Open-Field Host-Specificity Testing for Potential Biological Control Agents of Onopordum Thistles

Open-field host-specificity tests conducted in Greece and France demonstrated an absolute preference for Onopordum spp. over Cynara spp. by four potential biological control agents of Onopordum thistles that are weeds in Australia, the seed weevils, Larinus latus and L. cynarae, the seedfly, Tephritis postica, and the stem-boring weevil, Lixus cardui. A fifth species, the plant-hopper, Tettigometra sulfurea, showed a strong ovipositional preference for Onopordum spp., while nymphs hatching from eggs laid on Cynara spp. did not develop to adulthood. The genus Cynara is critical to host testing as it is the most closely related genus to Onopordum and contains economic crop plants. Rejection of Cynara spp. by the potential control agents justified the expense of continuing host-specificity testing against an expanded series of plants and has led to the authorization for field release of two of the above species, L. latus and L. cardui. Where key plant species can be identified they should be tested first, to reduce the wastage of resources that occurs when long test programs ultimately lead to rejection of candidate agents for biological control.