Douglas-Fir Beetle Biological Control

Dendroctonus pseudotsugae Hopkins

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: North America.

Range in North America: Throughout the range of Pseudotsuga menziesii in western North America, along the Pacific Coast through to the Rocky Mountains.

Plant hosts and damage: Primarily Pseudotsuga menziesii, but also recorded from Larix occidentalis, Tsuga heterophylla. Adults bore into bark, create egg galleries, and oviposit. Larvae feed between the bark and the wood. Heavy infestations kill the host tree.

Natural Enemies: Both vertebrate and invertebrate natural enemies are reported from D. pseudotsugae (Table 6). Nematode associates of this beetle have been reported by Thong and Webster (1983), but the nature of the associations for those species is not clear.

Table 6. Natural enemies of Dendroctonus pseudotsugae

Pest Status: This species is the most important bark beetle attacking Douglas-fir throughout the range of this tree in North America. The beetle usually breeds in felled, injured or diseased trees. Damage from endemic populations of this beetle is considerable but widely scattered. Outbreaks also occur, during which healthy trees are killed over extensive areas. Outbreaks are sporadic and of short duration along the Pacific coastal region from British Columbia to California, but kill large amounts of timber. These outbreaks usually follow extensive windthrow or fire, which provides abundant breeding resource for the beetle. In the Rocky Mountain region outbreaks also develop in trees felled by wind or snow or affected by drought, and are of longer duration.

In coastal Douglas-fir, outbreaks usually subside abruptly. Resistance of the host tree evidently keeps the beetle under control, as the beetles do not thrive in green timber. In interior areas tree stress is an important factor affecting the extent and duration of an outbreak.

Direct control of the Douglas-fir beetle has seldom been attempted. Salvage of at-risk trees in coastal areas is practical. In inland forests, prevention is practiced by thinning and harvesting.

Biological Control: Several studies have evaluated the role of natural enemies in populations of D. pseudotsugae. Rust (1933) notes the presence of predaceous mites in Ips spp. populations, and suggests that they are probably the cause of 63% egg mortality observed in D. pseudotsugae. Marsden et al. (1981) attributed 58% preimaginal mortality to entomophagous insects, including the predators Medetera spp. and Enoclerus sphegeus and the parasitoid Coeloides vancouverensis. The predator Medetera aldrichii consumed an average of 14.7 immature beetles in the laboratory (Nagel and Fitzgerald 1975). Infection of adult female beetles by the nematode Contortylenchus reversus resulted in lowered levels of protein in the haemolymph and a 20% reduction in the size of oocytes (Thong and Webster 1975). The abundance of larvae of the predators E. sphegeus and Thanasimus undatulus was significantly correlated with the abundance of attacks by D. pseudotsugae (Furniss et al. 1974) Karpinskiella paratomicobia parasitized adult D. pseudotsugae, causing a 67% reduction in fertility (Furniss 1968). Furniss (1967) reported six parasitic nematodes in adult beetles in Idaho and Utah. The report states that 93% of the beetles were infected, but the impact of the infection was not quantified. Hopping (1948) reports on the effectiveness of Medetera aldrichii as a predator of the beetle, and notes that only one other natural enemy, E. sphegeus, seems to be as effective,

Recommendations: As this beetle is a native pest, the introduction of additional natural enemies would have to be made from other species of related beetles. Some studies toward this end have been initiated against other Dendroctonus spp. (Miller et al. 1987), particularly focused around European or Asian species of clerid predators. None has yet met with success. Enoclerus sphegeus var. arachnodes preyed on the beetle in Chihuahua, Mexico, but not in Idaho, where the beetles was preyed on by Enoclerus sphegeus (Furniss and Cibrian Tovar 1980). The common braconid larval parasitoid Coeloides pseudotsugae was absent in Mexico, and might be profitably introduced there (Furniss and Cibrian Tovar 1980). This pest might be a suitable target to test the idea of introducing exotic natural enemies against native bark beetle pests (as might D. brevicomis or D. ponderosae).

Integrating knowledge of the biology and ecology of the predators with forest sanitation practices holds promise for integrating biological and cultural control. Ryan and Rudinsky (1962) described the field biology of the braconid parasitoid Coeloides brunneri, and discussed how the removal of infested logs from the forest, particularly late in the season, would remove overwintering parasitoids. They proposed felling small diameter trees late in the flight season of the beetle to favor late infestation as a means to promote populations of the parasitoid.

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