A native parasitoid, Kalopolynema ema (Schauff and Grissell) (Hymenoptera: Mymaridae), has begun utilizing a new host, Megamelus scutellaris Berg (Hemiptera: Delphacidae), the introduced biological control agent for waterhyacinth, Eichhornia crassipes (Mart.) Solms (Commelinales: Pontederiaceae). Parasitism rates varied between outdoor M. scutellaris rearing tanks and sentinel plants placed in the field, with parasitism rates in the tanks being higher.
Waterhyacinth, Eichhornia crassipes (Mart.) Solms (Commelinales: Pontederiaceae), is a floating aquatic plant that originated in lowland tropical South America, probably in the Amazon basin (Barrett & Forno 1982). It was first introduced into the U.S. in the late 1800s (Gopal 1987). Problems caused by E. crassipes in its adventive range are well documented (Center 1994) and result from its rapid growth, clonal propagation, and ability to re-infest via the seed bank or from plant fragments. Infestations deleteriously affect water traffic, water quality, infrastructure for pumping and hydroelectric operations, water use, and biodiversity (Schmitz et al. 1993). Additional problems include property damage during floods, water loss due to evapotranspiration, and increases in populations of vectors of human and animal diseases (Mack & Smith 2011).
Megamelus scutellaris Berg (Hemiptera: Delphacidae) was first released into Florida as a biological control agent for E. crassipes in 2010 (Tipping et al. 2014). This species is now considered to be established with more than 700,000 insects released in 15 counties in Florida (unpublished data). Five native congeners of M. scutellaris occur in Florida: M. davisi VanDuzee, M. palaetus (VanDuzee), M. gracilis Beamer, M. trifidus Beamer, and M. lobatus Beamer (Beamer 1955; Bartlett 2014). Megamelus davisi feeds on yellow pond lily (Nuphar advena [Aiton]; Nymphaeaceae), and its eggs are parasitized by Kalopolynema ema (Schauff and Grissell) (Hymenoptera: Mymaridae) (Fig. 1) (Wilson & McPherson 1979, 1981; Triapitsyn & Berezovskiy 2002). Triapitsyn & Berezovskiy (2002) noted that it is highly likely that K. ema can parasitize the eggs of other Megamelus species.
Nuphar advena is a common aquatic plant in southern Florida and has been planted widely at the University of Florida Ft. Lauderdale research station where the United States Department of Agriculture (USDA), Agricultural Research Service (ARS), Invasive Plant Research Laboratory (IPRL) is co-located. Megamelus davisi is one of several common herbivores found on N. advena at the station, and K. ema has been collected repeatedly from N. advena leaves. In 2014, K. ema was observed on E. crassipes plants that were growing in 10 × 20 m outdoor tanks at IPRL used to mass-rear M. scutellaris. Preliminary identification of K. ema was determined by C. R. Minteer using the keys in Triapitsyn & Berezovskiy (2002) and later confirmed by S.V. Triapitsyn. In order to first confirm and then estimate parasitism by K. ema, individual E. crassipes plants were collected without bias from the outdoor rearing tanks, soaked in a 2% solution of Safer® insecticidal soap to kill any surface insects, rinsed with fresh water, and placed individually in aquaria (28 × 31 × 40 cm). These aquaria were covered with fine mesh screens, placed in environmental chambers set at 27 °C and a 12:12 h L:D photoperiod, and monitored daily for the emergence of M. scutellaris and K. ema. Individuals of both M. scutellaris and K. ema were tallied and removed to prevent activities by either species that would alter parasitism estimates that were calculated by dividing the number of K. ema (K) individuals that emerged from a plant by the sum of the total number of M. scutellaris (M) and K. ema individuals that emerged from the plant.
% parasitism = K/(K + M)
Samples were collected from the outdoor tanks several times a month from Jan to Jun 2015 and from Oct 2015 to Jan 2016. Megamelus scutellaris and K. ema were the only 2 species observed emerging from the tissues of the E. crassipes plants in the aquaria, although we did observe midges, springtails, and spider mites on occasion. Parasitism rates of M. scutellaris by K. ema ranged between 29 and 78% and varied widely by month with no obvious pattern.
Parasitism rates were also investigated in the field by using sentinel plants of E. crassipes. Twenty M. scutellaris adults (1:1 sex ratio was assumed) were placed in aquaria with a single E. crassipes plant (sentinel plant) per aquarium in the laboratory for 2 d then removed. Sentinel plants were placed into 1 of 2 field sites for 2 d, returned to the laboratory and processed and monitored as before. Plants in the field were supported by a circular (diameter about 18 cm) Styrofoam float to support and mark the plant. The float was attached to a N. advena plant by using a twist tie to hold it in place. One field site consisted of small ponds (about 700 m2) at the University of Florida Ft. Lauderdale research station with populations of N. advena and cattail (Typha spp.; Typhaceae). The other field site was a canal in a South Florida Water Management District Storm Water Treatment Area with N. advena, as well as a population of E. crassipes within 40 m. Sentinel plants were deployed in field sites several times monthly from May 2015 through Jan 2016. Parasitism rates at the field sites ranged from 0 to 26% and were much less variable among months than the parasitism rates seen in outdoor rearing tanks.
Parasitism rates in the outdoor rearing tanks were much higher than those seen from sentinel plants placed in the field, which may be an artifact of the artificially high densities of E. crassipes and M. scutellaris present in the tanks. Although more research is necessary to investigate these differences, it is clear that K. ema can utilize M. scutellaris as a host in tank and field settings.
Thanks to Serguei Triapitsyn for confirming our original ID of K. ema and to Rudy Scheffrahn for developing the method we used to get clear pictures of the parasitoids. Partial funding was provided by the Florida Fish and Wildlife Conservation Commission.