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Imported Parasitoids
Thyme/iells /it/('o/a (Ochsenheimer), 81
Sixteen species of parasitoids have been reared from skipper populations in Quebec
since 1972 and only 3 species are common to the 22 species found in Ontario (Arthur
1962). As reported for Ontario total parasitism levels are very low and the only parasitoid
recovered consistently in any numbers is Itoplectis cotlquisitor (Say). Based on
observations over the last 7 years the importance of indigenous parasitoids in the
population dynamics of T. lin eo/a would have to be considered as negligible. While the
diversity of parasitoid species was lower in Europe, Carl (1968) reported high levels of
parasitism and felt that the principal larval parasitoids Pllryxe vulgaris Fall. and Rogas
tristis Westm., as well as the major pupal parasitoid Syspasis (=Slenichneumon)
scutellator (Grav.) would be suitable candidates for introduction into Canada. He
believed that S. scutellator would be the most promising due to the fact that it was well
synchronised with the skipper and, as it is univoltine, would not require alternate hosts
as do the larval parasitoids.
Specimens of P. vulgaris have been received regularly from Europe since 1972 (Table
13), but unfortunately shipments usually arrived when local skipper populations had
already pupated. Thus efforts were made to maintain laboratory cultures using Arlogeia
rapae (L.) larvae as an alternate host. Female parasitoids readily oviposted and parasitoid
larvae developed successfully, however difficulties in maintaining healthy host
colonies inhibited successful mass rearing of the parasitoid. In 1974, adults arrived soon
enough to be released directly in the field at Normandin (Table 14), but no P. vulgaris
have been recovered in the area. Releases of adults and parasitized A. rapae larvae were
made at Rawdon in 1974 and 1975 (Table 14), but were too late to coincide with the
presence of skipper larvae. However cabbages were quite widely cultivated around the
release site and an abundance of imported cabbageworm larvae were available as
alternate hosts. In subsequent sampling no skipper larvae parasitized by P. vulgaris were
recovered. It is of interest to note that P. vulgaris is an indigenous species and has been
reared from A. rapae in Ontario (Harcourt 1966), yet has never been reported attacking
T. lineola in either Ontario or Quebec. Carl (1968) suggested that different geographic
races of P. vulgaris, with restricted host ranges may exist, which would explain this
situation.
Shipments of S. scutellator have been received from Delemont since 1974 and as seen
in Tables 13 and 14, numbers were rather low in 1974 and 1975 but increased subsequently.
In an effort to increase the numbers collected, Dr Carl treated fifth-instar skipper larvae
with fluorescent powder and released them at the collection site at Plancher-Bas. It was
felt that as the last larval skin remains attached to the pupae individuals could be easily
located using a black light. While the recovery of marked individuals was poor, it was
observed that skipper pupae that had been parasitized were more readily located with a
black light than unparasitized ones, even without marking. Using this technique, night
time collections yielded higher numbers than in previous years. Adults held under
laboratory conditions throughout the winter (Table 13) were kept at 2 ± 1°C in plastic
boxes containing leaf litter and peat moss. Males died rapidly and although females lived
much longer only one survived until spring but died before T. lineola pupae were
available. From 1977 all adults not used in open releases were held on arrival in an
insectary and supplied with a 10% sugar solution as well as water. By mid-September all
males and some females had died. At this time all surviving females were placed in large
field cages containing logs, hay bales, and leaf litter. In 1977-78 there were no survivors
while in 1978-79 we recovered 5 of the 200 females in mid-June. These were fed and
held in the insectary until skipper pupae were available. One female stung the first pupa
that she encountered, remaining 45 minutes with her ovipositor in the host and upon
withdrawal, died. The stung pupa was held but no parasitoid emerged. The other 4
females however, showed no interest in skipper pupae. New hosts were provided each
day until the parasitoids died. All exposed pupae were held but only T. lineola adults
emerged. In 1979 the holding cages were destroyed during a storm and most parasitoids