Author: Scott Salom
Dr. Lee Humble, a scientist with the Canadian Forest Service in British Columbia who searches for natural enemies of HWA and the balsam woolly adelgid, observed that the small, little-known beetle, Laricobius nigrinus Fender (Coleoptera: Derodontidae), consistently feeds on HWA in western hemlock seed orchards. Drs. Scott Salom and Loke Kok (Virginia Tech) visited these seed orchards and, between 1997 and 2003, imported the beetles to Virginia for study under quarantine. They determined L. nigrinus produces one generation per year and undergoes diapause at the same time and for the same duration as HWA (Zilahi-Balogh et al. 2003 a, b). The predator will feed on other adelgids, but prefers to feed on HWA, and will complete development only on HWA (Zilahi-Balogh et al. 2002).
Lab studies show that L. nigrinus prefers temperatures between 12° and 15°C (54° to 59°F), and field studies in British Columbia and Virginia show it is active in the winter – a critical point, because HWA is also active during the winter (Fig. 12). Adults feed on all HWA life stages present from November to May, and begin laying eggs in HWA ovisacs (one per ovisac) in February (Fig. 13). Larvae feed exclusively on HWA eggs.
Laricobius nigrinus was removed from quarantine in September, 2000. The following year, Virginia Tech began conducting field evaluations and exploring ways to rear the insect on a large scale. In general, rearing HWA predators in the lab is labor intensive, mostly due to the enormous amount of fresh food required to maintain and build colonies. For L. nigrinus specifically, there are additional complications: They need cold temperatures; both the pupae and diapausing adults must live in soil; and the diapause period in lab-raised L. nigrinus must be in synch with the diapause period of HWA in the field. Complications aside, tremendous progress has been made, and the mass production of sufficient quantities of beetles needed for operational release appears achievable.
Field evaluations of L. nigrinus have been promising. Adults survive the Virginia winters in sleeve cages, and feed voraciously on HWA sistens. In the first field-release study of this predator, progeny produced by ovipositing adults in sleeve cages killed 50 percent more HWA progrediens than died naturally on untreated branches (Fig. 14). These experiments utilized 144 adults (one to three per branch) for only 10 days. Yet, during that time they yielded close to 12,000 predator eggs. Sampling to determine the establishment of L. nigrinus at this first release site began in fall 2003 and will continue for several years. In November and December 2003, 300 adult L. nigrinus were released at each of seven sites within the mid-Atlantic region. Releases will continue at increasing frequency over the next several years in an attempt to establish L. nigrinus.
A study was initiated in 2003 to evaluate the potential competitive interaction among two host-specific predators, L. nigrinus and S. tsugae, and the generalist, Harmonia axyridus. First-year results show the majority of L. nigrinus activity occurs earlier than that of both the other predators; both L. nigrinus and S. tsugae will feed on each others’ eggs, but only when HWA density is very low; and H. axyridus might not be as formidable a predator on the other two as was expected.
A worldwide search for additional Laricobius species began in 2002. As a result, two new species were discovered in China, one of which is currently being reared and studied under quarantine at Virginia Tech (Fig. 15).
Robbie Flowers, Lee Humble, L. T. Kok, Ashley Lamb, Warren Mays, Tom McAvoy, David Mausel, Gabriella Zilahi-Balogh.