The consequences of alternating diet on performance and food preferences of a specialist leaf beetle.

The food quality of a given host plant tissue will influence the performance and may also affect the preference behavior of herbivorous animals. As nutrient contents and defense metabolite concentrations can vary significantly between different parts of a plant and change over time, herbivores are potentially confronted with diet differing in quality even when feeding on a single plant individual. Here we investigated to what extent feeding exclusively either on young or old, mature leaves of Brassica rapa or on a mixed diet of young and old leaves offered in alternating order affects the larval performance, food consumption, and the host preference behavior of adult mustard leaf beetles, Phaedon cochleariae. Analyzing different leaf quality traits, we found lower water contents, no changes in C:N ratio but more than threefold higher glucosinolate concentrations in young compared to old leaves. Individuals reared on mixed diet performed as well as animals reared on young leaves. Thus, compared to animals feeding exclusively on highly nutritious young leaves, diet-mixing individuals may balance the lower nutrient intake by a dilution of adverse secondary metabolites. Alternatively, they may integrate over the variation in their food, using a previously assimilated resource for growth at times of scarcity. Animals reared on old leaves grew less and had a prolonged larval developmental time, although they showed increased consumption indicating compensatory feeding. Additionally, we found that experience with a certain diet affected the preference behavior. Whereas individuals reared exclusively on young leaves preferred young over old leaves for feeding and oviposition, we did not find any preferences by animals reared exclusively on old leaves or by females reared on alternating diet. Thus, in contrast to positive feedbacks for animals reared on young leaves, an integrative growth of diet-mixing individuals potentially leads to a lack of feedback during development. Taken together, our results suggest that different diet regimes can lead to comparable performance of mustard leaf beetles but experienced feedbacks may differ and thus convey distinct diet preferences.