Nutrient supply and accessibility in plants: Effect of protein and carbohydrates on Australian plague locust (Chortoicetes terminifera) preference and performance

In contrast to predictions from nitrogen limitation theory, recent studies
have shown that herbivorous migratory insects tend to be carbohydrate (not
protein) limited, likely due to increased energy demands, leading them to
preferentially feed on high carbohydrate plants. However, additional
factors such as mechanical and chemical defenses can also influence host
plant choice and nutrient accessibility. In this study, we investigated
the effects of plant protein and carbohydrate availability on plant
selection and performance for a migratory generalist herbivore, the
Australian plague locust, Chortoicetes terminifera. We manipulated the
protein and carbohydrate content of seedling wheat (Triticum aestivum L.)
by two means: 1) we increased the protein:carbohydrate ratio using
nitrogen (N) fertilizer, 2) we sought to increase carbohydrate
accessibility by grinding cell walls after drying the plants. Using a full
factorial design, we ran both choice and no-choice experiments to measure
preference and performance. We confirmed locust preference for plants with
a lower protein-carbohydrate ratio (unfertilized plants). Unlike previous
studies with mature wild grass species, we found that intact plants
supported better performance than dried and ground plants, suggesting that
cell wall removal may only improve performance for tougher or more
carbohydrate-rich plants. These results add to the growing body of
evidence suggesting that several migratory herbivorous species perform
better on plants with a lower protein:carbohydrate ratio.