Evaluation of the biological control agent, Lixus cardui, on Onopordum thistles: experimental studies on agent demography and impact

The life-cycle of the stem-boring weevil, Lixus cardui, was found to be closely synchronized to the phenology of its host plant, Onopordum spp., based on an experiment in which adults of this biological control agent were caged on individual plants in a natural population of these thistles. The mean lifespan of female L. cardui covered the whole 2-month period of growth and flowering of Onopordum stems, during which time they laid a mean of 40 eggs. Over 90% of these eggs were laid during the period most favorable for larval development and there was very little mortality of immature stages within the stems. Damage to the host plant, however, is primarily due to heavy adult feeding on leaf tissue of Onopordum plants and response surfaces were developed which related plant height and reproduction (numbers of viable capitula produced) to root size just prior to producing stems and the density of L. cardui. These data confirmed that L. cardui can have an impact at all densities (i.e., there is no threshold below which the plant compensates for feeding damage) and that very high densities can reduce height by over 60% and seed production by over 80%. The level of impact is also negatively related to plant size. While such experiments measure the impact of L. cardui, they do not allow for behavioral responses of the weevil to plant size and quality. Studies, in which L. cardui behavior is unconstrained, are needed to reflect these results to the real impact in the field.