Discriminating tastes: self-selection of macronutrients in two populations of grasshoppers

Authors: Fielding, Dennis; Defoliart, Linda

Submitted to: Physiological Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/5/2008
Publication Date: 9/20/2008
Citation: Fielding, D.J., Defoliart, L.S. 2008. Discriminating tastes: self-selection of macronutrients in two populations of grasshoppers. Physiological Entomology. 33:264-273.

Interpretive Summary: Increased carbon dioxide in the atmosphere from the burning of fossil fuels is expected to result in lower levels of protein, and higher levels of carbohydrates, in leaves of plants. To gain an understanding of how such changes in plants might affect insect pests, scientists at the Subarctic Agricultural Research Unit in Fairbanks, Alaska, conducted experiments to determine the optimal balance of protein and carbohydrates in the diets of grasshoppers from Alaska and from Idaho. When given a choice, the Alaskan grasshoppers consistently selected a diet consisting of about 9 parts protein to 10 parts carbohydrate. The grasshoppers from Idaho were much less selective. When restricted to diets with excess carbohydrate or excess protein, the Alaskan grasshoppers seemed to be more sensitive to diet quality, in terms of reduced growth rates, than were the grasshoppers from Idaho. We assume that the short growing season in Alaska makes it imperative for the grasshoppers to optimize their growth rate and so they more carefully select food with a protein:carbohydrate ratio that provides for maximum growth. Although it has been generally assumed that lower protein levels in plants due to increased carbon dioxide will be detrimental to insects, the results of this study indicate that in some cases the changes in protein:carbohydrate balance will be favorable to insect pests, but also that insect responses may vary regionally.

Technical Abstract: The capacity to self-select an optimal balance of macronutrients (protein and carbohydrate) was studied in two populations of Melanoplus sanguinipes F. (Orthoptera: Acrididae). One population was derived from subarctic Alaska, and the other from temperate Idaho, USA. During the 4th and 5th stadia, Alaskan grasshoppers consistently self-selected a diet centered on a 0.89 ratio of protein: carbohydrate, whereas protein and carbohydrate intake by the Idaho grasshoppers was contingent on the particular diet choices presented to them. When restricted to unbalanced diets, the Alaskan grasshoppers developed more rapidly than the Idaho grasshoppers regardless of diet composition. Weight gain was unaffected by diet in the Idaho population, whereas the Alaskan grasshoppers weighed less on diets with excesses of either protein or carbohydrate. Growth rates were dependent on diet in both populations, but the Alaskan grasshoppers showed a stronger response to macronutrient balance than did the grasshoppers from Idaho. Results suggest that demographic responses of insects to expected changes in plant chemistry due to elevated atmospheric levels of CO2, decreased protein and increased carbohydrate, will differ among populations.