Biological Control

Five biological control insects have been approved for release in the United States. Adults and larvae of the black-margined and golden loosestrife beetles (Galerucella calmariensis and G. pusilla, respectively) feed on buds and leaves, stunting plants and reducing seed production. Adults overwinter in the soil and vegetation surrounding infestations, and therefore soil cultivation within 50 yards (50 m) of the wetland edge should be avoided in the spring until adults move into the purple loosestrife plants. Adults disperse in the spring during the first few weeks of plant growth and lay eggs from May through June. The eggs incubate for 12 days. The three larval stages feed for about 14 days; pupate in soil beneath plants and in the spongy tissue of stems in standing water. Pupation lasts seven days. Adults of a second generation can emerge and disperse to new populations during July and August in warmer climates. This insect also feeds on, but does not reproduce on, the native winged loosestrife (L. alatum) and swamp loosestrife (Decodon verticillatus, only found in eastern North America), the introduced hyssop loosestrife (Lythrum hyssopifolium), and crapemyrtle (Lagerstroemia indica), neither of which are reported in Montana. The beetle can survive cold winter weather, but not continuously flooded areas, because adults need drier overwintering sites. Freshly overwintered adults collected by sweep net or hand picked can be transferred to new sites once egg deposition has begun. Release numbers from 250 to 500 adults are recommended to initiate a population. Loosestrife biomass decreased by 90 percent after release of these beetles in Oregon and Washington. Large plants can re-grow and flower after attack by the beetles. Studies indicate it takes three to five years after establishment for G. calmariensis to impact a purple loosestrife population.

Larvae of the loosestrife root weevil (Hylobius transversovittatus) feed continuously on the root storage reserves during the growing season reducing the plants ability to recover after defoliation. The adults feed on the foliage. The robust adults (8-12 mm long) emerge in mid- to late summer and live up to three years. Females lay one or two eggs in the soil or stems at the soil surface daily from June through August. Eggs hatch in 11 days. Larvae mine the roots from August through June, although the larval stage can last for two years. They pupate in the root crown during late spring and early summer. Adults are hand collected at night. As with the loosestrife beetles, this insect also utilized winged and swamp loosestrife in host specificity tests.

The loosestrife seed weevil (Nanophyes marmoratus) feeds on flower buds (larvae) and developing leaves (adults). Adults appear on plants in mid-May and lay eggs inside immature flower buds from June to September. The larvae eat stamens, petals, and ovaries in unopened buds. They form chambers in the damaged buds where they pupate. Adults are collected using a sweep net or by dislodging them from inflorescences with a stick onto a tray and capturing them with an aspirator. Releases of 100 to 200 adults are recommended. Severe defoliation by the loosestrife beetles can reduce inflorescence availability for weevil oviposition. Therefore release of the weevil is not recommended where there are heavy populations of loosestrife beetles.

Field release of the seed consuming weevil Nanophyes brevis, approved for release in North America, was postponed because the original shipment from Europe in 1994 was contaminated with a parasitic nematode.

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Last Modified: 08/12/2008