The Plant and the Insects
The text in this submission was adapted from my project manuscript which was submitted as part of my Zoology Honours course.
Water hyacinth (Eichhornia crassipes) (Mart.) Solms-Laub, is an invasive South American water weed that was originally imported into South Africa more than 100 years ago because of its beauty as an ornamental plant in ponds and aquariums (Guillarmod 1979). Much like the other major invasive water weeds in South Africa, water hyacinth has spread to many water sources throughout the country and has grown to become the most invasive water weed in South Africa (Cilliers 1991). Water hyacinth forms dense mats of vegetation that interfere with aquatic ecosystems, subsequently degrading the habitat of indigenous fauna and flora resulting in environmental, recreational and agricultural losses (Hill 2003).
Water hyacinth was originally controlled using herbicides and mechanical removal but because these forms of management damage indigenous biota, are expensive and offer only short term relief, biological control has become the preferred method of controlling water hyacinth as it is more efficient and sustainable than other means of control (Cilliers 1991, Hill 2003, Coetzee et al. 2011).
My study investigated the two species of biological control agents currently reared for water hyacinth at Waainek Mass Rearing Facility (WMRC), Rhodes University, Grahamstown. These include the water hyacinth mirid (Eccritotarsus catarinensis) and the delphacid plant hopper (Megamelus scutellaris). Both E. catarinensis and M. scutellaris are host-specific South American sap feeders that feed on the chlorophyll in water hyacinth. Continued feeding leads to a reduction in plant vigor and in some cases, death (Coetzee et al. 2005, 2011, Hernandez et al. 2011).
Eccritotarsus catarinensis is a 3mm long mirid that is found on leaves of water hyacinth, where it gathers nutrients through the uptake of chlorophyll from the leaf. The feeding process facilitates the leaf’s colour change from green to yellow and even brown, making the mirid important, because it weakens the water hyacinth and affords indigenous flora the opportunity to outcompete the invasive water weed (Coetzee et al. 2005).
Megamelus scutellaris is a 3.8-4.3mm long planthopper from Argentina which has found much success in South American studies because of its ability to combat the spread of water hyacinth (Hernandez et al. 2011). The adults exhibit wing dimorphism. There is a long winged form (macropterous), which is capable of flight and a short-winged form (brachypterous), which is not capable of flight.
Both insects are being reared at the WMRC for release in the field and are thus of great import to scientists because they offer the best method of controlling the spread of water hyacinth in South Africa.
Cilliers CJ. 1991. Biological control of water hyacinth, Eichhornia crassipes (Pontederiaceae), in South Africa. Agriculture, Ecosystems and Environment 37: 207-217.
Coetzee JA, Center TD, Byrne MJ, Hill MP. 2005, Impact of the biocontrol agent Eccritotarsus catarinensis, a sap feeding mirid, on the competitive performance of water hyacinth, Eichornia crassipes. Biological Control 32: 90-96.
Coetzee JA, Hill MP, Byrne MJ, Bownes A. 2011. A review of the biological control programmes on Eichhornia crassipes (C.Mart.) Solms (Pontederiaceae), Salvinia molesta D.S.Mitch. (Salvibiaceae), Pistia stratiotes L. (Araceae), Myriophyllum aquaticum (Vell.) Verdc. (Haloragaceae) and Azolla filiculoides Lam. (Azollaceae) in South Africa. African Entomology 19: 451-468.
Guillarmod AJ. 1979. Water weeds in Southern Africa. Aquatic Botany 6: 377-391.
Hill MP. 2003. The impact and control of alien aquatic vegetation in South African aquatic ecosystems. African Journal of Aquatic Science 28: 19-24.
Hernandez MC, Brentassi MJ, Sosa AJ, Sacco J, Elsesser G. 2011. Feeding behaviour and spatial distribution of two planthoppers, Megamelus scutellaris (Delphacidae) and Taosa longula (Dictyopharidae), on water hyacinth. Biocontrol Science and Technology 21: 941-952.