A simple laboratory assay was developed to evaluate substances and extracts for their repellent effect on the tick Ixodes ricinus L. The bioassay involved testing the locomotory activity of I. ricinus nymphs in a circular glass arena. The stimulus to be tested was applied onto the arena outside a circle line (4 cm diameter). One field-collected I. ricinus nymph was placed in the centre of the arena and the time spent before entering the treated area was compared with that recorded in suitable control experiments where no stimulus or the solvent alone was used. Apart from a DEETbased product that was used as a positive control, extracts of basil leaves (Ocimum basilicum L.) and leaves of various grasses were tested. The commercial DEET-product proved to be active and so was an acetone extract of basil leaves; the hexane extract of basil leaves and the acetone extract from leaves of mixed grasses had very little or no activity. The bioassay appeared to be suitable to assess the repellent activity of natural products. Basil seems to contain substances that are repellent to I. ricinus nymphs.
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