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Plantwise Technical Factsheet

shot-hole gall midge (Rabdophaga saliciperda)

Host plants / species affected
Salix alba (white willow)
Salix alba var. vitellina-pendula
Salix babylonica (weeping willow)
Salix caerulea
Salix fragilis (crack willow)
Salix fragilis x Salix alba
Salix fragilis x Salix triandra
Salix koriyanagi (Japanese basket osier)
Salix matsudana (Peking willow)
Salix purpurea (purple willow)
Salix triandra (almond willow)
List of symptoms/signs
Stems  -  galls
Stems  -  internal feeding
Stems  -  necrosis
Symptoms
The larvae of R. saliciperda develop in small chambers situated in the outer layer of the wood under the bark of willows; mainly Salix alba and Salix fragilis. The attacked parts are slightly swollen and initially covered with bark. Later, many small holes appear and the attacked parts seem to be shot, hence the common name of shot-holes gall midge. The openings are prepared (worked) by the mature larvae before their metamorphosis into pupae and they are finished by the pupae before the emergence of the adults. Usually the young stems and branches (1-15 cm in diameter) in the lower parts of willow shrubs and trees, are attacked. The larval chambers are usually grouped together and situated parallel to the axis of the stem. Each chamber is inhabited by one larva. After the emergence, the bark splits and a high number of emergence holes can be observed on the attacked area of the shoot. The damaged part of the stem may be 2 to 40 cm long and thicker than the unattacked part. The stem may increase in width if the females lay eggs on the stem where they emerge from, or close to this area. In such cases, the damaged part of the willow stem increases in successive years.

Prevention and control
The best method for control of this species is to cut out and burn the infected stems and branches. Also tarring the openings (shot-holes) can prevent the emergence of the adults (Barnes, 1951). In special cases, pyrethroid insecticides may be applied to the lower parts of the willow stems.
Impact
R. saliciperda has no economic impact on the willow trees and shrubs that grow in their natural habitats around ponds, along river sides and in wetlands. It has an economic impact in willow nurseries, although it was not listed as a pest by Nef and Perrin (1999). It usually attacks young willow stems in nurseries and this is of economic importance if they are used in the manufacture of baskets and other products.
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