European Elm Bark Beetle Biological Control

Scolytus multistriatus (Marsham)

From: Bellows, Thomas S. ,Carol Meisenbacher, and Richard C. Reardon, 1998, Biological Control of Arthropod Forest Pests of the Western United States: A Review and Recommendations, USDA, FS, FHTET-96-21.

Origin: Old World. The species is known from Europe, the Middle East, and northern Africa (CAB 1975).

Range in North America: Throughout North America on Ulmus spp.

Plant hosts and damage: Ulmus alata, Ulmus americana, Ulmus pumila, Ulmus procera, Ulmus parvifolia, and other Ulmus spp. Overwintering adults of this species feed on twigs and, if the adults are carrying infective stages of the Dutch elm disease pathogen, Ceratocystis ulmi, the tree is usually infected by this feeding. Adults then fly to a new host tree to bore into the bark (Wood 1982). Egg galleries are excavated between the bark and the wood. Larvae feed on the intercortex tissues. Severe infestations can kill trees, usually those under water stress, in the absence of Dutch elm disease (Brown and Eads 1966). Trees are usually killed by the fungal disease.

Natural Enemies: Pathogens, predators and parasitoids have been reported from this species (Table 10). Additional parasitoids are known in Europe (Thompson 1943, Herting 1973).

Table 10. Principal species of natural enemies reared from, or associated with Scolytus multistriatus

¹Reared in association with S. multistriatus
² Laboratory record
³ Reared from infestations of S. multistriatus together with two other Scolytus species

Pest Status: There may be up to three generations per year in the United States (Hanula and Berisford 1984). This bark beetle is most renowned as a vector of Dutch elm disease, a fungus of Old World origin that can kill otherwise healthy elm trees. Trees become infected when beetles carrying infective stages of the fungus feed on twigs, before boring into the bark to reproduce. Many strains of the disease-causing fungus are known. Some fungal strains are more virulent than others, and many lead to death of the tree. Different species of Ulmus have different susceptibilities to infection. The greatest significance of this pest in North America is in the urban landscape, where many Ulmus spp. have been widely used as shade and ornamental trees.

This beetle species was accidentally introduced into eastern North America early in the 20th century. Populations eventually spread to the West Coast. In California, populations initially did not vector Dutch elm disease (Brown and Eads 1966, the causative agent is presumed to have been absent at that time). Nonetheless, these populations were still of sufficient intensity that concentrated attack on water-stressed trees could lead to tree death.

The disease-causing pathogen has become widespread and is a major cause of damage to elms. Trees are infected widely throughout the continent, but quantitative estimates of regional losses were not located. Westwood (1991) reports on management costs and losses in Manitoba, and equates a small percentage loss of elm trees in the region to Can$43 million. Cost of management in Manitoba averaged Can$1.5 million annually since 1981. Interest in control has focused both on controlling the vector (S. multistriatus) and on controlling the fungal disease.

Biological Control: Natural enemy introductions into North America were initiated in 1965 (Clausen 1978), when the braconid Dendrosoter protuberans was introduced from Europe. Millions of individuals were released in infested areas, and the species is widely established. The pest beetle is also attacked by other natural enemies (Table 10) in Europe. The species is susceptible in the laboratory to many different pathogenic fungi and nematodes. In laboratory and field studies, pathogenic nematodes are capable of killing all stages of the beetle (Poinar and Deschamps 1981). Adult beetles can be sterilized by some species of nematode (Oldham 1930).

Although several pathogens are reported as infective S. multistriatus and other Scolytus species, Mazzone and Peacock (1985) consider that efforts to find effective pathogens of this pest have been essentially futile. Investigations by Jassim et al. (1990) demonstrated pathogenicity by certain fungi, but these authors thought it unlikely that treating elm trees with these pathogens would reduce larval populations through mycotic infections.

The few field studies that have been conducted have identified braconids, either Coeloides scolyticida (Fransen 1931) or D. protuberans (Maksimovic 1979, Schroder 1974a) as the principal parasitoids in Europe. Parasitism of 73% was noted in Yugoslavia in 1972-1973 (Maksimovic 1986). Hajek and Dahlsten (1985) found total mortality of about 80% in California populations, with parasitism contributing 2% mortality.

Recommendations: While S. multistriatus is clearly a species of major importance, our knowledge of its population dynamics of is very limited. Quantitative studies on factors affecting population rates of increase are lacking, so determining the significance of natural enemies relative to other biotic (host-plant health) and abiotic factors remains difficult. Population studies in both the United States and areas where the beetle is native are needed to determine whether mortality caused by natural enemies is still lacking in the United States. If this is the case, additional natural enemies should be sought and released. The area evaluated for seeking additional natural enemies should include populations of the beetle in the Middle East. If population dynamics and mortality (related to natural enemies) on the two continents are equivalent, other strategies for control should be applied.

Among the possible strategies for control are the augmentation or application of pathogenic fungi or nematodes (Finney and Walker 1977) to elm trees in high value areas. Sanitation and related practices will continue to be important in limiting losses to the beetle and to Dutch elm disease (Westwood 1991). Biological control of the pathogen could be pursued, either through seeking antagonists to the pathogen (Gemma et al. 1984, 1985) or through efforts to reduce its virulence (Brasier and Gibbs 1975, Pusey and Wilson 1979). Genetic engineering of genus Ulmus has been proposed to improve disease resistance (Mazzone and Peacock 1985).

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