Author: Carole Cheah
Explorations for natural enemies of HWA began in Japan in 1992. Of the several predators collected from adelgid-infested Japanese hemlocks, a tiny lady beetle, Sasajiscymnus tsugae (Sasaji and McClure) (formerly Pseudoscymnus tsugae Sasaji and McClure [Coleoptera: Coccinellidae]), from Osaka prefecture (approximately 34° N) on the northern island of Honshu (Sasaji and McClure 1997) proved to have the greatest potential for biological control of HWA (Fig. 6). (Note: The lady beetle, Pseudoscymnus tsugae, was recently reclassified as Sasajiscymnus tsugae by Vanderberg .)
Sasajiscymnus tsugae belongs to the Tribe Scymnini, a group of small coccinellids, less than 3mm long, that are specialist predators of aphids, scales, mealybugs, and adelgids. The adult is black, about 2mm long, and both larvae and adults feed on all stages of HWA. Females lay eggs singly or in small clusters in concealed locales on hemlock foliage, buds, cones, and stem crevices.
There are four larval instars and a pupal stage, with 70 percent of the total pre-adult development time taking place during the mature fourth instar. Eggs hatch in 6 days at 25°C (77°F) and 10 days at 20°C (68°F); maturation to adult takes 24 days at 25°C and 40 days at 20°C (Cheah and McClure 1998.)
Sasajiscymnus tsugae prefers adelgids to aphids, and can complete development on HWA as well as the balsam woolly adelgid, pine bark adelgid, and Cooley spruce gall adelgid. A lab colony of Sasajiscymnus tsugae was established in Windsor, Conn., in 1994; S. tsugae were free-released in North America in 1995. Subsequent studies at The Connecticut Agricultural Experiment Station showed it to have a high lifetime fecundity. Females lay between 64 and 513 eggs (avg. 280) over a period of 5 to 30 weeks (avg. 14 weeks) (Cheah and McClure 1998). Adults have long life spans (longer than one year with overwintering). Sasajiscymnus tsugae is bivoltine, producing two generations per year in the northeastern United States (Cheah and McClure 2000).
Other field studies have shown that S. tsugae can reproduce after release, disperse locally, survive heat waves, overwinter, and establish in a variety of different hemlock habitats in Connecticut and other states (Cheah and McClure 2000). It responds well to laboratory mass rearing on HWA-infested foliage collected from the field between fall and mid-summer.
Field releases of S. tsugae began in Connecticut in 1995 and elsewhere in 1999. Since 1995, over 1-million S. tsugae have been released on more than 100 sites in 15 eastern states, from South Carolina to Maine. The releases were made possible largely through the mass rearing efforts of the Phillip Alampi Beneficial Insects Laboratory of the New Jersey Department of Agriculture, and EcoScientific Solutions, LLC, the North Carolina Department of Agriculture, and Clemson University.
Studies conducted in Connecticut have established that a significant synchrony exists between the life cycles of S. tsugae and HWA (Cheah and McClure 2000) (Fig. 7). Adult beetles emerge from overwintering sites in hemlock forests in March and April. Females generally mate before the onset of winter and begin oviposition on the HWA sistens generation in April, when daytime temperatures average 15°C (59°F). Females lay eggs throughout the spring into mid-summer on both the sistens and progrediens adelgid generations. A smaller second generation of S. tsugae is also produced on progrediens, with new adults emerging from mid-August into September. Incubation periods and larval development times vary and depend on seasonal spring temperatures. Larvae feed on all stages of HWA (Cheah and McClure 1996) and the first generation of adults generally emerges in June or July. Adult S. tsugae can survive the late summer period by feeding on dormant settled adelgid nymphs and can be found in hemlock forests during the late summer and early fall. Adults were found on hemlock foliage during mild winters in the northeast (Cheah and McClure 2000). In sleeve cages, S. tsugae survived minimal daily winter lows of -21.6°C (-7°F) in northern Connecticut, and -21°C (-5.8°F) in north central Maine (Cheah and Donahue 2003).
At four forested sites in Connecticut and Virginia, initial field releases of S. tsugae on un-bagged branches have shown good potential for producing localized reductions in pest levels. Between 2,400 and 3,600 adults were released, 30 to a branch per site, onto established HWA infestations in May, 1997. In October, HWA densities were significantly lower on release branches than on non-release control branches, 500 meters away. Adelgid densities were 47-83 percent lower on exposed, un-bagged branches in the release area, compared to both bagged branches, which previously had been enclosed to exclude both native and introduced predators, and un-bagged branches in control areas (McClure et al 2000). Sampling for S. tsugae with a beating sheet, following augmentative releases in subsequent years, indicated successful overwintering, reproduction, and local dispersal, both laterally and vertically into the upper canopy (Cheah and McClure 2000).
In Connecticut it appears the long-term impact of S. tsugae releases on hemlock health varies with the type of site. Long-term monitoring (4 to 7 years) of S. tsugae release sites in Connecticut shows that hemlock recovery on some sites can be hampered by several factors, including soil characteristics (rocky or shallow), drought stress, and infestations of either elongate hemlock scale or hemlock borer. Also, the probability of effective prey reduction within 3 years after a predator release is influenced by the magnitude of the adelgid infestation prior to the release. In sites with stressed trees and initially high adelgid and scale infestations, S. tsugae appears less able to establish itself rapidly enough within a few years of release to slow hemlock decline. However, in some sites previously infested hemlocks with dieback have demonstrated a remarkable ability to recover and produce new shoots, especially under favorable environmental conditions, such as abnormally moist or cool springs and summers (Cheah and McClure 2000).
In better quality woodland sites, adelgid densities have remained low and the trees have shown recovery after 4 to 7 years. Recent winter extremes (2000 and 2003) have dramatically reduced adelgid populations, which has favored predator impact. It is evident that the winter mortality of the sistens generation plays a significant role in adelgid population dynamics in spring and summer.
Technical Assistance: Beth Beebe, Mary (Klepacki) Frost, Robert Ballinger, Steven Lamoureux, John Winarski, Richard Horvath, Peter Trenchard, Xavier Asbridge, Steven Sandrey (Connecticut Agricultural Experiment Station).