Biology, seasonality, and impact of Altica litigata Fall on purple loosestrife, Lythrum salicaria L., in northeastern Tennessee

Purple loosestrife, Lythrum salicaria L., an exotic, wetland perennial, forms large, monotypic stands throughout many temperate regions of the U.S. and Canada. This invasive plant replaces native vegetation, degrades wildlife habitats, and obstructs waterways. A large population of a native, herbivorous insect, Altica litigata Fall, was found feeding on purple loosestrife in upper eastern Tennessee, and both larvae and adults fed on foliage and caused plant damage. Thus, a two-year research project was initiated in 2000 to assess biology, seasonality, and impact of this North American flea beetle, A. litigata, on seed production, plant height, and leaf consumption of purple loosestrife in northeastern Tennessee. The study site was located in Unicoi County, Tennessee, at a wetland along the banks of South Indian Creek where a small infestation of purple loosestrife exists, and was monitored two times weekly from April to September. The site was divided into three subplots, and adult flea beetle densities were determined weekly by sweep-net sampling. Life stage information was obtained from laboratory and field observations and direct sampling. To assess impact of adult feeding on numbers of seed capsules, seeds/capsule, and plant height, plants were placed into four damage rating categories: 1) none, 2) minor, 3) moderate, and 4) major. Ten additional plants, extensively damaged by larvae, also were evaluated. Plant height, number of seed capsules/lateral, number and lengths of seed-bearing laterals, and number of seeds/capsule were measured. In the laboratory, leaf consumption by adults was quantitated using a leaf area meter. Adults (0, 2,4, or 6 male or female) were fed foliage, which was measured and replaced every 2 days. Estimated numbers of beetles required for defoliation of purple loosestrife were determined. All life stages of A. litigata were found in association with L. salicaria. Eggs and adults were first found on 18 May, and larvae were first documented as on 8 June, but had been present in the field for several weeks. Developmental times for eggs, larvae (three instars), and pupae were 3-5 days, 13-16 days, and 7-10 days, respectively. In the field, larval feeding was extensive; skeletonized foliage appeared "burned" and reduced plant viability and flower maturity. Larval damage caused large reductions in seed production (the larval-damaged plants produced 3,003 seeds/plant stem, compared to the non-damaged plants that produced 67,258 seeds/plant stem). The larval-damaged plants averaged only 52 seed capsules/plant stem, while the non-damaged plants were at least 10x greater, averaging 685 seed capsules/plant stem. The extensive larval damage resulted in a 20% reduction in plant stem height when compared to the non-damaged plants (with mean plant heights of 144 and 179 cm for larval-damaged and non-damaged plant stems, respectively). Adult females consumed ca. 2x more leaf area than males (ca. 0.60 cm² and 0.32 cm², respectively). Based on these results, 39 male or 20 female beetles would consume an entire leaf( with an average leaf area of 12 cm²) in a two-day period. Combined leaf consumption by adults and larvae during their entire lifetime could cause significant damage to purple loosestrife. Synchronization of larval A. litigata and the host flower budding stage of L. salicaria is a positive attribute of this potential native biological control agent. Altica litigata herbivory had a negative impact on seed production and plant height. This work will be the first to document biology, seasonality, and impact of A. litigata on purple loosestrife. This incidence of A. litigata also represents a new state record. A better understanding of the biology, life history, and impact of A. litigata on L. salicaria may lead to its use as a biological control agent. As a native species its use should have less negative environmental consequences than an introduced species. Further research is needed to investigate the biological control potential of A. litigata on purple loosestrife.