Defoliation of alders (Alnus glutinosa) affects herbivory by leaf beetles on undamaged neighbours

The effects of defoliation of alder (Alnus glutinosa) on subsequent herbivo ry by alder leaf beetle (Agelastica alni) were studied in ten alder stands in northern Germany. At each site, one tree was manually defoliated (c. 20% of total foliage) to simulate herbivory. Subsequent damage by A. alni was assessed on ten alders at each site on six different dates from May to Sept ember 1994. After defoliation, herbivory by A. alni increased with distance from the defoliated tree. Laboratory experiments supported the field resul ts. Not only leaf damage in the field, but also the extent of leaf consumpt ion in laboratory feeding-preference tests and the number of eggs oviposite d per leaf in another laboratory test were positively correlated with dista nce from the defoliated tree. Resistance was therefore induced not only in defoliated alders, but also in their undamaged neighbours. Consequently, de foliation of alders may trigger interplant resistance transfer, and therefo re reduce herbivory in whole alder stands.