Efficiency of Suction Sampling for Rhinocyllus conicus 1 and a Comparison of Suction and Visual Sampling Techniques

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Abstract
Efficiency and reliability of a suction device for assessing Rhinocyllus conicus populations were determined and compared with current visual research techniques. A suction machine collected a greater proportion of weevils from thistles ranging in height from 6 cm to 1 m but was less efficient than visual searches on rosettes or plants in excess of 1 m. A single, 2-sec suction sample was as efficient as multiple samples with durations up to 6 sec. Horizontal approaches were superior to up-sweep or vertically oriented suction samples and collected 98.6 ± 1.3% of adult weevils in recapture tests. R. conicus population density increased with plant height; correlation coefficients were highest (r = 0.95) when plant height variability was greatest and all thistles except rosettes were sampled. Although ca. 15% of the potentially viable eggs per primary bloom may be dislodged during suction sampling, removal of old, dried-out egg casings resulted in counts of eggs remaining on thistle blooms which were generally more indicative of actual weevil populations.