A latitudinal gradient in herbivore resistance in common sunflower, Helianthus annuus (Asteraceae)

Abstract

Background and aims – The intensity of herbivory is expected to decline with increasing latitude. As herbivory varies spatially and over time, a reliable method of assessing the intensity of herbivory is to examine the degree of herbivore resistance in the plant community. Latitudinal gradients in resistance to herbivory were examined in wild populations of common sunflower, Helianthus annuus. Materials and methods – Seeds from 23 different latitudes, ranging from 20 to 44°N, were obtained from the USDA’s Germplasm Resources Information Network. Plants were grown in a greenhouse for nine weeks. At that time, the size (height, leaf length, number of leaves) and resistance of each plant to herbivory (determined through a bioassay using a generalist herbivore, Helicoverpa zea was assessed. Key results – Resistance to herbivory decreased significantly with latitude, while plant size, as indicated by height, was positively correlated with latitude and negatively correlated with both temperature and resistance to herbivory.Conclusion – Populations from lower latitudes exhibited elevated resistance to herbivory and slower growth, suggesting first, that herbivory is more intense at lower latitudes and second, that there is a tradeoff between growth and defense.