Water hyacinth first invaded Africa’s Lake Victoria in 1989, quickly spreading and forming thick mats around the shoreline. In an attempt to control it, a weevil, Neochetina spp. was introduced in 1995. In 1997 and 1998 the water hyacinth began to decrease drastically, seeming to clearly indicate a success for the weevil. But around this same time, El Nino weather patterns occurred over the Pacific Ocean with repercussions that could be felt over Lake Victoria. From these facts has sprung a debate over whether the decline of the water hyacinth was due to the weevils or the El Nino-related weather.

The main point of disagreement between the two opinions about water hyacinth’s decline is the meaning of the time between the introduction of the Neochetina spp. and the noticeable decline in water hyacinth. Wilson et al. (2007) states that although the El Nino-associated weather, namely increased wind and wave patterns on the lake and decreased light availability, added further stress for the water hyacinth to deal with, they were just a mercy stroke for already doomed plants that were fatally damaged by the weevil. They believe that the time difference was a normal occurrence for a biocontrol agent, no more than a “lag time” while it became established and that it was coincidence that it really began to take effect at the same time as the El Nino event. Williams et al. (2007) on the other hand assert that although the weevils did their part, the decline of water hyacinth on Lake Victoria would not have occurred without the timely El Nino event. They say that while the weevil did some damage to the water hyacinth, it would not have declined as it did without the El Nino weather patterns.

Both articles present valid arguments and it seems clear that the both the weevils and the El Nino weather had a part in the decline of the water hyacinth. In light of the new satellite images from NASA’s Earth Observatory, depicting a resurgence of water hyacinth, it is clear that the biocontrol was a short term solution. Williams et al. provides the best explanation for this resurgence of water hyacinth: “unstable host populations may well lead to unstable controlling herbivore populations,” that is, once the water hyacinth had declined by a significant amount, the weevils lost their food source and their population dropped too low to effectively control the invasive species.

References:
NASA Earth Observatory. 2007. Water Hyacinth Re-invades Lake Victoria. http://earthobservatory.nasa.gov/IOTD/view.php?id=7426. Viewed 11 Sept 2010.

Williams, A. E., R. E. Hecky, and H. C. Duthie. 2007. Water hyacinth decline across Lake Victoria- Was it caused by climatic perturbation or biologial control? A Reply. Aquatic Botany 87: 94-96.

Wilson, J. R. U., O. Anjuonu, T. D. Center, M. P. Hill, M. H. Hill, M. H. Julien, F. F. Katagira, P. Neuenschwander, S. W. Njoka, J. Ogwang, R. H. Reeder, and T. Van. 2007. The decline of water hyacinth on Lake Victoria was due to biological control by Neochetina spp. Aquatic Botany 87: 90-93.