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Branches, twigs and stems, especially main shoots, are attacked by S. populnea. In newly planted forests, it mainly attacks the main shoots. As the trees grow, some lateral branches may become suitable for attack, but the insect still attacks the main shoots much more than the lateral branches.
Where S. populnea attacks, a gall is formed in the shape of a spindle. The infested branches are prone to withering and wind-breaks, which result in a deformed crown. If S. populnea attacks the stems of saplings, it can kill the whole tree. After being attacked by S. populnea for 3 to 5 years, small trees will result as the forest ages.
Good control of larvae and pupae of S. populnea was achieved by the release of Sclerodermus guani in conditions of sparse understorey vegetation, low altitude (200-300 m), high temperature (22-30°C) and low humidity <90% RH) (Zhang, 1994). In an experiment conducted during 1996 in Jilin, China, the introduction of S. guani resulted in effective (64.5%) control of S. pupulnea. The financial cost of this control strategy, however, was high (Pi et al., 2001).
In a field trial, the application of Metarhizium anisopliae against S. populnea resulted in 94.1% mortality (Fan et al., 1988). Pilot tests with a conidial suspension of M. anisopliae subsp. anisopliae, strain Ma83, gave 70.8% control of S. populnea (Fan et al., 1990).
The protection and utilization of natural enemies such as woodpeckers may be a good way of controlling the pest. In a study in Finland, the few natural enemies of S. populnea together destroyed 29% of the eggs, larvae and pupae, and seemed to play a relatively large role in regulating numbers of S. populnea (Pulkkinen and Yang, 1984).
The occurrence of S. populnea is closely related to the environmental conditions, especially the silvicultural methods used. Planting large areas of pure poplar forest creates good conditions for the pest to thrive, and therefore planting mixed forests including resistant species suitable to the site is useful. When planting poplars, 3-year-old saplings with an obvious stem should be used. Existing pure forests should be restructured to improve the quality of the forest, as pure poplar forests are very liable to be infested by S. populnea.
Spray treatment with phenthoate or chlorpyrifos during the period of adult emergence produced high insect mortality (>91%) (Shin and Shung, 1987).
A programme comprising 5-8 applications of organophosphate insecticides with systemic, ovicidal or contact action against different stages is recommended as effective against S. populnea (Attard, 1979). The cost of the treatment was considerably lower than that for treatments used hitherto.
Other Control Methods:
Sources of pest infestation should be cleared; infested branches, dead trees and wind-thrown trees should be removed, and weak trees should be regenerated. Galls can be cut from late autumn to early spring.
The flight ability of S. populnea is poor, and artificial trapping can be used against the pest. During the peak period of adult eclosion, trapping adults can reduce the population.
The eggs can be cracked with hammers or stones to decrease the quantity of larvae emerging.
S. populnea causes a high loss of timber. For a 2- to 4-year-old tree, one gall on the main shoot can decrease the increment in height by 25%. Two galls will decrease height increment by 45%, three galls by 70%, and four or more galls can prevent growth of the main shoot in that year or cause the whole tree to wither and die (Zhang and Zhang, 1990).
In Jilin Province, China, where S. populnea is a major insect pest of poplars, S. populnea occurs in more than 20,000 hectares of poplar forest and has much impact on the social economy and ecology.