Control Methods and Technical Key Points for the Chestnut Gall Wasp

The chestnut gall wasp Dryocosmus kuriphilus Yasumatsu belongs to the order Hymenoptera, family Cynipidae, and is also known as the chestnut tumor wasp. It is widely distributed in chestnut-growing areas, where its larvae damage the new buds of chestnut trees. When chestnut buds sprout in spring, the damaged buds fail to sprout and gradually swell to form galls, sometimes with deformed small leaves growing on the galls. In severe cases, the tree vigor weakens and branches or the whole tree may die.

Morphology: adult body length 2.5-3 mm; head and abdomen dark brown; thorax swollen, jet black and glossy; wings transparent, wing veins black; legs yellow, hind legs relatively well developed. Larva body length about 2.5 mm, milky white, spindle-shaped, stout, legless.

Occurrence pattern: 1 generation per year. It overwinters as first-instar larvae within the host buds. When chestnut trees sprout in spring, small galls appear on the twigs and leaves of new shoots, and in late May the larvae mature and pupate inside the galls. In early June adults emerge, bite through the galls, go out to activity and lay eggs; after the eggs hatch, the larvae damage the buds for a period and form oval chambers at the damaged sites, then begin overwintering in late September. In March of the following year the galls swell. The natural enemies of the chestnut gall wasp are mainly parasitic wasps, with many species, chiefly the Chinese long-tailed chalcid wasp.

Control methods: (1) In winter, remove branches with galls during pruning, improve ventilation and light penetration within the canopy, and at the same time strengthen cultivation management to enhance tree vigor and promote new shoot growth. In severely damaged chestnut groves, heavy pruning may be adopted, cutting off all parts except the positions at the base of branches where dormant buds are borne; fruit bearing can recover after 1 year. (2) Chemical control. In severely damaged chestnut groves, in early June when adults are at peak occurrence, spray the canopy with 50% fenitrothion emulsifiable concentrate, 40% dimethoate emulsifiable concentrate at 1500 times dilution, or 80% dichlorvos emulsifiable concentrate at 2000 times dilution, which can kill large numbers of adults. In addition, in chestnut groves with dense canopies, dichlorvos smoke agents may also be used for fumigation killing during the peak adult occurrence period.

Appendix: There is another species, the chestnut ball gall wasp (Diplolepis japonica Ash.), which forms galls on young twigs; its control methods are the same as those for the chestnut gall wasp.