An ecological study of an attempt at biological control of Noogoora burr (Xanthium strumarium)

Abstract

The recent introduction of two cerambycids, Mecas saturnina and Nupserha vexator, for the biological control of Noogoora burr, Xanthium strumarium, a weed of sheep pastures in subtropical eastern Australia, has led to the development of a verbal model of the effect of these introductions on the populations of the weed.

The model takes into account the dependence of X. strumarium on late spring and summer rainfall, the overwinter survival and germinability of the burrs, and the effect of plant density and growth conditions oil the number of stems and burr production per plant. The fact that only one cerambycid survives in each rootstock, no matter how many stems are attacked, and the small estimated reduction in burrs per plant produced by an individual cerambycid larva (15 for N. vexator, 35 for M. saturnina), indicated that successful biocontrol depended on high cerambycid populations.

On taking into account the egg production of each cerambycid, it was concluded that, although high beetle populations could develop in dense, strongly growing X. strumarium stands in coastal and subcoastal regions, they would cause only minor reductions in plant density. They would be more effective against poorly growing stands and at sites with low overwinter burr survival, provided that stands occur consistently each year. These conclusions are dependent on the adaptability of the cerambycids to the climates of the Noogoora burr-infested region. a factor not considered.