Life history, survivorship and parasitoid complex of the jumping gall wasp, Neuroterus saltatorius (Edwards), on Garry oak, Quercus garryana Douglas
Date
1995
Authors
Smith, Joanna Louise
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Abstract
Garry oak, Quercus garryana Douglas, ecosystems on Vancouver Island currently occupy <1 % of their original distribution and are threatened mainly by urban encroachment and industrial development. The recent population increase of an oak gall wasp, Neuroterus saltatorius (Edwards) (Hymenoptera: Cynipidae) has caused extensive leaf necrosis and foliar scorching and has added to the concern for the long-term health of this native species. Cynipid communities tend to be regulated by parasitoids (Hymenoptera: Chalcidoidea) and as a result, are normally found at non-damaging levels. The high gall wasp infestation levels and rapid spread throughout Q. garryana stands on Vancouver Island might have arisen because of the accidental introduction of this species (c.1986) without the associated parasitoid community. The objectives of this study were to examine the life history and describe the mortality factors of N. saltatorius.
Eight sites on southern Vancouver Island, reflecting a range in infestation ages, were surveyed in 1992-94. Gamic females were larger, emerged-over a shorter period of time and had a greater number of smaller eggs than agamic females. Agarnic galls began to drop during June and the peak drop occurred from the end of June to the middle of July; pupae and adults were found in galls in the soil by the middle of September.
Three chalcidoid species were recovered from the garnic generation and ten emerged from the agamic generation; Mesopolobus sp. nr. longicaudae Donganlar was the most common in the gamic generation and Aprostocetus pattersonae (Fullaway) and Amphidocius schickae (Reydon and Boucek) were the most common in the agarnic generation. Amphidocius schickae and Aprostocetus verrucarii (Balduf) were also identified as hyperparasitoids. The parasitoid complex of N. saltatorius was found to have a high degree of overlap with other oak gall wasps found in B.C. prior to this infestation but peak parasitoid emergence occurred after most of the agamic galls had fallen to the ground.
Gamic generation survivorship was very high; mortality factors principally affected survivorship in the agamic generation. Contrary to most studies, it was found that chalcidoid parasitoid attack was not a large contributor to host mortality possibly due to poor synchrony of parasitoid activity with N. saltatorius development. The greatest sources of mortality for agamic generation galls on the foliage were failed development, fungal infections and desiccation. Survivorship in the agamic generation was found to be lowest in populations of N. saltatorius at older infestation areas. The low number of viable galls collected at older sites might indicate that mo1tality from several sources will cause a decline in local jumping gall wasp populations.