Federal Research Station, Schloss 335, 8820 Wädenswil, Switzerland.
Host-plant acceptance in the cabbage root fly is mainly determined by contact chemoreception. Like in many other crucifer specialist, glucosinolates stimulate oviposition in the cabbage root fly. Recently another group of compounds, the so-called CIF’s, has been identified and these compounds are thought to be much more powerful oviposition stimulants in this fly than the glucosinolates are. They are estimated to be present in the leaves of crucifers in extremely small amounts and are detected by highly specific receptor cells in a pair of ventro-medial C sensilla on the fifth tarsomere. It is unclear why the cabbage root fly relies so much on such a minor group of compounds. The existence of specialist receptors suggests a strong selection pressure on CIF detection since such receptors are unlikely to evolve by chance. However, the cabbage root fly also oviposits when CIF’s are not present and about 15% of the C5 sensilla seem insensitive to CIF. A possible explanation for this phenomenon will be discussed.