Other Purple Loosestrife Biocontrol Agents
Galerucella spp. Are not the only biological controls approved for release in the U.S. There are also two weevils approved as biocontrol agents, both of which attack different parts of the plant. One weevil’s larvae consume the roots of the loosestrife, and one species attacks the buds specifically, resulting in a distinct reduction of seed production. All of the biocontrol agents take time to establish, but they all have different advantages in different types of loosestrife infestation and so can pick up the slack where another biocontrol species might not do as well.
Hylobius transversovittatus, a nocturnal weevil that feeds on purple loosestrife and attacks its root systems. It can survive in a variety of environments, and can withstand periodic flooding as well. These weevils overwinter as adults, just like the Galerucella beetles, and the adults consume the leaves and stems of the loosestrife. As with Galerucella beetles, the true damage from Hylobius comes from the larvae, who feed on the roots of the loosestrife for up to several years, also using the roots to pupate (pupation can also last several years). Hylobius larvae can affect shoot growth, seed output and plant biomass, but it can take several years for larval root feeding to have a large impact on an established infestation of loosestrife.
This weevil is native to Europe, and was introduced to test sites in Virginia, starting in 1992. Establishment can take up to 6 years, and control can be spotty with this weevil, but it is a common purple loosestrife predator in its native habitat, and can survive a significant amount of flooding (though not permanent flooding) as well, making it a durable biological control agent.
Hylobius is a dark brown weevil with tufts of white hair running down its bag in two lines. It has a proboscis, reddish legs and is a little under a half-inch long.
Nanophyes marmoratus is another weevil that overwinters as an adult and emerges as purple loosestrife begins to grow. They feed on flower buds and lay their eggs in the tips of flower buds. Larvae consume the flowers, pupate in the buds and emerge in August of the same year. The emerged adults feed on the leaves of the loosestrife plant and hibernate once winter sets in. The seed output of purple loosestrife is significantly reduced, as each infested bud does not produce any seeds.
Nanophyes was released in several parts of Oregon in 1994, and seems to be most effective in areas with low plant density; Galerucella are less effective in less-dense loosestrife patches, so Nanophyes is often released in those sites. The research on Nanophyes effects on loosestrife populations is not as abundant as the research on Galerucella beetles, largely because Nanophyes takes more time to establish and spreads less rapidly than Galerucella does.
Nanophyes are very, very small, only 2-3 mm in length. They have a long snout, are reddish-brown and have light-colored markings on their back.
Releasing multiple types of biocontrol agents can result in competition between the agents, so choosing the right agent for your site is a sensible approach to using biological control. Our sites were very rarely flooded, and had high densities, so Galerucella made sense for a release agent. However, your needs may be met through another biocontrol agent, so research into what has been used in areas that are similar to your infested site is worth your time.