The effect of Puccinia jaceae var. solstitialis on the yellow starthistle biological control insects Eustenopus villosus and Chaetorellia succinea

Six insect biocontrol agents have been introduced, with limited success, for managing the invasive plant yellow starthistle (Centaurea solstitialis). In 2003, a recently introduced fungal rust agent, Puccinia jaceae var. solstitialis, was approved for release in California. The presence of the rust in this multi-species complex was evaluated for its effect on performance of the two most common insect biocontrol agents for yellow starthistle, Chaetorellia succinea and Eustenopus villosus. To accomplish this, yellow starthistle was planted in 1 m2 plots in monocultures at three densities and in a competition replacement series with wild oat (Avena fatua) in both 2006 and 2007. Twenty seedheads were dissected from each plot to evaluate the effect of P. jaceae on seedhead maturation and insect attack rate. In the replacement series experiment in 2007 and in 2006 and 2007 combined, P. jaceae caused a 35% and 20% increase, respectively, in the proportion of mature seedheads compared to total seedheads. However, there were no significant differences in the density experiment in either year or in the replacement series experiment in 2006. Although P. jaceae appeared to have a slight effect on yellow starthistle seedhead maturation, there was no effect of the rust on seedhead attack rates of either insect biological control, regardless of the experiment or year. These results indicate that P. jaceae does not interact significantly with the insect biological control agents for yellow starthistle.