Plant Quality and Grasshopper Feeding: Effects of Sunflower Condition on Preference and Performance in Melanoplus Differentialis

Abstract

The feeding of the grasshopper Melanoplus differentials was observed in fields of wild and cultivated sunflower, Helianthus annuus, in central Texas. Observation of 227 feeding grasshoppers revealed a strong bias toward wilted, damaged, diseased, or senescent leaf tissue or inflorescences over turgid, undamaged leaf tissue. Only 1% of the grasshoppers fed on undamaged leaf tissue, even though it was more abundant than other tissue types. The preference for girdled, wilted leaves was studied in laboratory experiments using excised leaves to simulate the leaves preferred in the field. In binary choice tests, grasshoppers of all instars and both sexes of adults preferred wilted leaves to turgid ones. In long—term growth experiments, grasshoppers offered a mixed diet of wilted and turgid leaves from hatching through oviposition had increased survival, growth rate, fecundity, and egg mass as compared with their turgid—leaf diet sibs. The growth difference appears in the first instar and is due to both increased consumption and increased efficiency of conversion of ingested food by mixed—diet grasshoppers. Excess water, while not responsible for the preference, may in part be responsible for the growth effect. Other chemical differences between the leaves may also have a role. These results and similar observations on other plants and grasshoppers suggest a strong selectivity by these generalists and may have implications for their population dynamics. Increased performance on stressed plant tissue is not found under all conditions nor in all herbivores, and possible reasons for this are discussed.