Size variation in wild Bactrocera tryoni (Diptera: Tephritidae): only a small amount is related to the host fruit

Body size is an indicator of fitness in fruit flies (Diptera: Tephritidae), with larger males and females having increased mating success and egg production, respectively. Based on laboratory studies, the larval diet and its nutritional composition are considered the most important factors influencing juvenile growth and subsequent adult body size. However, a very limited literature is less clear on the importance of larval diet as an adult size modifier of wild fruit flies. Bactrocera tryoni (Froggatt) is a polyphagous species with larvae feeding within the fruit of over 200 plant species. To test the impact of larval host on adult size in wild individuals of this species, we collected naturally infested fruit from the field and then reared out adults in the laboratory. The wing length and dry body weight of emergent flies were measured. Linear mixed models were used to analyse the difference in size depending on fruit type, larval density, sex and their interactions. Separate measurements of body size, wing length and dry body weight were significantly influenced by the larval host fruit collected from the field. However, only 7% and 24% of the variation of the wing discal–medial cell length and dry body weight, respectively, could be explained by larval host fruit type with high individual body size variation both within and between host fruit types. Fruit protein content was positively correlated, and water content negatively correlated, to wing discal–medial cell length. Unexpectedly, the parameters of wing length and dry body weight were only weakly correlated with each other. The results support the large body of laboratory-based fruit fly diet work, which identifies protein as a key component of the larval diet, but do not support a general assumption that larval host is the key driver of adult body size variation in wild frugivorous tephritids. Rather, and in agreement with a small number of laboratory-based trials, the importance of larval diet to adult fitness appears to become less important as the larval feeding environment becomes more complex.