EFFECTIVENESS OF GALL INDUCERS IN WEED BIOLOGICAL-CONTROL

Gall inducers are favoured as biocontrol agents of weeds because they tend to have a narrow host range. Six insect and one nematode gall ind ucer used in Canada are described in terms of their biology, gall morp hology, gall physiology, and effectiveness in weed control. The specie s differ in plant organ attacked, requirement for moisture, whether th e galls are induced by secretions or by severing xylem, and effectiven ess, which in part relates to the ability of the gall to import nutrie nts. The most powerful galls divert assimilates from other sinks via a gall's vascular system joined to that of their host. One of our examp les also has mechanisms to compensate for reduction of turgor during d rought. Two of the gall inducers enhance their nutrient supply by seve ring xylem in a plant nutrient sink. One, in the short-term sink of a thistle capitulum, obtains about a quarter of its assimilates at the e xpense of other capitula. The other, in the long-term sink of a rosett e root, approximately halves seed production. Hypotheses are presented to explain various aspects of gall development and function.