Mixed diets may produce suboptimal performance in the grasshopper Melanoplus differentialis

Compensatory feeding mechanisms promoting the selection of a balanced diet in generalist insects are well understood.  In this study I presented the generalist grasshopper Melanoplus differentialis with two suitable host plants and asked if they would feed on the plant offering higher fitness or would be driven to switch among the two plants by compensatory mechanisms, even if performance suffered.  In 13 different trials using common cultivated plants, insects offered two host plants in excess fed on both species without exception.  Survival was generally higher on the mixed diet treatment than on either plant alone.  Males also had higher relative growth rates and final adult mass on mixed diet treatments with few exceptions.  However, in females the mixed diet produced lower or intermediate relative growth rates in 3 of 13, and lower or intermediate final adult mass in 5 of 13 trials.  The results suggest that nutritional regulating mechanisms may operate differently in males and females of this species and that under some conditions compensatory feeding may result in lower fitness than functional specialization, even without specific host-plant adaptations.