Of the many methods used to control invasive species, the use of biocontrol, which often results only in the addition of another harmful invasive species to an ecosystem, is certainly one of the most controversial. Occasionally, however, this method successfully reduces the effects of the original invader. Some, including Wilson et al. (2007), speculate that one such success story may have taken place in Lake Victoria, where Neochetina was used as biocontrol on water hyacinth, an invasive plant in many parts of the world. This plant, which lived on Lake Victoria’s surface and ultimately altered its entire ecosystem, began to decline shortly after the introduction of the weevils. This decline also coincided with changing climatic conditions caused by El Nino, leaving many uncertain of where credit for the plant’s decline belongs.
Examining the effects of weevils and El Nino’s weather conditions on water hyacinth in other areas with similar climates ecosystems may result in the most accurate determinations of why the dwindling of water hyacinth occurred. Wilson et al. (2007) have observed that, in areas with a climate similar to that of Kenya, such as West Africa and Papua New Guinea, the introduction of Neochetina has successfully slowed the invasion of water hyacinth, while low sunlight levels present because of El Nino did not prevent growth, leading us to believe that Neochetina caused the decline. However, Williams et al. (2007) assert that low sunlight levels, flooding, and waves caused by El Nino caused the demise of water hyacinth in Lake Victoria.
I agree that “changes in plant quality [including those caused by climatic changes] can affect the efficiency of weevils and a rapid deterioration of plants can lead to an early decline in weevil populations … such that plants can recover” (Williams et al., 2007). However, like Wilson et al. (2007), who acknowledge the effects of these natural Neochetina population fluctuations, I do not believe that the weevil population was disturbed severely enough to negate its effect on the water hyacinth. The water hyacinth was likely overcome by the combined destructive forces of Neochetina and El Nino. Perhaps the recent reinvasion of water hyacinth in Lake Victoria, as shown by images from the NASA Earth Observatory (2007), will allow us to determine whether Neochetina can successfully slow water hyacinth invasion in as a large body of water as Lake Victoria without the weakening of the plant by El Nino.
NASA Earth Observatory. 2007. Water Hyacinth Re-invades Lake Victoria.http://earthobservatory.nasa.gov/IOTD/view.php?id=7426. Viewed 27 January 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. Katagaria, 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.