Proximity to host plant of a congener determines parasitism of a waterhyacinth biological control agent by a native parasitoid

•Megamelus scutellaris is parasitized by a mymarid wasp that primarily utilizes a native delphacid.•Parasitism rate of M. scutellaris decreases farther away from the parasitoid’s primary host.•Parasitism was not density dependent suggesting it will not interfere with biological control. The biological control agent Megamelus scutellaris Berg (Hemiptera: Delphacidae) was released in Florida in 2010 to improve control of the invasive aquatic plant, Pontederia (Eichhornia) crassipes (Mart.) Solms (Commelinales: Pontederiaceae). A native egg parasitoid, Kalopolynema ema (Schauff and Grissell) (Hymenoptera: Mymaridae), was documented utilizing M. scutellaris, potentially limiting its effectiveness as a biological control agent. An experiment to determine if the distance from K. ema’s native host impacted parasitism rate of M. scutellaris was conducted in artificial ponds with naturally occurring populations of the native insect species. Models indicated that parasitism decreased with distance from the native host and with increasing M. scutellaris density. Although parasitism increased with K. ema density, rainfall, and temperature, parasitism was low with over half of the test plants indicating no signs of parasitism on M. scutellaris. The lack of a density dependent response by K. ema suggests it will not interfere with biological control of P. crassipes in Florida.