Invasive plants are one of the strongest drivers of species extinctions. Biological control offers a sustainable and safe means of long-term population reduction of their target. Herbivores introduced for the control of invasive plants interact with the native community in addition to the top-down control they exert on their host plant. Frequently this results in their reduced efficacy, but they may also interact in non-direct pathways such as apparent competition between shared parasitoids. Waterhyacinth (Eichhornia crassipes) is perhaps the world’s most invasive aquatic plant. Large-scale invasions impede travel by boats and aquatic vertebrates, and cause stagnant water suitable for Aedes mosquitoes. Megamelus scutellaris (Hemiptera: Delphacidae), a planthopper, was introduced in 2010 to Florida from Argentina for the control of waterhyacinth. This species can overlap in certain aquatic habitats with a native congener, M. davisi that feeds on Nuphar lutea. Both planthoppers are attacked by a stenophagous native egg parasitoid, the fairy wasp Kalopolynema ema (Hymenoptera: Mymaridae). To gauge whether M. scutellaris can provide a subsidy to K. ema that might produce apparent competition with M. davisi, we set up 1m x 1m plots of E. crassipes infested with M. scutellaris within ponds containing N. lutea, M. davisi and K. ema. We then measured parasitism of M. davisi on N. lutea and the abundance of K. ema at 1m, 5m, 10m, and 20m from E. crassipes for 11 consecutive months.
We found no significant relationship between the distance from E. crassipes and associated M. scutellaris and the incidence of parasitism in M. davisi. Although K. ema utilized M. scutellaris during this study, our data indicated that biotic resistance from K. ema is reducing M. scutellaris populations, rather than M. scutellaris providing a subsidy capable of causing apparent competition with M. davisi via a shared parasitoid.