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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 Capitol Regional District and environs.
Investigations and surveys designed to determine the ecological
interactions affecting the jumping gall wasp were accordingly
updated.
Preliminary seedling trials indicated that Garry oak from British Columbia was six times as likely to suffer from infestation by N. saltatorius than were trees of the same species with a California provenance (infestations of 90% <36/40> and 15% <6/40> respectively). Conversely, Californian Garry oak was approximately ten times as likely to suffer attack by Phylloxera sp. nr. glabra than seedlings with a British Columbian provenance (infestations of 22.5% (9/40) and 2.5% (1/40) respectively).
Investigations of relative phenological development disclosed no readily apparent relationship between early bud- break and gamic and agamic oviposition preference. Nor is there any evidence that agamic and gamic oviposition on a particular leaf is inversely related.
Data from dissections of 600 overwintered agamic galls recovered from duff indicated rates of predation varying from 5% to 50% (average: 32%) and parasitism varying from 5% to 19% (average: 11.5%) with survivorship from 5% to 70% (average: 21.5%) at the six principal study sites.
The dissection of 1,200 gamic galls revealed a rate of parasitism of from 0% to 3% (average: 2%) at the six principal study sites which agrees well with levels of parasitism as determined by rearings.
Examination of 4,561 galls on tagged leaf clusters showed that the failure rate for developing galls decreased (from 69% in 1993 to 58.4% in 1994). However, the increase in the percentage of maturing galls was offset by a decrease in survivorship from 73.5% in 1993 to 54.1% in 1994.
Dissections of 5,708 agamic galls from weekly trap collections showed increased mortality due to parasitism and undiscerned factors (such as host-feeding) in 1994 compared to the previous year (parasitism: 9.8% in 1993, 15.3% in 1994; unknown mortality factors: 34% in 1993, 46.5% in 1994). Similarly dissections 7,156 of agamic galls obtained by leaf-rubs exhibited parasitism of 22.1% and unknown mortality factors of 56.7%.
Soil predation and parasitism studies indicated that negligible parasitism occurs once galls have dropped from leaves.
Rearings of gamic galls confirm that Aprostocetus pattersonae and Mesopolobus nr. longicaudae were responsible for the bulk of parasitism of the gamic generation of galls. Total parasitism of the gamic generation was 1.25% in 1993 and 1.5% in 1994 according to rearing data.
Combined rearing results from agamic galls recovered from funnel-traps and leaf-rubs yielded the following percentages of total parasitoids reared: Amphidocius schickae 63.8%, Aprostocetus new sp. 8.7%, Aprostocetus pattersonae 26.0%, Aprostocetus verrucarrii 1.5%, and Brasema sp. 1.5%.
Sticky trap catches were not found to represent the same relative abundance of each species as indicated by rearing data. Peak populations of adult parasitoids, as measured by sticky trap catches and rearings, occurred from late August to mid-September for most parisitoids ( A. pattersonae experiencing an additional peak in mid-July).
The rate of gall drop appears to have been similar in 1993 and 1994 the majority of gall-drop occurring before mid-July.
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