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UBC Theses and Dissertations

A study of acorn feeding insects : filbert weevil (Curculio occidentis (Casey)) and filbertworm (Cydia latiferreana (Walsingham)) on Garry oak (Quercus garryana) (Dougl.) in the southeastern Vancouver Island area Rohlfs, Doris Andrea

Abstract

The Garry oak (Quercus garryana) Dougl. is the only oak native to British Columbia and is one of the more distinct and stately trees growing in the Greater Victoria area. The Garry oak meadow ecosystem is unique and rich, with the largest number of rare plant species of any ecosystem not only in British Columbia, but in Canada. Since acorns are vital to maintain future generations of Garry oak trees, the insects that attack and damage these acorns are of interest. Garry oak acorns were collected from June to September in 1996, 1997 and 1998 to determine the abundance and spatial distribution of acorns infested by the filbert weevil (Curculio occidentis (Casey)) (Coleoptera: Curculionidae) and filbertworm (Cydia latiferreana (Walsingham)) (Lepidoptera: Olethreutidae) at 10 locations in the Greater Victoria area. The biology of C. occidentis was studied by laboratory rearings from 1997 through 1998. Both C. occidentis and C. latiferreana infested a large portion of acorns in 1996, 1997 and 1998, exhibiting an inverse trend of infestation level to crop abundance. This trend was more evident for the filbertworm, than the filbert weevil. Acorn crops in 1996 and 1997 were light, and in 1998 was heavy, a mast year. Dissection of 10,879 acorns showed the combined infestation rates by the two insects were 80.7%, 75.0% and 51.3% in 1996, 1997 and 1998, respectively. Of these acorns, the filbert weevil consistently infested more acorns than the filbertworm. Significantly more filbert weevil infested acorns occurred in the lower- than the middle- and upper-portion of the sample trees in 1998, but not in 1997. The proportion of acorns infested by the filbert weevil and filbertworm did not vary by compass direction (south, northeast and northwest) in either 1997 or 1998. Strata infestations were not examined in 1996. Although infestation by these two insects was high, it was shown that even moderately damaged acorns, with up to 50% damage to the cotyledon, still have the potential to germinate. In 1996,1997 and 1998, 51.4%, 49.5% and 77.6% of acorns, respectively, had less than 50% damage to the acorn cotyledon. Filbert weevil adults emerge in June through September to oviposit into the forming Garry oak acorns. Laboratory reared larvae feed, on average, for 5 Vi weeks, completing 4 larval instars during this time. The larvae overwinter in the 4th instar, and pupate in the spring. The pupal stage last approximately 12 days, with the callow adults requiring an additional 10.3 days to harden and gain full coloring. Females are larger than males in body length, body width, and rostrum length. Filbertworm eggs were rarely seen during this study. Larvae were observed in acorns beginning mid-July and emergence holes were found approximately one month later. Control measures for the filbert weevil and the filbertworm are not recommended because even moderately damaged acorns are capable of germinating. Thus, the impact of damage by these two insects is less than it appears. The natural cycle of poor and mast crops of Garry oak acorns acts as a natural control for these two insect populations.

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