Invasive and Exotic Species

Hylobius transversovittatus
Goeze Common name: Loosestrife root weevil
Order: Coleoptera
Family: Curculionidae

Hylobius transversovittatus is a large, reddish-brown weevil, 0.4 to 0.6 inches (10 to 14 mm) in length (Figure 14). First released in 1992, it was the first of the loosestrife biocontrol insects to be introduced. It is now established at many sites across the United States and Canada. This weevil is nocturnal and long-lived as an adult (two to three years or longer).

In spring, overwintering adult weevils appear shortly after purple loosestrife shoots sprout. Loosestrife root weevils are most active at night, but can be found on their host plants in the early evening and morning hours or during cool, overcast, or rainy days. During warm, sunny days, they hide in the litter, often at the base of the plants. Adults begin feeding on foliage and young stems, and within two weeks, mating and oviposition begin, which can last until September. Females deposit single, white,

Figure 14. Adult Hylobius transversovittatus. (UGA0002033)

Figure 15. Hylobius transversovittatus eggs in a purple loosestrife stem (UGA1291016).

Young larvae hatch in about eleven days and start to mine the root hairs (if hatched from eggs laid in the soil) or into the stem. They later feed on the outside of the root, and then mine into the center of the root where they continue to feed for one to two years (Figure 16). Mature third instar larvae move to the upper part of the root to pupate. The new loosestrife root weevil generation emerges from late June to October.

Figure 16. Hylobius transversovittatus larvae in a purple loosestrife: a) stem, and b) root. (Left to right: UGA1291017 and UGA 1291018)

Figure 17. Schematic lifecycle of Hylobius transversovittatus. Solid colored bars represent the length of activity for each of the life stages, with adult overwintering presented by the patterned bars. A single generation can take one to two years to mature.

Impact:

The rate of attack (number of larvae feeding in a root) and damage to the plant varies depending on the size and age of the rootstock, the density of purple loosestrife plants, and the number of weevils at the site. Usually, only one or two larvae feed in the same root, but as many as 20 larvae have been found in a single, large root.

Root feeding by loosestrife root weevil larvae can be very destructive to the roots, especially at high larval densities. Small rootstocks can be severely damaged or killed. Large roots can withstand substantial feeding pressure over several years before significant damage becomes noticeable. However, attacked plants are shorter, have reduced root and shoot biomass, and produce fewer seeds.

The loosestrife root weevil is tolerant of a wide range of environmental conditions. However, where water levels fluctuate, females may not be able to lay eggs at the base of the plant. Larvae may drown in roots that are submersed in water for a prolonged period of time (several months). For these reasons, the loosestrife root weevil should not be released in permanently flooded sites.

The loosestrife root weevil can be used effectively with the loosestrife leaf beetles. The combined effects of the three biocontrol agents is expected to be greater than the effects of either insect alone. For example, purple loosestrife regrowth following defoliation by Galerucella leaf beetles is greatly reduced when plants are also attacked by the root feeder.

Comments:

A semi-artificial diet has been developed to facilitate mass rearing of the loosestrife root weevil. Refer to the rearing purple loosestrife biocontrol agents section in Chapter 3 of this manual for more information about the semi-artificial diet.

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