Galerucella calmariensis and G. pusilla are members of the leaf-feeding family of beetles called Chrysomelidae. Native to Europe and Asia, these beetles are currently being used to help control purple loosestrife in the United States and Canada.
Both beetles look very similar and have nearly identical life cycles. They range from 3-6 mm in length and are about half as wide as they are long. Coloration is light brown or tan, sometimes with a dark stripe on each side of the insect when viewed from the top. It is not possible to differentiate between the two species in the field due to variability in key characteristics.
In spring (late April or May) overwintering adults emerge from soil and litter below old purple loosestrife plants to feed for several days on new foliage and then begin to reproduce. After mating, females lay eggs in masses of 2 to as many as 20 eggs per grouping, which may be placed low on the stem, at leaf axils, or on leaves. The egg-laying period occurs from mid-May to mid-July with the peak in May and June. An individual female may lay up to 500 eggs during this period. Larvae emerge from eggs in 7-10 days and feed on tissues of developing shoots and leaves. When larvae commence feeding adults may disperse, migrating to other nearby plants. Larvae feed for about three weeks, going through several instars before moving down into the soil or plant litter to pupate. Where water levels are higher, larvae may pupate inside the stem by burrowing into aerenchyma tissue (plant tissue containing cells composed primarily of air giving many aquatic plants buoyancy). Upon emergence the new adults harden in seven to ten days, then feed until moving into the litter or soil below to overwinter. This period of emergence occurs typically from July to September. The total maturation time from egg to adult is approximately 30-40 days.
The two Galerucella species inhabit loosestrife throughout the plant's natural range in Europe and Asia where, along with other natural enemies, they help keep purple loosestrife in check. Both species will lay eggs only on purple loosestrife and are specific feeders as well. The Galerucella beetles cause significant damage to purple loosestrife. Adults inflict a shothole feeding pattern eating small (1-2 mm) holes through foliage. Newly hatched larvae feed in growing shoot tips, killing them. Larger larvae produce a windowpane feeding pattern on the leaves, eating the softer tissues and leaving the tougher, less digestible veins.
Larval damage to flower and shoot buds reduces plant growth and inhibits flowering. Adult and larval leaf damage greatly reduces the photosynthetic capability of purple loosestrife, possibly leading to reduced starch stores in the roots which can result in winter plant mortality. Photosynthetic inhibition results in reduced stem height and root length, both essential to overall plant vigor. The resultant weakening and/or death of the loosestrife plants provides an opportunity for previously out-competed native plant species such as cattails, grasses, and sedges to return.
The beetles can fly between plants or plant clusters and both larvae and adults float, allowing current or wind to move them to nearby plants. Unfortunately, long-term standing water may be detrimental to Galerucella's development since pupation is difficult under these conditions.
It is expected that the establishment of permanent stable populations of G. calmariensis and G. pusilla on purple loosestrife will reduce this plant to levels that may be tolerated by North American wetland ecosystems.
- Donald Sebolt, Michigan State University
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