Control efficiency of leaf beetle, Ophraella communa, on the invasive common ragweed, Ambrosia artemisiifolia, at different growing stages

Ophraella communa LeSage (Coleoptera: Chrysomelidae), an effective biological control agent of A. artemisiifolia, was unintentionally introduced into China. To understand the biological control potential of O. communa on A. artemisiifolia, plant height, the number of branches and leaf control index of A. artemisiifolia injured by the beetle were determined. The results showed that plant height and number of branches decreased and leaf control index increased with the increasing initial release density of O. communa adults, when they were released at 90-100-cm plant height stage in 2008 and 60-70-cm stage in 2009. Even when O. communa was released at a low average density of 1.07 adults per plant at early growth stage of A. artemisiifolia in 2009, it could significantly suppress plant height and number of branches and led to a higher leaf control index compared with the herbicide treatment with Roundup. Prior to the fructicative period, the leaf control index of A. artemisiifolia by O. communa was up to 1.0 on the 47th day after the average initial release of 12 beetles per plant in 2008 or on the 85th day after the average release of 1.07 beetles per plant in 2009, which was significantly higher than that in the herbicide treatment. We suggest that the initial release average density of O. communa adults should be >= 1.07 beetles per plant at the early growth stage, or >= 12 beetles per plant at the late growth stage for an effective control of A. artemisiifolia in the field.