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Conrad Bérubé |
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ABSTRACT
The jumping gall wasp, Neuroterus
saltatorius (Edwards), continued to cause significant amounts
of foliar scorching of Garry oak throughout the Capital Regional
District in 1995. Population densities of both the gamic and
agamic generations declined considerably in 1995. However, the
severity of scorch remained at levels similar to those observed
in 1994. Results of surveys measuring annual changes in gall wasp
damage, population density and mortality factors at six sites are
presented for 1995 and compared to data collected in 1993 and
1994.
Preliminary data from seedling trials in 1994 indicated that Garry oak from British Columbia provenances were six times as likely to suffer from infestation by N. saltatorius than were trees of the same species from a California provenance (infestation occurred on 90% [36/40] and 15% [6/40] of the trees from British Columbia and California respectively). This distinction was not as clear in 1995: data indicated an incidence of infestation by N. saltatorius of 81.3% (26/32) for BC Garry oak seedlings compared to 64.5% (20/31) for seedlings from California. However, the severity of infestation was substantially higher on British Columbia stock, 23.5 galls/leaf, compared to California stock, 9.0 galls/leaf.
Although the incidence of infestation by P. glabra on Garry oak stock from both provenances was similar in 1995, the levels of percent infestation were considerably higher in each case than those recorded in the previous year. A marked increase in incidence of infestation by P. glabra on Q. garryana seedling stock from British Columbia was recorded in 1995, 50.0% [16/32] as compared to 2.5% [1/40] in 1994. Incidence of infestation on California Garry oak in 1995 was 58.1% [18/31], also up from 1994 levels of 22.5% [9/40].
Gamic generation gall-production data from the six study sites indicated that the population of gamic galls in 1995 was greatly reduced from levels recorded in the previous year (0.24 galls per leaf in 1995 as compared to 1.52 galls per leaf in 1994). Similar, although less extreme, tendencies were noted for the agamic generation in 1995 (having a rating of .44 [corresponding to between 0 and 10 galls per leaf]) compared to 1994 (having a rating of 1.01 [corresponding to 11 to 33 galls per leaf]).
Dissection of 1,200 gamic galls showed that the rate of parasitism in the gamic generation at the six principal study sites varied from 0% to 9.5% (average: 3.4%) in 1995 as compared to a range of 0% to 3% (average: 2%) in 1994. Given the spatially patchy distribution of gamic galls, these data agree fairly well with levels of parasitism obtained from rearings of gamic galls: 0 to 14.5% (average: 5.3%) in 1995; 0 to 5.5% (average 1.5%) in 1994.
Examination of 9,508 oviposition stings, gall abscission scars, partially developed galls and fully developed adhering galls on tagged leaf clusters showed that the failure rate for developing galls remained at levels similar for those recorded at study sites in 1994 (69% in 1993; 58.4% in 1994; 59.6% in 1995).
Dissections of 5,104 agamic galls from weekly trap collections showed that greatly increased mortality due to parasitism occurred in 1995 compared to the previous years of the study (9.8% in 1993, 15.3% in 1994, 32.0% in 1995) while mortality from unknown factors varied (34% in 1993, 46.5% in 1994, 29.1% in 1995). Similarly, dissections of 1,496 mature agamic galls rubbed from leaves showed parasitism, at 34.4%, increased in 1995 from 22.1% in 1994. Mortality due to unknown factors was 40.6% in 1995 compared to 56.7% in 1994.
Rearings of gamic galls confirmed that Aprostocetus pattersonae (Fullaway) and Mesopolobus nr. longicaudae Doganlar were responsible for the bulk of the parasitism observed in the gamic generation. Total parasitism recorded in the gamic generation rearings was 1.25% in 1993, 1.7% in 1994 and 5.3% in 1995.
Rearings of agamic galls recovered primarily from funnel- traps and supplemented with galls rubbed from leaves yielded the following relative percentages of each species reared: Amphidocius schickae Heydon and Boucek 36.1%, Aprostocetus new sp. 24.7%, Aprostocetus pattersonae (Fullaway) 33.1%, Aprostocetus verrucarrii (Balduf) 2.7%, and Brasema sp. 3.5%. The relative percentage of Amphidocius schickae decreased in 1995 compared to 1994 while Aprostocetus new sp. increased proportionately. Total parasitism recorded in the agamic generation rearings was 6.8% in 1994 and 8.5% in 1995
The timing of gall-drop appears to have been similar in 1995 and 1994 with the majority of gall-drop having occurred by the first week of July. Funnel- trap catches of agamic galls decreased dramatically this year compared to 2 years previous: an average of 7612.3 galls per trap per year were collected in 1993 as compared to 3922.7 galls per trap per year in 1995.
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