Suppressing Broad-leaved Dock by Combining Root-boring Larvae and Plant Competition

The combination of competition by forage plants and herbivory by native root-boring moth larvae can significantly reduce the growth of small broad-leaved dock plants in permanent grasslands.

Broad-leaved dock (Rumex obtusifolius L.) is one of the most problematic weeds in intensively managed grasslands in Europe. Non-chemical control measures are in high demand, especially on organic farms. Two European moths, Pyropteron chrysidiforme and P. doryliforme (Sesiidae), have therefore been proposed as candidates for the biological control of broad-leaved dock, as their larvae are root borers and overwinter in the taproot. So far as is known, however, the impact of the larvae is too small to have a significant control effect on mature broad-leaved dock. Agroscope researchers were therefore looking for ways to enhance the larvae’s effect on the dock plants.

A factorial field experiment was established to investigate the combined effect of the root-boring larvae and competition from common ryegrass (Lolium perenne L.) on dock plants. Because ryegrass is a well-known competitor of broad-leaved dock and suppresses small dock plants in particular, dock plants of differing root size were planted in plots with and without ryegrass competition. The plants were then inoculated with eggs of either P. chrysidiforme or P. doryliforme, while the remaining third of the plots served as an untreated control. A total of 432 R. obtusifolius roots were planted. After one season, the total stem, leaf and root mass of the dock plants were measured. The roots were dissected to quantify the number of larvae and the damage caused by them.

Synergistic effects of ryegrass and larvae on small broad-leaved dock

Although competition from ryegrass significantly suppressed both the stem and leaf mass as well as the root mass of the broad-leaved dock, plants from initially small roots were more strongly suppressed by ryegrass than those growing from large roots. The combination of competition from ryegrass and inoculation with moth eggs strongly reduced the growth of the small plants or caused them to die off. By contrast, broad-leaved dock not subjected to competition from ryegrass suffered almost no negative effects from the larvae. This shows a synergy between ryegrass and moth larvae, with the combined impact being greater than the sum of the individual effects. Competition from ryegrass also affected the herbivores themselves: in the presence of competition, the infestation rate and number of larvae per root mass were higher, resulting in more larvae in the small dock roots.

The results show that plant competition and root-boring larvae synergistically suppress broad-leaved dock, but that they only do so effectively in the case of small dock plants. The competition from grasses reduces available resources, while the dock plants are further weakened by the root-boring larvae. The application of Pyropteron species to control broad-leaved dock in established grasslands thus has potential, but is not at present commercially available owing to the complexity of the cultivation process.