Host Specificity of Three Potential Biological Weed Control Agents Attacking Flowers and Seeds of Lythrum salicaria (Purple Loosestrife)

Lythrum salicaria is a Eurasian herbaceous perennial that has become a serious invader of wetlands in the United States and Canada. Dense monospecific stands replace a diverse native flora resulting in the degradation of these wetland habitats. There are presently no satisfactory means of control. Biological control offers the most promising method of resolving this problem. A root-mining weevil and two leaf-feeding chrysomelids from Europe were released in North America in 1992. The host specificity of three additional flower- and seed-feeding species was investigated. The two weevils, Nanophyes marmoratus and N. brevis, have a wide geographic and ecological range. Both develop exclusively on Lythrum salicaria within its native European range and were found to be highly host specific during screening tests. Minor adult feeding was observed in no-choice tests on a few other species within the Lythraceae. Successful larval development was restricted to purple loosestrife. The only known field host of the third species, the gall midge Bayeriola salicariae, is purple loosestrife. Oviposition and successful larval development of B. salicariae in cages and the open field occurred on potted test plants of another three Lythrum species. The introduction of N. marmoratus and N. brevis into North America is expected to further reduce seed output and lessen the competitive ability of purple loosestrife. Their introduction was approved in 1994.