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.

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