SUMMARY
Macrodactylus pumilio Burm. (Coleoptera: Scarabeidae) and Naupactus cervinus (Boh.) (Coleoptera: Curculionidae) are considered primary pests in citrus crops in Brazil, causing damage to plants and decreasing productivity. However, few studies investigate the ecology of these insects. In this context, the use of stable isotopes analysis (SIA) emerges as an alternative technique to conventional studies of behavioral ecology because it is faster and may explain feeding behavior based on the food source for each species. Field sampling and laboratory experiments were carried out to examine the changes of carbon and nitrogen stable isotope ratios (d13C and d15N) among pests and host plants (C3 citrus and C4 grasses), providing means to examine trophic interactions. Beetles were collected at the municipality of Gavião Peixoto, São Paulo State, identified and kept at 5 °C in saturated saline solution until the SIA. Two patterns for both species were found: d13C value for N. cervinus was -23.6 ‰ and -13 ‰ for M. pumilio, indicating similarity between the results of d13C of N. cervinus and citrus plants (-26 ‰) and dependence on grasses (-12 ‰) for M. pumilio individuals. The mean d15N value was 4.3 and 5.8 ‰ for citrus plants and grass leaves, respectively, and the mean d15N value was 4.4 ‰ for N. cervinus and 4.9 ‰ for M. pumilio. The results showed a higher affinity of N. cervinus for citrus roots since the larval stage compared with the alternative diet on M. pumilio.