The effect of nutrients and inflorescence damage by insects on fruit-set by Banksia spinulosa

We examined the role of added macronutrients (N, P, and K) and prevention of damage to inflorescences (ovule/seed predation and rachis damage) by insects in determining levels of fruit-set in Banksia spinulosa at two sites in southeastern Australia. Insecticide application eliminated damage to developing inflorescences (primarily by the lepidopteran, Arotrophora canthelias) and application of macronutrient fertilizer significantly increased nutrient concentrations in leaves at both sites. Only joint application of nutrients and insecticide increased fruit-set per infructescence significantly above that of the control, indicating that the effect of nutrients on fruit-set depends on the intensity of insect damage to inflorescences. The proportion of inflorescences that developed fruits differed between sites and higher levels of rachis damage at one site may partially explain the lower proportion of inflorescences that fruited. Neither nutrient addition or inflorescence damage influenced the position of fruit-set within an infructescence. Our results suggest that (1) macronutrient availability and inflorescence damage interact in determining levels of fruitset in B. spinulosa, and (2) other factors, such as pollen or micronutrient availability, may limit it at some sites. These results highlight site-specific variation and interaction in the factors that limit plant reproduction.