Mosses and the production of Chinese gallnuts

Abstract

Chinese gallnuts are galls that form on tfie-leaves of sumac trees (Rhus spp) in response to attack by aphids in the Eriosomatinae, for which mosses form obligate winter hosts. Walls of the gallnuts may comprise up to 70% tannins and yield compounds of considerable commercial value, notably tannic acid and gallic acid. Gallnut production can therefore have a major impact on the local economy in rural areas of China.

At least 14 types of gallnut have been described, each caused by a different species of aphid. A wide range of mosses act as winter hosts. Of greatest importance commercially is the true-horned gallnut which forms on R. chinensis in response to attack by Schlechtendalia chinensis, an aphid that overwinters on members of the Mniaceae, particularly Plagiomnium acutum. There are six generations of S. chinensis in each annual cycle. Reproduction is principally viviparous and parthenogenetic, except for a single sexual generation produced after spring migrants fly from mosses to the sumac trees in April and early May.

True-horned gallnuts are traditionally harvested from semi-natural woodland on rocky hillsides. Approximately 1-2% cover of P. acutum is adequate to give high production of gallnuts provided that it is uniformly distributed, and favourable distribution and abundance of the moss is achieved by artificial transplantation. Attempts are being made to produce gallnuts by growing sumac trees in agricultural fields, but success is lirnited by the difficulty of establishing Plagiomnium spp in the fields. One solution is to cultivate the moss in plastic bowls which are kept for most of the year in sheds. In October each year the bowls are placed outside under the sumac trees for several weeks and autumnmigrant aphids fly into the moss colonies. The bowls are kept in the shed during the winter, the moss mat is stripped from the bowls and placed under the trees in April, and spring-migrant aphids then leave the moss and fly to the trees. The moss regenerates in the bowls during the summer. We are investigating the growth and reproductive ecology of P. acutum as an approach towards increasing gallnut production.