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A number of insect species coexist in ecosystems, with each species occupying a certain range of space and hosts to ensure its population development. In the long evolution course, species at a same trophic level have established close relations with the aim of reducing mutual negative impacts while enhancing positive interactions (such as facilitation). However, there were few studies regarding the inter-specific facilitation in insects.The brown Planthopper (BPH) Nilaparvata lugens (Stal), small brown planthopper (SBPH) Laodelphax striatellus (Fallen), and whitebacked planthopper (WBPH) Sogatella furcifera (Horvath) are important rice insect pests. In Asia, they have taken a turn since 1960s to arise as the most serious pests of rice, and particularly in the past decade both BPH and SBPH have caused serious damage to rice simultaneously at heading stage. In this study, density-related intra- and inter-specific effects of BPH and SBPH, as well as interactions of these effects with host varieties (Shanyou 63 and Xiushui 11), were studied in the laboratory to clarify the ecological mechanism of simultaneous outbreaks of the two rice planthopper species. The main results are represented as below.1. There were significant differences in the population parameters between BPH and SBPH. As compared with SBPH, BPH had significantly shorter female and male nymphal development duration, and significantly higher nymphal emergency rate, brachypterous rate and fecundity, indicating that BPH has significantly higher population increase capacity than SBPH. At the same time, there was significant intra-specific competition in both planthopper species. As initial density increased, the intra-specific competition resulted in prolonged nymphal development duration, lower emergence rate (survival rate), brachypterous rate and fecundity, and shorter female body length and longevity. These intra-competitions interacted significantly with planthopper species and host variety, with the more intensive competition occurring in N. lugens as well as on a more suitable host variety.2. When the two planthopper species coexisted, the inter-specific interaction led to shorter nymphal development duration, higher emergence rate, brachypterous rate and fecundity, and longer female body length and longevity, which indicated that there was an inter-specific facilitation. These inter-specific effects interacted significantly with planthopper species and host variety, with the facilitation being more evident on L. striatellus, as well as on a less suitable host variety.3. The comparison of main biological parameters of the BPH and SBPH feeding on healthy rice plants and rice plants previously fed by the same or another species indicated that there were also indirect intra-specific interactions between the two planthopper species. The SBPH feeding on the rice plants previously fed by BPH had shorter nymphal developmental duration, higher emergency rate, higher female longevity and fecundity as compared to those of SBPH feeding on healthy rice plants and rice plants fed by SBPH, except the female longevity on Xiushui 11. By contrast, none of these parameters changed significantly for BPH feeding on the rice plants previously fed by SBPH.4. The BPH-SBPH coexistence can also affect vertical distributions of the planthoppers on rice plants, and such an impact was related to planthopper species and host variety. When coexisting with BPH on Shanyou 63 plants, SBPH tended to move to lower parts of plants, with the percentage of individuals staying within 15 cm from the base increased from 17% to 88%, as compared to the situation without BPH-coexistence.5. Finally, the mechanism responsible for density-related intra-and inter-specific interactions in rice planthoppers, as well as evolution significances of these interactions, were discussed. Such density-related intra-and inter-specific interactions in BPH and SBPH probably have evolved as a strategy which favors population development in rice planthoppers. When the population of a single species reaches a certain density, intra-specific competition would occur to reduce population growth rate and thereby to reduce the risk of population collapsing. At the mean time, the inter-specific facilitation in rice planthoppers has also evolved as a mutual beneficial strategy, which helps to enhance population development of each of the species sharing a same host plant species. However, the mechanism involved in inter-specific facilitation of planthoppers is to be clarified in future.
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