Invasive and Exotic Species

Chapter 1: Biological Control Agents

Biological Control of Hemlock Woolly Adelgid

Scymnus Lady Beetles from China - Michael E. Montgomery

From: Cheah, C., M. E. Montgomery, S. Salom, B. L. Parker, S. Costa, and M. Skinner, 2004. Biological control of hemlock woolly adelgid. USDA For. Serv. FHTET-2004-04, Reardon, R. and B. Onken (Tech. Coordinators), 22pp

Exploration for natural enemies of HWA was undertaken from 1995 through 1997 in the Chinese provinces of Yunnan, Sichuan, and Shannxi, at the edge of the Tibetan Plateau (Yu et al. 1997, Wang et al. 1998, Zhao et al. 1999) (Fig. 8). Three species of hemlock grow in this steep, rugged, mountainous terrain at altitudes between 2,500 and 3,000 meters (8,000-10,000 feet). The hemlocks usually occur as a scattered, minor component in forests of mixed deciduous and coniferous species (Mongtomery et al. 1999).

Figure 8. Map of China showing three provinces (red arrows) from which Scymnus spp were collected.

More than 54 species of lady beetles were collected with more than half of the species new to science (Yu et al. 2000). Two-thirds of the lady beetles collected are in the coccinellid subfamily Scymninae, which, with more than 600 species, is the largest subfamily of lady beetles. Its members are small, pubescent, and dull colored. They are fairly prey specific and usually feed only on one family in the order Homoptera. Several species have been used successfully for biological control (Hagen et al. 1999). The two groups of lady beetles in the Scymninae subfamily that seem most promising for biological control of HWA are in the genera Sasajiscymnus and Scymnus (subgenus Neopullus). Both are endemic to Asia and have not been found in North America.

Figure 9a Scymnus camptodromus Yu and Liu - Adult(s) Photo by Guangwu Li Chinese Academy of Forestry

Figure 9b Scymnus sinuanodulus Yu and Yao - Adult(s) Photo by Guangwu Li Chinese Academy of Forestry

At each of three collection sites in China, the most abundant lady beetles feeding on HWA were previously unknown species of Scymnus (Neopullus) (Coleoptera: Coccinellidae). (These lady beetles were also observed to feed on an adelgid found on Pinus armandii [Yu 1999]). The beetles were subsequently identified as Scymnus camptodromus Yu et Liu, Scymnus sinuanodulus Yu et Yao, and Scymnus ning-shanensis Yu et Yao (Fig. 9). The biology and host range of each of the three Scymnus species were studied in China and, after importation to the United States, in quarantine.

Figure 9c Scymnus ningshanensis Yu and Yao - Adult(s) Photo by Guangwu Li Chinese Academy of Forestry

Host range tests showed Scymnus adults prefer to feed on adelgids, but will feed to a limited extent on aphids (Montgomery et al. 1998, Butin et al. 2002). Scymnus adults will feed on all life stages of HWA; however, Scymnus larvae (first instar) will survive only if they can feed on HWA eggs.

The life stages of the three lady beetles are similar (Fig. 10). In nature, all three beetles lay many single eggs in bud-crevices, beneath the ovisac wool, or in other concealed locations. The eggs of S. sinuanodulus and S. ningshanensis hatch in about 10 days; the eggs of S. camptodromus enter diapause and do not hatch until the following spring. Diapause makes it difficult to rear S. camptodromus in the laboratory, making it less attractive as a candidate for biological control than either S. sinuanodulus or S. ningshanensis. All three beetles pass quickly through four larval instars and a pupal stage (Lu et al. 2002), with total development time from egg hatch to adult between 25 and 30 days at room temperature, 20°C (68°F) (Wang et al. 2000, Montgomery et al. 2002). Generally, newly matured adults overwinter one season before laying eggs, and unlike Sasajiscymnus tsugae, they produce only one generation per year.

Figure 10a Egg(s), Egg inserted into budscale Photo by Lynn Jones and Nathan Havill USDA Forest Service	Figure 10b Larva(e) Photo by Lynn Jones and Nathan Havill USDA Forest Service
Figure 10c Pupa(e), Masked with defensive secretion at tip of setae Photo by Lynn Jones and Nathan Havill USDA Forest Service	Figure 10d Adult(s), Mating Photo by Lynn Jones and Nathan Havill USDA Forest Service

Egg laying by the Scymnus beetles is dependent on the availability of prey. Beginning within one week after emergence from overwintering, the beetles lay one to two eggs per day for 4 to 6 weeks, followed by a decline of approximately one month due to a lack of prey. However, if prey should happen to be available, the egg-laying period can last for 7 months (Zhao et al. 1998, Lu and Montgomery 2001, Mongtomery et al. 2002).

Sleeve cages are being used to evaluate the impact of S. sinuanodulus and S. ningshanensis on the population dynamics of HWA. Adelgid woolly masses on a hemlock branch are counted in April, when the overwintering sistens generation of HWA starts to lay eggs. After counting, either a lone female beetle or a male/female pair is placed in a mesh bag, which is then affixed to the branch tip infested with around 300 adult adelgids, far more than the beetles are expected to eat (Fig. 11). Bags with no beetles serve as controls. In late June, when the progeny of the sistens are mature, the bagged branches are brought to the laboratory; the adelgids and lady beetles are counted and compared to the original counts (Butin et al. 2003).

Figure 11. Sleeve cage used for field study of Scymnus lady beetles
Lynn Jones and Nathan Havill, USDA Forest Service

In these field studies, the HWA populations were lower in the bags containing Scymnus beetles than in the control bags. However, the extent of the impact of the lady beetles appears to be affected by the condition of the adelgid population prior to the branches being bagged. If the adelgid population is low and rapidly increasing, the beetles dramatically reduce the rate at which the HWA population increases; if the population is declining because of a reduction in host nutritional value, the impact of the lady beetles on the HWA population is relatively small. By preventing low HWA populations from increasing, the Scymnus beetles stabilize the population, which in turn prevents the adelgids from damaging hemlock. Preconditioning the beetles for several weeks with simulated winter conditions prior to placement in the field might influence the beetles effectiveness against HWA.

The Connecticut Agricultural Experiment Station and the Phillip Alampi Beneficial Insects Laboratory, New Jersey Department of Agriculture, are cooperating with the USDA Forest Service to mass produce Scymnus beetles. As of this writing, a mid-summer (2004) release of newly reared, non-overwintered adult S. sinuanodulus beetles is underway in the Southern Appalachians.

Acknowledgments

Thanks and appreciation for assistance in collecting, rearing, and evaluating Scymnus lady beetles is extended to: Defu Yao, Houping Liu, Hongbin Wang, Guoyue Yu, Xueqin Xu, Hengxiao Guo, Carol ODell, Wenhua Lu, Julie Slowik, Jenny Ogrodnick, Lynn Jones, Nathan Havill.

[  Contents  ]  [  Previous  ]  [  Next  ]  [  Home  ]