Loosestrife/Galerucella calmariensis L.
Common name: Black-margined loosestrife beetle
Galerucella pusilla Duftschmidt
Common name: Golden loosestrife beetle
Galerucella calmariensis and G. pusilla are two nearly identical species of leaf-feeding beetles (Figure 8). Both beetles are light brown in color, but G. calmariensis (Figure 8a) usually has a black triangle or black line on the thorax while in G. pusilla (Figure 8b), the line is thin or nearly absent (these characters develop fully only in overwintered beetles). The beetles are the same size— about 0.15 to 0.3 inches (4 -6 mm) in length—and have the same habits. Released in 1992, these two leaf beetles were the second loosestrife insects to be introduced in the United States and Canada for biological control of purple loosestrife. They are now widely established in more than 30 states and 8 provinces where purple loosestrife is a problem.
In early spring, adult beetles emerge from their overwintering sites in plant litter. They live eight to ten weeks after they emerge in the spring, feeding at first, and then mating and laying eggs. Adult feeding on loosestrife leaves and young shoot tips results in a characteristic “shothole” damage of the plants (Figure 9). Mating and oviposition begin in late May or early June. A single female can lay as many as 400 eggs in her lifetime. Eggs are laid in batches of one to ten on leaves and stems (Figure 10). Each egg is covered with a black line of frass (fecal deposit).
Larvae hatch from eggs after about one week and move to leaf buds where they remain well-concealed as they feed. As they get older and larger, larvae openly feed on leaves and stems. The larvae of the loosestrife beetles look like tiny caterpillars with black heads and yellowish bodies (Figure 11). After completing three instars, mature larvae move into the litter beneath purple loosestrife plants to pupate. On flooded purple loosestrife they pupate in the spongy tissue (called aerenchyma) that develops on the flooded portion of the stem.
Development time, from egg to adult, is 30-40 days (Figure 12). New adults emerge between mid-June and mid-July, feed for a limited time to accumulate body fat, and then seek overwintering sites in the leaf litter.
Both loosestrife beetle species usually have only one generation each year, but a partial second generation may occur in warm regions where the first new generation adults emerge before mid-June (before the summer solstice). At this time of year, warm temperatures and long day lengths may trigger beetles to mate and produce a second generation. If a second generation develops, new adults may emerge as late as the end of August.
Adult loosestrife beetles are very good fliers and they can easily find new patches of purple loosestrife on which to feed and reproduce. Loosestrife beetle adults are known to disperse 2-4 miles a year. Although loosestrife beetles survive under a range of conditions, they do not thrive in shade or in areas where water levels fluctuate dramatically (e.g., dam reservoirs).
Larvae feed on the underside of leaves, stripping the photosynthetic tissue off while leaving the upper leaf cuticle and epidermis intact, creating a “window-pane” effect (see Figure 13). At lower larval densities, plants are less severely damaged. At high densities (greater than 4-5 larvae/inch stem or 2-3 larvae/cm of shoot length), stems can be defoliated. Large loosestrife beetle populations can defoliate 100 percent of the plants in an infestation many acres/hectars in size. While plants may recover after defoliation and flower late in the year, in some areas loosestrife beetle feeding has completely suppressed flowering. Plants that regrow after defoliation are often shorter and bushier than normal, unattacked plants.
Predators and parasites of loosestrife beetles have also been reported in North America, although the specialist wasps that attack these species in Europe were carefully excluded when beetles were imported. Adult loosestrife beetles can be parasitized by a nematode that feeds and develops inside the beetle, eventually killing it. Other native predators of loosestrife beetles are ladybeetles, true bugs, predaceous beetles, spiders, and possibly birds, frogs, and lizards.