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Conrad Bérubé |
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ABSTRACT
The jumping gall wasp, Neuroterus saltatorius
(Edwards), continues to cause foliar scorching to Garry oaks
in the Capital Regional District and environs and appears to have
resurged this year to levels higher than in 1995. Investigations
and surveys designed to determine the ecological interactions
affecting the jumping gall wasp were accordingly updated.
Gall-production data indicate that the population of gamic galls
in 1996 was greatly increased from levels of the previous year
(1.64 galls per leaf in 1996 as compared to 0.24 galls per leaf
in 1995) but similar in level to 1994. Similar, although less
extreme tendencies were noted for the agamic generation: average
agamic gall-loads per leaf per tree increased considerably in
1996 as compared to 1995 (with respective ratings of .88
The dissection of 1,200 gamic galls revealed a rate of parasitism
of the gamic generation from 0% to 5% (average: 0.83%) at the six
principal study sites which, given the spatially patchy
distribution of parasitoids in the gamic generation, agrees
fairly well with levels of parasitism as determined by rearings
of gamic galls of 0 to 0.5% (average: 0.16%).
Examination of agamic galls on tagged leaf clusters showed that
the failure rate for developing galls decreased slightly to 43.9%
in 1996 (from 69% in 1993; 58.4% in 1994; 45.6% in 1995).
Dissections of 5,104 agamic galls from weekly trap collections
showed that mortality due to parasitism, as calculated by
protocols set in 1993, was little changed in 1996 compared to the
previous year (parasitism: 9.8% in 1993, 15.3% in 1994, 32.0% in
1995, 30.3% in 1996) as were unknown mortality factors (unknown
mortality: 34% in 1993, 46.5% in 1994, 29.1% in 1995, 28.7% in
1996). However, calculations were done in which weekly percent
parasitism was considered in light of the contribution of that
week's gall-drop to total gall-drop for each site. This procedure
should give a better estimation of the condition of the galls
when they entered the leaf duff. In these terms percent
parasitism was lower in all years than was previously calculated
and fell slightly in 1996 compared to the previous year (10% in
1994, 18.1% in 1995 and 15.3% in 1996). As reflected in the drop
in total values percent parasitism fell at the majority of sites
in 1996 as compared to 1995.
Rearings of gamic galls confirm that Mesopolobus nr.
longicaudae was primarily responsible for parasitism of
gamic generation galls in 1996. Total parasitism of the gamic
generation was 1.3% in 1993, 1.5% in 1994, 5.3% in 1995 and 0.8%
in 1996 according to rearing data.
Rearing results from agamic galls recovered from funnel-traps and
leaf-rubs yielded the following percentages of total parasitoids
reared:
Amphidocius schickae 64.0%, Aprostocetus new
sp. 6.9%,
Aprostocetus pattersonae 20.7%, Aprostocetus
verrucarrii 2.7%, and Brasema sp. 4.6%.
Numbers (and percentages) of Amphidocius schickae
increased in 1995 compared to 1994 while Aprostocetus new sp.
decreased proportionately.
The rate of gall-drop appears to have been similar in 1996 and
1995, the majority of gall-drop being completed by the second
week of July.
Predators of Phylloxera sp. nr. glabra were
collected using visual and beat sheet survey techniques.
Pilophorus sp, Harmonia axyridus , Anthocoris sp. ,
Campyloneura sp. , Vedalia bipunctata and
Forficula auricularia comprised the majority of specimens
recovered by both survey methods.
No correlation could be ascertained between rates of gall-drop
and parasitism with precipitation and temperature data or other
factors for the years during which data was collected. Likewise,
no consistent trends could be discerned in data-- the infestation
of N. saltatorius continues to afflict Garry oaks and
cause scorching at moderate to high levels.
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2007 Conrad
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