Species differences in reproductive timing and egg load in two fly species adapted to different host plants

Selection acts on reproductive timing and egg number to maximize reproductive success based on available resources. Species often vary in these life history traits due to differences in trait variation and local selection pressures, especially when species are adapted to different environments. Yet, species may share similar life history traits given common correlations, such as a positive relationship between body size and fecundity. In this study, we tested whether species adapted to different environments differ in their reproductive timing and egg load using two species of Tephritid fruit flies: Rhagoletis pomonella adapted to downy hawthorn and Rhagoletis suavis adapted to black walnut. In both species, older females were more likely to produce eggs and have more eggs. However, reproductive timing differed between species; hawthorn flies produced eggs earlier than walnut flies. In contrast to the common association of larger body size with earlier reproduction and higher egg counts, we did not find that adult female body size predicted reproductive timing in either species. In hawthorn flies with earlier reproductive maturity, body size did not affect egg number. For walnut flies with later reproductive maturity, larger females produced more eggs. Our findings suggest that while the time to accumulate resources and develop eggs is a common constraint, reproductive strategies are likely shaped by adaptation to specific host plants. Our study highlights the complex interaction between selection pressures and trait correlations in shaping life history traits across species.