BENEFITS AND RISKS OF IMIDACLOPRID-BASED MANAGEMENT PROGRAMS FOR HEMLOCK WOOLLY ADELGID

Hemlock woolly adelgid, Adelges tsugae (Annand) (HWA) (Hemiptera: Adelgidae), has caused widespread decline of eastern hemlock, Tsuga canadensis (L.) Carriére. This collaborative retrospective analysis with Great Smoky Mountains National Park (GRSM) provides research-based management guidance on imidacloprid treatments and nontarget risks to aquatic systems.

Imidacloprid and olefin concentrations were assessed in foliage from different diameter at breast height (DBH) size hemlocks 4 – 7 yr post-imidacloprid treatment. Imidacloprid concentrations were below the LC₅₀ [lethal concentration] for HWA, but olefin was above the LC₅₀ 4 yr post-treatment. HWA populations were suppressed, and hemlock canopies were healthy. Treatment efficacy can last up to 7 yr post-treatment.

Hemlocks from the larger (61 and 76 cm) size classes generally had higher concentrations of imidacloprid and olefin. Concentration data from foliage of different size hemlocks were used to develop a model to optimize dosages based on tree diameter. Smaller (< 30 cm) and larger (> 63 cm) hemlocks can be given lower imidacloprid doses, while maintaining > 80% HWA mortality in hemlocks.

Impacts to stream water quality and aquatic macroinvertebrates were assessed in ten streams in hemlock conservation areas. Water samples were collected upstream and downstream from conservation areas and in nearby control streams. Imidacloprid was present in seven downstream locations in concentrations from 28.5 to 379 ppt, which is below USEPA aquatic life benchmarks. Aquatic macroinvertebrate bioassessments were conducted at downstream and upstream locations, and downstream baseline data were available. Diversity measures at downstream samples did not vary from those at upstream and baseline samples. Imidacloprid treatments did not negatively affect aquatic macroinvertebrate communities.

Imidacloprid treatments suppress HWA populations for up to 7 yr. Dosages can be optimized based on the DBH size of the hemlock. Imidacloprid risks to aquatic systems for this HWA management program were minimal and within USEPA benchmarks.