The use of bio-control has always been a controversial topic. While it is more natural and eco-friendly compared to other methods of invasive species control, it has sometimes created unforeseen negative impacts, and its efficacy is disputed. For Africa’s water hyacinth-infested Lake Victoria, the weevil was introduced as a biological solution, but its long term impact is uncertain. A third party, the weather, has further complicated matters.
Both Wilson et al. (2007) and Williams et al. (2007) acknowledge that a combination of weevils and weather affected the water hyacinth population in the late nineties, but they offer their own theories as to how much each factor actually contributed.
Wilson et al. (2007) emphasize the weevils’ role. The beetles punch holes in the plants as they feed, allowing the plants to become waterlogged and slowly start sinking. Wilson et al. (2007) claim that the wave action associated with El Nino simply expedited this sinking process. The water hyacinth population declined drastically in 1999, four years after the weevils’ introduction. Other documented cases of bio-control also saw a 3-5 year latent period before any results were notable.
In contrast, Williams et al. (2007) claim that the weather served a more prominent role in the decline of the water hyacinth. According to their paper, constant low lighting in 1997/1998 weakened the plants, and the weevils’ effect on the water hyacinth might be overstated. Williams et al. (2007) also raise the issue that “unstable host populations may well lead to unstable controlling herbivore populations.” Because water hyacinths serve as food and shelter for weevils, reducing the water hyacinth population also reduces the lake’s carrying capacity for weevils.
Even though Wilson et al. (2007) might take a more general approach, I feel that they offer better evidence and stronger theories. They analyzed the opposing side’s data on light vs. plant mortality, deeming it inconclusive, and they explained unstable populations through “population regulation.” Wilson et al. (2007) suggest that a spike in hyacinth numbers is normal, that it does not necessarily mean weevil failure. This makes sense to me because populations are not static; they are rather dynamic, as satellite photos suggest (NASA Earth Observatory 2007).
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.