Suitability of two root-mining weevils for the biological control of scentless chamomile, Tripleurospermum perforatum, with special regard to potential non-target effects

The biology and host range of the two root-mining weevils Diplapion conflue ns Kirby and Coryssomerus capucinus (Beck), two potential agents for the bi ological control of scentless chamomile Tripleurospermum perforatum (Merat) Lainz, were studied in the field in southern Germany and eastern Austria, and in a common garden and under laboratory conditions in Delemont, Switzer land from 1993 to 1999. Both weevils were univoltine, and females started t o lay eggs in early spring. Diplapion confluens had three and C. capucinus five instars. Larvae of both species were found in the field from mid-April until the end of July; later instars preferentially fed in the vascular cy linder of the shoot base, root crown or root. Although larvae of both speci es occupy the same temporal and spatial niche within their host plants, the y occurred at all investigated field sites together, and showed a similar d istribution within sites. No negative or positive interspecific association was detected. Host-specificity tests including no-choice, single-choice, a nd multiple-choice tests under confined conditions, as well as tests under field conditions with natural and augmented insect densities revealed that both herbivores were specific to plant species in the tribe Anthemideae. Ho wever, their development to mature larva or adult on several cultivated pla nts, as well as on one plant species native to North America, rendered them unsuitable for field release in North America. It was concluded that to in vestigate non-target effects reliably, host-specificity tests with biologic al control agents should be carried out under a variety of conditions, part icularly with augmented insect densities, as are expected to occur naturall y after release.