SW2: Where There’s A Weevil, There’s A Way
By: Evan Schwartz
By the late 1990’s, Water hyacinth became the most problematic invasive aquatic species in Africa’s Lake Victoria. As it spread, the plant began to clog waterways, impede fishers, destroy native flora and fauna, and turn the lake into a cesspool of stagnant water. However, after reaching unprecedented levels in 1998-2000 (Wilson 2007), the hyacinth population began to decrease rapidly. Two major explanations appear for this decrease.
Wilson et al. (2007) present the argument centering on the importance of Neochetina spp, or weevils. The weevil is a natural predator of the water hyacinth, and feeds primarily on the large, green leaves. The weevil then deposits its larvae inside the stalk of the plant, which they hollow out during their development. By attacking the plant from all angles, the weevil is clearly able to reduce the population of the hyacinth while increasing its own numbers for further control. Wilson et al. acknowledge that the El Nino conditions also play a role in the reduction of the hyacinth; however, this only served to weaken and redistribute the plants across the lake (Wilson et al. 2007).
The Williams et al. (2007) reply to the Wilson article centers on the importance of the El Nino weather system. They assume the position that while the weevil population clearly reduced the amount of water hyacinth in the lake, the weather system reduced the amount of available light to the area, significantly weakening the invasive plants (Williams et al. 2007). They also assume that a severe reduction in hyacinth severely weakens the stability of the weevil population, causing an unnatural flux in weevils, as well as an opportunity for the plants to recover (Williams et al. 2007).
After considering both arguments, I am under the impression that weevils are the major contributor to the reduction of the water hyacinth population. The stormy conditions caused by El Nino would attack anything within the region, and weaken both populations. Weather conditions can decimate populations for a period of time, but eventually the pre-disaster species will assume original levels. It is clear that the weevils are the major factor in hyacinth control, as they decimate the plant from all points of attack: the roots, leaves, stems, etc. After considering satellite imagery (NASA 2007), the weevil populations need to be regulated in high levels in order for the hyacinth to be kept in check.
NASA Earth Observatory. 2007. Water Hyacinth Re-invades Lake Victoria. http://earthobservatory.nasa.gov/IOTD/view.php?id=7426. Viewed 20 Jan 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 biological control? A reply. Aquatic Botany 87:94-96.
Wilson, J. R. U., O. Ajuonu, T. D. Center, M. P. 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.