Comparative cold tolerance and climate matching of coastal and inland Laricobius nigrinus (Coleoptera: Derodontidae), a biological control agent of hemlock woolly adelgid.
Laricobius nigrinus (Coleoptera: Derodontidae) was first collected near the coastal city of Victoria, British Columbia, Canada for release as a biological control agent to suppress tree-killing densities of hemlock woolly adelgid, Adelges tsugae (Hemiptera: Adelgidae), in the eastern United States. Beetles established in warm areas of the invaded range of A. tsugae, but had a low probability of establishment in cold areas. With the goal of locating beetles with greater cold-tolerance, we collected hundreds of adults in the northern Rocky Mountains. To support planned releases of these inland L. nigrinus, the cold tolerance of field-collected coastal (Seattle, WA) and inland (Coeur d'Alene and Moscow, ID) adults and climate match index scores (CLIMEX v.2) of these collection areas and parts of the eastern United States were compared. We found that individuals of inland L. nigrinus were more cold tolerant than those of coastal L. nigrinus, based on higher survival in a winter field cage study in Massachusetts and a lower supercooling point in a laboratory assay. Inland L. nigrinus also had higher survival than coastal L. nigrinus after an 18 h exposure to sub-zero temperatures in another laboratory assay. Areas of the eastern United States infested with A. tsugae and receiving L. nigrinus releases matched the climate of Seattle and Coeur d'Alene reasonably well, but Coeur d'Alene had higher index values over a considerably larger area than Seattle. Accordingly, inland L. nigrinus appears preferable for release in the colder portions of A. tsugae's invaded range in the eastern United States.