Drosophila suzukii (Diptera: Drosophilidae), which is also called spotted-wing drosophila (SWD), is originally from Asia, and has become a severe invasive pest in South America and Europe since 2008 [1, 2], causing huge economic losses in the fruit production industry . Usually, Drosophila suzukii lay eggs within ripe and ripening fruit by using a sclerotized and serrated ovipositor that can penetrate and lead to physical damage of fruit skins [4, 5]. According to the literature, D. suzukii can lay up to 600 eggs (around 400 eggs on average) during their life span, and the minimum development time from oviposition to adult under optimal temperature is 8 days . As for environmental adaptability, D. suzukii has a wide range of tolerance to climatic conditions, with a reproduction temperature from 10 °C up to 32 °C . In addition, the activity and development temperature range are usually between 20 to 25 °C . This pest also has wide host range, and up to now, there have been more than 150 species reported as hosts for D. suzukii [9, 10]. Due to this, it is almost impossible to find an effective way to eliminate this pest in a short time, for different hosts have non-identical ripening times, and wild hosts as well as ornamental plants can serve as refuges, so that D. suzukii can persist through survival challenges such as the low temperature in winter .
Currently, Drosophila suzukii has successfully invaded many areas and countries. The use of ovipositors can cause physical damage to intact fruit, and the larvae can speed up the process of decay, which provides an ideal food source to other phytophagous insects, so competition is almost inevitable with other Drosophila species, such as Drosophila melanogaster. Usually, competition includes intraspecific competition and interspecific competition. The outcome of competition depends on the relative strength of interspecific competition and intraspecific competition. Both types of competition are a struggle for survival for every individual because of limited environmental resources . To plant-eating insects, the importance of interspecific competition to community structure has been discussed by many ecologists [13, 14]. Interspecific competition widely exists among insects [15, 16]. The research by Denno analyzed 193 pairs of competitor species, and concluded that the interspecific competition affects the abundance and distribution of plant-eating insects . The interspecific competition between two species is an interspecific interaction that could lead one species population to be reduced as the other species increases, which has great effect on species coexistence, habitat partitioning, food resource partitioning, and species replacement .
According to the niche theory, closely related species overlap in resource needs, so the competition can be very strong when two species first meet . A previous study concluded that, in a lab environment, the presence of D. melanogaster can significantly reduce emergence and egg laying of D. suzukii through interspecific competition . This is because most male Drosophila can produce a pheromone called cis-vaccenyl acetate (cVA) that used in courtship, aggression, and aggregation signaling, which has a disruptive effect on D. suzukii . The number of successful matings is reduced when male D. suzukii encounter this pheromone. Similarly, in previous work, we found that when adults of D. melanogaster and D. suzukii were reared together for three generations, the whole population of D. suzukii would eventually die. Thus, in this study, we attempted to explore more alternative potential factors.
The current study shows that both D. suzukii and D. melanogaster have strong appetites for rotten fruit, and have identical feeding niches. For acquiring resource, two species might show competition, which might cost time, energy and increase risk for injury. When interspecific competition of two flies occurs, more time and energy might be used for competition, resulting in decreased in egg laying, and individuals might suffer physical damage, affecting the longevity of adults. Decreases in both egg laying and adult longevity could be important factors leading to the whole population of D. suzukii would be finally dying. Therefore, in our experiments, we investigated the effect of interspecific competition on the longevity and fecundity of the two flies. In addition to adult competition, larval-larval competition between D. melanogaster and D. suzukii was also studied. Besides, that, reproductive interference is also a kind of interspecific sexual interaction that the female reproductive success of one species is reduced because of the interference by another species . The frequency of conspecific courtship and mating, fecundity of females, and fertility of eggs can be influenced by reproductive interference, and time, energy, nutrients, or gametes can be wasted in heterospecific sexual interactions, causing fitness loss for the individuals involved. Reproductive interference might be a potential factor resulting in the population reduction of D. suzukii, and would be examined in this study.