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.

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Water hyacinth (Eichornia crassipes)

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).

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Feeding damage dealt to water hyacinth by biocontrol agents

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).

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Water hyacinth mirid (Eccitotarsus catarinensis)

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.

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Delphacid planthopper (Megamelus scutellaris)

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.

References

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.

 

The frog, the plant and the insect

Unlike last year which saw me producing one project in pursuit of my zoological degree, this year is quite different. This year I am expected to produce two projects. Both fall within the field of herpetology but that’s about all they have in common. My other project which ventures to unravel the secrets of spotted skaapsteker distribution patterns’ is a genetics project. The project that I will explain now is a biological control project.

Unlike last year however, this project doesn’t detail the effect of water-stressed aquatic weeds on insects but rather the effect of painted reed frogs on water hyacinth biocontrol agents. Biological invasions are common place in the world, with many invasive species being introduced into new ecosystems both intentionally and unintentionally. Some are very bad for the local ecosystem and its animal and plant inhabitants, but some invasions confer no negative effect on the environments.

Water hyacinth and it’s spread through South African aquatic ecosystems is neither good nor neutral, one could venture to call it a parasite on the nation’s natural water resources. Water hyacinth is a menace because unlike indigenous water weeds, it has no natural enemies and thus it grows out of control, thereby chocking species of indigenous plant life, which struggle to keep up.

Water hyacinth is a very real threat to South Africa’s water bodies, and although there are many ways of combating it’s spread, very few are successful. Herbicidal control kills the water hyacinth but kills indigenous plants and animals too. Mechanical removal is painstaking and ineffective because the smallest piece of plant can re-establish somewhere else, if it is not killed completely.

These failures led scientists to look at water hyacinths’ natural enemies for answers. Biological control involves bringing in enemies from the plants natural environment and introducing the foreign enemies in our ecosystems, granted they do not pose any threat to local fauna or flora. In doing this the plant is exposed to an organism that can recognise, eat and thereby deplete its density.

In water hyacinths case, this natural enemy is not one but many, but the ones we are going to focus on is a plant hopper (M. scutellaris) and and a mirid (E. catarinensis) species from Brazil. Both biocontrol agents have been relatively successful in controlling water hyacinth. Both biocontrol agents are currently being reared at the Waainek Mass Rearing Centre at Rhodes University. The problem is, recently, painted Reed Frogs (Hyperolius marmoratus verrucosus) have been found in the mass rearing pools, the same pools that rear the insects.

The project thus aims to determine whether the frog predates upon the two insects. If the frog does in fact incorporate either of the two insects into its diet, it could affect the Waainek Mass Rearing Centres’ ability to rear and distribute the insect, and if they cannot rear the insect in high enough quantities, then water hyacinth will continue to take over South African waterways without restriction from the biocontrol agents.