Polydrusus impressifrons Gyllenhal (Coleoptera: Curculionidae: Entiminae)
Hybrid poplars (Populus spp.) are propagated by inserting 12–15 inch cuttings next to each water emitter in a denuded 160 or 271 acre (65 or 110 hectare) field. The number of stems/hectare depends upon the final product: 500 for sawlogs, 1,750 for pulp, and 2,750–5,500 for biomass. Adult pale green weevils are strong fliers. They immigrate into these newly planted areas from adjacent mature stands where they are not considered serious pests to mature trees. Adult pale green weevils are considered reestablishment pests, damage is more likely when propagation follows a harvest of an earlier rotation of trees. Our objectives are (1) to alert growers that the pale green weevil is a potential reestablishment problem for propagation using cuttings, and (2) to suggest how to protect these new plantings from weevil attack.
Curculionidae is one of the largest insect families, with 20 subfamilies. Within hybrid poplars in eastern Oregon and eastern Washington, Polydrusus and the strawberry root weevil, Otiorhynchus ovatus L. belong to the broad-nosed weevil subfamily Entiminae (Figure 1). Cryptorhynchus lapathi L., the poplar-and-willow borer, another pest of poplars, is in the subfamily Molytinae. Finally, the bark beetle subfamily Scolytinae is represented by Xyleborinus saxesenii (Ratzeburg), commonly known as the fruit-tree pinhole borer. All four of these Coleoptera species are found in hybrid poplar plantings in the Pacific Northwest.
The pale green weevil has a wide range of hosts but prefers poplar, birch, willow, apple, and pear; however, it also feeds on elm, linden, locust, rose (Parrott and Glasgow 1916a), and yew (Windels and Flaspohler 2011).
The pale green weevil is native to Germany and France. It was first reported from a nursery in Geneva, New York, in 1906 (Parrott and Glasgow 1916a). It was first collected in hybrid poplars grown in eastern Oregon in 2004.
Partial life history studies were conducted in New York in 1916 (Parrott and Glasgow 1916a; Pierce 1916). Washington State University researchers in the Pacific Northwest conducted a more recent and detailed study (Niedbala 2013). Adults emerge from the ground at the base of host trees starting in late April or early May. The adult female weevil is slightly larger (5.5 mm) than the male weevil (4.25 mm). Adults are black, covered with greenish or sometimes yellowish scales (Figure 1); hence their common name: pale green weevil. They immediately begin to mate (Figure 2A) and females lay eggs (Pierce 1916). There is no report that the weevils need to feed before mating; however, they feed vigorously in the first week or more after their emergence. Eggs are laid in crevices, holes in the bark, or in wounds on the tree surface; however, there is nothing in the literature stating an attraction to any volatiles given off from the wounds. Eggs are found deposited on the sides of the trees exposed to the sun (Parrott and Glasgow 1916b). Eggs are laid singly or in groups (Figure 2B and C) of up to 85 (Parrott and Glasgow 1916b) and are approximately 0.5 mm in length. Under laboratory conditions these eggs hatch in 13 days at 25°C (77°F).