COMPARISON OF THE SEASONAL OCCURRENCE OF TRICHOSIROCALUS HORRIDUS (PANZER) (COLEOPTERA: CURCULIONIDAE) IN VIRGINIA BETWEEN 1981–83 and 1979

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Abstract
Trichosirocalus horridus (Panzer) (Coleoptera: Curculionidae), a ceutorhynchine weevil introduced for control of Carduus thistle, is well established in Virginia. Its preferred host, Carduus thoermeri Weinmann (musk thistle), was collected from two sites with large populations of the weevil. Sampling was carried out over a 20-month period between 1981 and 1983. Whole plants were dug up and returned to the laboratory for dissection to determine the seasonal occurrence of T. horridus. The results were compared with those of an earlier study in Virginia which was based on non-destructive sampling soon after the weevil became established. In the current study all immature stages were detected at least 1–3 months earlier in the fall than previously reported, and the adult stage was found on the host plant from September through July. Detection of the different life stages of T. horridus for longer durations was also the result of larger established populations of the weevil at the sampling sites. Other new findings include the discovery that T. horridus overwinters in the egg, larval, and adult stages, that the overwintering site for the adult is decaying organic matter and the soil surface in the vicinity of the rosettes, and that viable eggs are found in non-photosynthetic as well as green leaves. The large proportion of eggs found in the rachis of nonphotosynthetic leaves suggests that sampling for eggs based on green leaves only can be misleading.