Natural enemies of Trapa spp. in northeast Asia and Europe

Trapa natans is an aquatic plant native to the Old World. In the late 1880s this plant was found to be naturalized in the eastern United States where it has become an important weed. To evaluate the potential of biological control to reduce this weed, surveys for natural enemies of Trapa spp. were made in Northeast Asia and Europe. In addition, populations of Trapa japonica in South Korea were monitored for one season to evaluate the occurrence and impact of insect natural enemies. Among the insects found, the leaf beetle Galerucella birmanica was the most common and damaging species in Asia. It can cause complete defoliation of whole populations of plants. Nymphuline pyralid moths were also common and at times damaging. Both the beetle and the moths feed and develop on unrelated plants and so have no potential as biological control agents of T. natans in North America. Two Nanophyes weevils were found in Asia which feed in petiole floats of the leaves. They are thought to be specific to Trapa but were not observed to be damaging. Low level populations of polyphagous Homoptera were common. Chironomid midges were also frequently associated with the plants, but for the most part were filter feeders, not herbivores. The greatest impact on the monitored plants in South Korea was by the leaf beetle G. birmanica which defoliated most of the plants, causing the mats of plants to sink. In Europe, a similar and related insect fauna was found, but none of the insects was very damaging to the plants. One Italian weevil, Bagous rufimanus, feeds within the fruit stems and might be more damaging at higher than observed population levels. Some diseases were seen on some plants in both regions. European populations of T. natans have declined significantly, stimulating conservation efforts. Literature records of Trapa insects from warm areas are presented. These could be of interest if T. natans spreads into warmer areas of North America.