Assessing larval food quality for phytophagous insects: are the facts as simple as they appear?

1. We argue here that host plant quality affects many life-history traits of herbivorous insects and these traits often interact. Studies that look only at a limited number of traits often fail to determine the overall effect of plant quality on larval performance and adult fitness. Parameters such as mating success and adult longevity are frequently neglected even though they are affected by larval feeding and are crucial to overall fitness. 2. To illustrate this, we examined a whole suite of life-history traits of the moth Lobesia botrana after rearing larvae of this grape pest on three different grape cultivars. 3. Development time, mating success, fecundity, egg size and fertility were differentially affected by the cultivar on which the larvae were reared. Our results highlight that decreased fecundity can be balanced by an increase in egg size and fertility. 4. Throughout this study, we emphasize that drawing conclusions about larval food quality can be spurious when based on too few life-history traits. Therefore, it is essential to study all fitness related life-history traits to fully understand the effects of larval food quality on herbivore fitness.