**Background:** Two invasive drosophilid fruit flies, Drosophila suzukii (spotted-wing drosophila) and Zaprionus indianus (African fig fly), co-occur in the Neotropics including Brazil. D. suzukii has a serrated ovipositor allowing it to infest healthy fruits, while Z. indianus is a secondary pest requiring damaged fruits. Understanding their competitive interactions is important for pest management, as their co-occurrence may influence population dynamics and damage potential.
**Methods:** Field collections of overripe strawberry fruits were conducted in Atibaia, São Paulo, Brazil in October and December 2020 (130 and 150 fruits respectively) to assess co-occurrence. In the laboratory, eight egg densities (8, 10, 20, 40, 60, 100, 200, or 400 eggs per cup; equivalent to 0.55–27.59 eggs/g of diet) were tested under both intraspecific (single species) and interspecific (both species together) conditions. Egg-pupa viability, adult emergence, development time, and fecundity were measured. Leslie matrix projections over 10 time-steps were performed for low (8 eggs), medium (60 eggs), and high (400 eggs) densities. Two-choice oviposition bioassays tested whether D. suzukii females preferred diets previously infested with conspecific or heterospecific eggs at densities of 1, 3, 7, or 15 eggs per diet.
**Key Results:** Field collections showed Z. indianus was overwhelmingly dominant: in the first collection (836 adults), 82.77% were Z. indianus, 1.43% D. suzukii, and 15.78% other Drosophila species. In the second collection (948 adults), 83.33% were Z. indianus, 2.74% D. suzukii, and 13.92% other species. Under intraspecific competition, both species showed density-dependent decreases in egg-pupa survival and adult emergence. D. suzukii consistently showed higher survival than Z. indianus across all densities. Under interspecific competition, D. suzukii showed linear density-dependent decreases, while Z. indianus showed a bell-shaped curve with optimal performance at densities of 40–60 eggs. Fecundity under intraspecific competition decreased with density for both species. However, under interspecific competition, Z. indianus fecundity was significantly higher than D. suzukii at all densities tested (approximately 40 eggs/female for Z. indianus vs. 10 eggs/female for D. suzukii at peak). Development time increased with density for both species; D. suzukii showed similar development times in intra- and interspecific conditions (approximately 10–14 days), while Z. indianus had longer development under interspecific competition (13.63–20.51 days) compared to intraspecific (13.46–17.34 days). Leslie matrix projections showed D. suzukii had increasing oscillations at low and medium densities and decreasing oscillations at high densities under both competition types. Z. indianus showed cyclic oscillations at medium intraspecific density but decreasing oscillations under interspecific competition at low density. In two-choice bioassays, D. suzukii females showed no significant preference between egg-infested and control diets for either conspecific or heterospecific eggs at any density tested (all P > 0.05).
**Clinical Implications:** This study demonstrates that co-occurrence with Z. indianus does not negatively affect D. suzukii survival or fecundity, while Z. indianus actually benefits from the presence of D. suzukii, producing more offspring. This asymmetric relationship helps explain the high field abundance of Z. indianus relative to D. suzukii. For pest management, these findings suggest that control tactics targeting D. suzukii should account for the fact that Z. indianus may exploit D. suzukii oviposition sites, potentially maintaining pest pressure even when D. suzukii is controlled. The neutral effect of Z. indianus on D. suzukii oviposition behavior means that management strategies cannot rely on competitive displacement. Understanding these competitive dynamics is essential for developing integrated pest management approaches in regions where both species co-occur, particularly in soft fruit production systems.