by Mark S. McClure and Carole A. S - J. Cheah, Valley Laboratory, The Connecticut Agricultural Experiment Station, Windsor, Connecticut
Among the most widespread and effective predators of hemlock woolly adelgid, Adelges tsugae (Annand), in its native homeland of Japan is a previously non-described coccinellid of the genus, Pseudoscymnus. In 1992 Dr. Mark McClure collected this ladybird beetle from adelgid-infested hemlocks in 13 of 37 forests and at 11 of 37 ornamental sites in 12 prefectures throughout Honshu, Japan. Drs. McClure and Dr. Hiroyuki Sasaji have described and named this new beetle, P. tsugae. Beginning in 1995 more than 100,000 adult beetles have now been released in infested hemlock forests in Connecticut, New Jersey and Virginia to evaluate P. tsugae as a biological control agent.
The egg of P. tsugae is about 0.48 mm long by 0.25 mm wide and is oval and reddish-orange in color within an opalescent sheath. Eggs are often laid singly or in small groups in cracks and crevices in the bark and in bud scales. Larvae change from reddish-brown to gray and grow from about 1.1 mm to 2.7 mm during their four instars of development. The pupa is reddish-brown and is about 1.9 mm long and 1.1 mm wide. The newly emerged adult is light golden brown before darkening to jet black within a day. Adults are oblong to oval-shaped and are about 1.7 mm long by 1.1 mm wide. The body is entirely black and is pubescent on its dorsal surface.
Adults and larvae were found on branches of Tsuga diversifolia and T. sieboldii infested with A. tsugae throughout Honshu, Japan; adults were collected from sweep net samples of a grassy meadow in Fukui Prefecture, Japan; and all life stages were gathered from T. canadensis at release sites in Connecticut and Virginia.
Pests Attacked (Host Range)
P. tsugae is only known to attack A. tsugae in nature. However, laboratory experiments revealed that P. tsugae can also feed and develop on other adelgid species including balsam woolly adelgid, A. piceae, Cooley spruce gall adelgid, A. cooleyi, and pine bark adelgid, Pineus strobi.
The life cycle of P. tsugae is well synchronized with that of its prey. Both insects have two generations each year in the field. Spring egg laying by adult beetles normally coincides with peak egg laying and hatching of adelgids. Furthermore a second generation of beetles occurs in June around the time that the second generation of adelgids does. Also, when adelgids are inactive for about 14 weeks during the summer, adult ladybugs are able to survive by feeding on dormant young adelgids. Three or more generations of P. tsugae can be reared each year in the laboratory under controlled temperature conditions.
Adults and larvae of P. tsugae are highly mobile and voraciously feed on all life stages of A. tsugae. Each beetle larva consumes about 500 adelgid eggs or from 50 to100 adelgid nymphs, depending upon the size, during its development. Adults can live for more than one year and may consume about 50 adelgid nymphs each week during times of peak feeding activity. Each female beetle lays nearly 300 eggs in her lifetime.
P. tsugae is an important predator of A. tsugae in Japan; it killed from 86 to 99% of the adelgids at the 24 sites were it occurred. Experiments conducted at four sites in Connecticut and one site in Virginia from 1995 through 1997 revealed that releasing relatively few adult beetles (2,400-3,600) into an infested hemlock forest reduced adelgid densities by 47-88% on release trees in only 5 months. In the field P. tsugae mated, reproduced and dispersed from release trees into the surrounding hemlock forest, and established. It overwintered successfully for three years under a wide variety of climatic conditions. These studies indicate that P. tsugae has excellent potential for biological control of A. tsugae.
For general information about conservation of natural enemies, see Conservation in the Tutorial section on this site, Feature Article on conservation in Volume II, No. 1 of Midwest Biological Control News.
P. tsugae is not available commercially. Research is underway to streamline the mass rearing of P. tsugae which for now remains extremely labor intensive. If P. tsugae proves to be a successful biological control agent for hemlock woolly adelgid, the rearing and release efforts will be intensified so that beetles can reproduce and spread on their own from relatively few release sites throughout the entire adelgid-infested area which now includes 11 eastern states.
Cheah, C.A. S-J. and M.S. McClure. 1996. Exotic natural enemies of Adelges tsugae and their potential for biological control. Proc. First Hemlock Woolly Adelgid Review, USDA For. Serv., FHTET 96-10, pp. 103-112.
Cheah, C.A. S-J. and M.S. McClure. Life history and development of Pseudoscymnus tsugae (Coleoptera: Coccinellidae), a new predator of the hemlock woolly adelgid, Adelges tsugae (Homoptera: Adelgidae). Environ. Entomol. (in press).
McClure, M.S. 1997. Biological control in native and introduced habitats: Lessons learned from the sap-feeding guilds on hemlock and pine. pp. 31-52. In D.A. Andow, D.W. Ragsdale, and R.F. Nyvall (eds.). Ecological Interactions and Biological Control. Westview Press, Boulder, Colorado.
McClure, M.S., and C. A. S-J. Cheah. Released Japanese ladybugs are multiplying and killing hemlock woolly adelgids. Frontiers of Plant Science. 50(2): (in press).
Sasaji, H. and M.S. McClure. 1997. Description and distribution of Pseudoscymnus tsugae sp. nov. (Coleoptera: Coccinellidae), an important predator of hemlock woolly adelgid in Japan. Ann. Entomol. Soc. Amer. 90:563-568.