboratory before release.
(a) Ecology
The biology of T. horridus was reported by Trumble & Kok (1979) and a bibliography
of the weevil compiled by Trumble & Kok (1980). The weevil is found from Italy to
Poland, in Turkey and the USSR. The adults have been found feeding on a number of
thistle genera and the larvae developed on several of these in the laboratory (Kok 1975),
however, in the thistle garden at the Regina Research Station, they have only been found
on Carduus spp. and not on either European or native Cirsium spp. Thus the field host