How to find a needle in a haystack – host plant finding of the weevil Ceratapion onopordi

The weevil Ceratapion onopordi Kirby (Coleoptera: Apionidae) shows a mutualistic interaction with the rust fungus Puccinia punctiformis (Str.) Röhl (Pucciniaceae). One of the weevil’s host plants, the thistle Cirsium arvense (L.) Scop. (Asteraceae), is also the host of the rust fungus. It has been argued that weevils prefer rust‐infected thistle shoots for egg deposition and consequently should be able to detect such shoots. Olfactory, visual, and gustatory orientation was tested using a four‐chamber olfactometer, a visual testing arena, and feeding choice tests. Whereas the weevils used olfactory cues to find their host plants, visual orientation does not seem to be important. Rust‐infected thistle shoots were not preferred over healthy shoots in any of the tests. We conclude that host plants infected with rust fungi, which are rather rare in the field, appear to be found more or less by chance.