The importance of understanding host plant phenology in the biological control of Acacia longifolia*

Abstract

SUMMARYIn order to assess the effect of an introduced gall wasp on the weed Acacia longifolia, it was necessary to study the vegetative growth and reproductive potential of uninfested trees. The growth of branches on uninfested trees was studied over one year. The results yielded a growth phenology which is notably different from the growth phenology of A. longifolia described by Milton (1980). Milton (1980) observed no flushing, but a gradual increase and decline in growth during spring and late summer, respectively. The present study showed distinct flushing, and a marked lack of growth during spring. Examination of the effect of reproduction on the vegetative growth of A. longifolia explains the disparity. A. longifolia reproduces during spring. Comparison of phyllode addition during spring with pod production for three successive years at one site, and for two years at another, shows that phyllode addition is low during years of high pod production, and vice versa. Between trees, phyllode addition is also inversely related to pod production within a reproductive season. Because the present growth phenology was done in a year of high pod production, growth during spring was negligible, in contrast with Milton's (1980) phenology which was done in a year of low pod production and high phyllode addition. Depending on the year in which the plant was studied, its reproductive potential and vegetative growth could have been under‐ or overestimated by as much as 62% and 90%, respectively. This study shows that the phenology of weeds with an annual cycle needs to be examined for at least two years in order not to under‐ or overestimate their reproductive or growth potential. This is especially important when assessing the efficacy of biological control agents.