Impact of competition from wheat and below‐ground herbivory on growth and reproduction of scentless chamomile, Tripleurospermum perforatum (Mérat) Laínz

This study investigated the effect of competition from wheat and below ground herbivory on the growth and reproduction of scentless chamomile, Tripleurospermum perforatum (Mérat) Laínz (Asteraceae), a target plant for classical biological control. Field cages, in which scentless chamomile was planted, were established in 1994 near Neuenburg (Rhine Valley, Germany), and two weevil species, Diplapion confluens Kirby and Coryssomerus capucinus (Beck), were released alone (40 adults each) or together in a substitutive design (20 adults each). Wheat was sown alone or in combination with both herbivores as an additional stress factor. To test the cage effect on plant growth, an uncaged control was set up. At the end of the experiment, caged control plants were about 20 cm (18%) higher, and allocated more biomass to shoots and less to roots and seeds than uncaged plants. Competition from wheat reduced the number of shoots per T. perforatum plant from 7.6 to 3.2. As a consequence, the biomass of scentless chamomile plants that competed with wheat was reduced by 39% and reproductive output by 49%, compared with plants that were grown without competition. At the end of the experiment, 70–80% of T. perforatum plants had been attacked by an average of two to three D. confluens and 60–70% of plants by one to two C. capucinus . At these densities that were lower than expected, the two herbivores alone or in combination had no significant effect on any of the plant parameters measured. However, in the presence of wheat, herbivory further reduced the number of shoots per plant. We conclude that control of scentless chamomile using insect biocontrol agents will be more likely to be successful if agent densities are high, and agents are released in habitats where scentless chamomile is competing with other plant species.