The Nutritional Physiology of a Hemipteran Omnivore: The Power of Protein and Diet-Mixing

Abstract

Nutrition is an important factor in insect survival, growth, and reproduction, as it has a strong impact on all physiological processes. Insect nutrition research has consistently demonstrated that two macronutrients, protein and digestible carbohydrates, are the two most important nutrients driving overall performance, as they provide amino acids needed (for protein synthesis and structural growth) along with energy to fuel these two processes. Insect herbivores acquire protein and digestible carbohydrates from plants. Omnivores acquire these nutrients by eating both plants and other soft-bodied arthropods. Hemipterans are insects with sucking mouthparts but in general their nutritional physiology and ecology is poorly understood. In this thesis I used tarnished plant bugs (Lygus lineolaris, TPB), an economically important crop pest, as a model organism to investigate the impacts that dietary protein (p) and carbohydrate (c) content has on their survival, development, and feeding behavior. Additionally, I designed experiments to behaviorally assess TPB diet-mixing behavior.

To do this, an artificial diet was created that allowed the manipulation of its protein-to-carbohydrate (p:c) ratios so that a wide variety of diets could be manufactured that ranged from carbohydrate-biased to protein-biased. We used these diets in a series of no-choice and choice experiments to test how diet p:c content affected TPB performance as well as investigate diet mixing behavior (Chapter II). Next, a second series of no-choice and choice experiments were conducted using natural foods and a single high-protein artificial diet. Survival and growth was measured, including on mixtures of natural foods and artificial diet. A nutritional analysis of the natural foods was conducted to help inform TPB diet performance and feeding behavior (Chapter III).

The results from these experiments demonstrated that TPB performance was enhanced on protein-biased diets and decreased dramatically as the carbohydrate content of the diets exceeded 10%. It also showed that the performance was best on a 4:1 p:c ratio. For the natural foods, performance was variable, but in general TPBs did best on the treatments with higher protein content. The choice experiments demonstrated that diet-mixing, both with pairing of artificial diets and a natural food and single artificial diet, almost always improved performance compared to the best no-choice treatments. Finally, the results from the behavioral choice experiments demonstrated that TPBs prefer feeding on the protein-rich artificial diet but still generally feed on the alternate artificial diet or natural food, likely as a mechanism for regulating their nutritional intake, while diluting the intake of plant toxins. These results have strong implications for future research, as they indicate that TPBs likely rely more on omnivory in the field than previously thought, and thus will require higher protein diets than those in current use. They also indicate that diet-mixing increases performance, which may contribute to their emergence as secondary pests in many economically important agroecosystems.