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Beat Forster
Swiss Federal Research Institute WSL
Swiss Forest Protection
Zürcherstrasse 111
CH - 8903 Birmensdorf
Phone: +41 44 739 23 67
Fax: +41 44 739 22 15
Beat Wermelinger
Swiss Federal Research Institute WSL
Forest Entomology
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CH - 8903 Birmensdorf
Phone: +41 44 739 22 58
Fax: +41 44 739 22 15
Article
| Author(s): | Beat Forster, Roland Engesser, Franz Meier |
| Editorial office: | WSL, Switzerland |
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The oriental chestnut gall wasp has reached the Ticino
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Fig. 1 - Last year’s and new galls on a
chestnut tree branch. Photo: Swiss Forest Protection |
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Fig. 2 - A detailed view of the galls. Photo: Swiss Forest Protection |
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Fig. 3 - A cross section of a gall showing white larvae (arrow) Photo: Swiss Forest Protection |
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Fig. 4 - Female gall wasps Photo: J.A. Payne, www.forestryimages.com |
In May 2009 the oriental chestnut gall wasp (Dryocosmus kuriphilus), which
originates from China,
was discovered in the Canton Ticino. This is the first time this invasive species has been observed in Switzerland.
After being informed about the unusual infestation by a private tree owner the forestry service of Ticino and the cantonal plant protection service realized, as the shoots began to appear in May 2009, that the oriental chestnut gall wasp, which is considered to be very invasive, was already established in many areas of Mendrisio and Lugano. However, the worst known case, up to now, is the one in Mendrisio. It is not known if the gall wasp flew into southern Ticino from Lombardy or whether it was brought in, from Italy, amongst plant material.
Affected European chestnut trees have pronounced gall occurrence on shoots, leaves and blossoms. Due to the biological development of the insect (see box) and the intensity of the infestation it could be deduced that the inspected 15 m high tree had become infested in 2007. A strong infestation reduces the formation of fruit and shoots and after several years leads to a crippling of the tree. Larger chestnut trees however do not die but such an infestation can lead to a 75% reduction in chestnut production.
Plant trade encourages spread
The oriental chestnut gall wasp, which originated in China, has now spread world wide:
- 1941 Japan
- 1963 Korea
- 1974 USA
- 1999 Nepal
- 2002 Italy
- 2005 France
- 2005 Slovenia
It is expected that it will soon spread to the north of the Alps. This could occur by infected nursery plants or propagations being transported illegally or through the flight of the females. Initially infestations in a region only spread a few kilometres per year. Later with the aid of the wind they can spread widely.
Control measures
In Europe very little is known about the effectiveness of any control measures. Neither mechanical nor chemical measures have been able to dam the spread of infestations as the wasp larvae are well protected from insecticide inside the gall. In nurseries, and at other sites of infestation, the only possibility available is to cut out and destroy the galls during the initial stages of infestation. It is important that contaminated plant material, such as young plants, be contained.
In Japan biological measures using a Chinese parasitic wasp have been successful. These wasps have now been introduced into Italy, where they have become established. However, at present, too little is known about the effects on European animal and plant species from using this parasitic wasp. How chestnut production will be affected by these measures will only be evident when more knowledge has been collected about chestnut trees infested by the oriental gall wasp.
Report your observations to the plant protection services
The oriental chestnut gall wasp is classified as an A2 quarantine organism by the EPPO (European and Mediterranean Plant Protection Organisation). A new infestation must be reported to the cantonal plant protection service. No chestnut plants may be taken from infestation areas and or moved within the infestation area itself. This massively restricts not only fruit production, but the grafting and planting of young chestnut trees.
Biology of the oriental chestnut gall wasp
Only sweet chestnut trees (Castanea sp.) act as host plants for the oriental gall wasp. Between June and August the 2.5 to 3 mm gall wasp female (fig. 4) lays up to 30 eggs in newly formed buds. An egg is only 0.1 mm in size. The tiny white larvae hatch in late summer or autumn and over-winter in the buds. At this point in time there are no visible symptoms. The larvae become active during budding the following year. The tree produces 0.5 to 2.5 cm smooth walled, multiple chambered galls, whose colouring ranges from light green to pink (fig. 1 to 3), on infested young shoots, inflorescences and leaves. After a few weeks of feeding pupation (dark brown pupae) takes place inside the gall. The new generation of wasps swarm out in that same summer. The plant’s organs, which have been attacked by the over-wintering larvae, do not fully develop due to deformation caused by the larvae. Individual shoots can die off. Galls are often to be found in the crowns of large trees.
Translation: Dawn Meister (Affoltern a. A.)
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