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Neot!i/Jrio" satifer (Geoffroy), 333
level and does not generally increase to outbreak levels again. Eventual decline of an
outbreak was related to starvation of late-feeding larvae, parasitism of pre-spinning
larvae by Exenlerus spp. and predation and parasitism of cocooned larvae. When
outbreaks occur in stands of larger trees, they last longer and are associated with varying
amounts of prolonged diapause (Lyons 1977a).
Until 1958 the recorded insect parasitoid complex of N. sertifer consisted of 14 native
primary parasitoids, 3 native hyperparasitoids, and 4 established European parasitoids.
In the following decade another 14 indigenous parasitoids were recovered; recovery was
made of Drino bolremica Mesn., another of the European species introduced during the
1930s (McGugan & CoppeI1962); and one new exotic species, Lophyroplectus lutealor
Thunb., was established (Griffiths & Lyons 1968). In the period under review only two
species have been added to the parasitoid list. One of these, Exenterus affinis Roh., a
native ichneumonid, was recovered from N. sertifer as early as 1964, but was not
separated from the other species of Exenlerus attacking this host until 1977. It is
recovered infrequently (Lyons 1977b). The second species is Exenterus abruptorius
(Thunb.), a European parasitoid of N. sertifer, introduced in the 1930s but not recovered
since the initial release. Lyons (1977b) recovered two specimens in a cocoon collection
made in Dufferin County in May, 1972. Since then five more E. abruptorius from four
other localities have been identified after examination of over 6 000 pinned specimens of
Exenterus obtained from N. ser/ifer collections throughout southwestern Ontario since
1952. Further discussion of this species is given in the following section. Another
recovery of the rare exotic species D. bohemiea was recorded in 1972 from a collection
made at Gore Bay on Manitoulin Island.
Dispersal of L. IUleator from the releases made in 1962 and 1964 (Griffiths & Lyons
1968) has been followed closely. By 1969,7 years afterthe 1962 release in Grey County,
recoveries had been made up to 19.3 km east of the release point. However, by 1971, 7
years after the 1964 Norfolk County release, the greatest dispersal was only 3.4 km
(Griffiths 1973). Rose (1976) reported that by 1975 L.luteatorhad spread throughout the
whole of the N. sertifer distribution area east and north of the Grey County release
point. It is possible, however, that the northern dispersal into the Bruce Peninsula was
the result of a release made near Hepworth, Ontario, in 1970 (Griffiths & Lyons 1980).
Rose noted that L. luteator had spread 305 km in an easterly direction in the 13 years
since its release. There had been little or no movement south or west from the 1962
release point and little further dispersal from the 1964 release in Norfolk County.
Investigations on the biology of the principal parasitoids of N. sertifer and the
interactions between them have continued during the last decade. It was found that
females of Pleolophus basizonus (Grav.), which oviposit in cocooned pre-pupae, show
a strong rejection of hosts containing larvae or pupae of its own species, although they
do not discriminate between unattacked hosts and those containing eggs of its own
species (Griffiths 1972). Also, it was found that P. basizonus females show a strong
avoidance of cocoons in which there are eggs, larvae, or pre-pupae of the larval
parasitoid L. lutealor (Griffiths 1976) and avoid cocoons containing developing E.
abruptorius and Dahlbominus fuscipennis (Zelt.). On the other hand, D. fuscipennis
does not avoid hosts containing P. basizonus or E. abruptorius and is usually successful
in competition with them. Exenterus abruptorius, E. nigrifrons (formerly E. canadensis
Prov.) and E. amielorius (Panz.) all superparasitize and none avoids attacking hosts
containing eggs of either of the other two species. Neither E. abruptorius nor E.
nigrifrons has an advantage over the other when competing on the host, but E. amictorius
is usually successful when competing with the other species. This information on
Exenterus spp .• plus data on the three species obtained in insectary experiments over 6
years, was incorporated into a simulation model to investigate the interactions of the
three parasitoids and their host. More recently a more sophisticated simulation model of
N. sertifer and the living and non-living factors that influence it has been developed. It