Entomophaga 30(3): 279-286

Biological control using the Cyrtobagous salviniae weevil is a popular control method for giant salvinia. Both the effectiveness of biocontrol and the growth rate of the plant are reliant on environmental conditions such as nitrogen levels and temperature.

Forno and Bourne (1985) examined how temperature, nitrogen content of the plant, and density of weevils altered the effectiveness of the weevil. The results showed that high-density populations of weevils (4 adults/bud) at high temperatures had significant damage to the plant. However, plants with low-density populations (1 adult/bud) developed buds faster than control plants with no insects. Varying nitrogen levels in plant tissue, however, did not influence plant development. The study suggested that high temperatures and high population density is the weevil are more important for reducing giant salvinia growth than a high nitrogen content.

Sep
20
Filed Under (SW3) by Michael Di Nunzio on 20-09-2010

Michael Di Nunzio

09/13/2010

Water hyacinth has posed a problem for Lake Victoria since first being reported there in 1989. The plant forms dense mats of vegetation that inhibit the movements of fishermen, block sunlight to native plants, and obstruct irrigation systems.  The invasive weeds can also deplete the water’s oxygen levels, suffocating the indigenous flora and fauna of the lake and in turn disrupting the local ecosystem. To control the hyacinth populations, invasive weevils (Neochetina spp.) were introduced with the intention of suppressing the noxious weed (Williams et al. 2007).

Using satellite image samples, Wilson et al. (2007) estimated the proliferation of water hyacinth over Lake Victoria and fluctuations in the plant’s presence over time, finally presenting their data in Aquatic Botany. However, Williams et al. (2007) warned with a rebutting article that this method of gathering data is oversimplified for such a complex environment. In spite of the dispute, both parties agreed that hyacinth levels dropped after the 1998 El Niño disturbed the lake. Following the initial decline came a steady rise until 1999 when hyacinth levels again began to decrease dramatically. From 2000 until 2002 hyacinth levels remained suppressed to under 5000 hectares of biomass over the lake’s entirety (Wilson et al. 2007).

Wilson et al. (2007) reasoned that the drop in 1999 was a result of the control weevils introduced in 1995 becoming effective after four years of relative dormancy. They also noted that the weevils control the weed by lowering its buoyancy and sinking it, and El Niño could have facilitated this process with wind and wave action. Because El Niño would inevitably blow some hyacinth into new areas, Wilson et al. (2007) suspected that local reports of hyacinth resurgences might have actually been false. Valid reports of resurgence may have resulted only if weevils died due to a lack of buoyant hyacinth, leaving the plant temporarily uncontrolled. According to Wilson et al. (2007), there was no substantive evidence to link low light levels with any of the withdrawals of hyacinth as Wilson et al. (2007) surmised.

Williams et al. (2007) placed less emphasis on the importance of the weevils in regards to water hyacinth control. Rather than biocontrol being a significant factor, they claimed El Niño more likely pulled hyacinth from the shoreline and destroyed it with wave action. This theory was both plausible and agreed seamlessly with the data showing a decline in 1998. Furthermore, the 2000 to 2002 nadir in hyacinth was thought to be a fleeting product of the weevil’s efficacy after 1999 and “suboptimal light” (Williams et al. 2007). Finally, Williams et al. (2007) pointed to the River Kagera as an ideal means of future resurgence, as hyacinth from this region is untainted with weevils and can float freely into the lake. This meant that there would be a delay before biomass control could take effect.

While Wilson et al. (2007) offered the more optimistic outlook on the data set, Williams et al. (2007) were unfortunately the most realistic. Williams et al. (2007) provided the most coherent argument, and aptly paralleled the situation in Lake Victoria with that of sub-tropical climates plagued by water hyacinth. They assumed that the lack of hyacinth is a part of a cyclic process involving a balance between weevils and weeds that will invariably lead to hyacinth resurgences. Wilson et al. (2007) tended to make unlikely excuses for all reported instances of resurgence, rather than offering any real insight into the possible validity of their reasoning. The satellite images from MODIS vindicate the argument of Williams et al. (2007), as resurgence obviously took place by 2006 (NASA 2007). Thus the relationship between adequate light, the presence of weevils, and the predominance of hyacinth must be a continued subject of study at Lake Victoria if definite conclusions about the hyacinth resurgence cycle are to be drawn. However, Williams et al (2007) seems to have lead us in the right general direction.

References:

NASA Earth Observatory. 2007. Water Hyacinth Re-invades Lake Victoria. http://earthobservatory.nasa.gov/IOTD/view.php?id=7426. Viewed 12 Sep 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

Sep
13
Filed Under (SW3) by Brianca King on 13-09-2010

Water Hyacinth, or Eichhornia crassipes, is among the world’s most noxious invasive weeds (NASA Earth Observatory 2007). The plant arrived in Africa in the late 1800’s and made a home in Africa’s largest lake, Lake Victoria.

Biological control, the use of organisms that are natural predators, parasites, or pathogens to control an environmental pest, was introduced in 1995 and weevils were released onto different parts of Lake Victoria (Wilson et al. 2007). A few years after the introduction of the weevils in 1997 and 1998 El Nino hit and here is where the controversy begins. According to Wilson et al.(2007) the weevils were the main cause for the decline of the Water Hyacinth and El Nino was a small aid to the problem. Weevils reduce the plant buoyancy and allow for bacteria and secondary fungi to cause severe damage to roots (Wilson et al.2007). Williams et al.(2007) says that El Nino had the greater affect on the reduction of the Water Hyacinth by accelerating the decline through direct effects.  El Nino produced a low light climate. Low light levels do not cause instant mortality but prolonged sub-optimal light will reduce growth and reproduction rates and relatively increase the effect of other debilitating influences (Williams et al.2007). Both sides used satellite images to make graphs to support their arguments as well as the work of other researchers. The graphs depicted in each article show the increase and decrease of the Water Hyacinth in Lake Victoria over time.

Williams et al.(2007) provided a better argument for the decline of the Water Hyacinth. The approach taken by Williams et al.(2007) was more realistic in that it did not oversimplify the issue and acknowledged that Lake Victoria is a complex aquatic ecosystem and that any synchronicity across such a large waterbody is unlikely to occur at the biological scale(Williams et al.2007). This was a direct response to the condensed graph provided by Wilson et al.2007 used to show the rise and decline of the Water Hyacinth in Lake Victoria. Williams et al.(2007) also acknowledges that weevils are effective and mentions their success in other situations across the world but makes it clear that El Nino had a more direct effect on the Water Hyacinth population which is evidenced by the graphs in both articles. Furthermore satellite images from the NASA Earth Observatory showed a resurgence of the Water Hyacinth in 2006 proving that Williams et al.(2007) was correct in saying that the weevil population would not stabilize and the Water Hyacinth would return.

References:

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.

Sep
13
Filed Under (SW3) by Max Castillo on 13-09-2010

Ever since first being recorded in 1989, the water hyacinth plant has plagued Lake Victoria in Africa. The plant is causing substantial problems for the local ecosystems, such as reducing fishing levels, threatening biodiversity and tampering with the many transport routes across the lake. Because of these issues, measures to try to reduce the hyacinth’s population have been taken, but much controversy exists questioning whether  man or nature removed the vast majority of the plant from the lake. Much of the debate spawned from two conflicting research articles, which attempted to clarify whether the seasonal El Niño storm had destroyed the hyacinth or if  the introduction of hyacinth-eating weevils caused the plant’s demise.

Wilson et al. first proposed that through biological control the hyacinth populations started decreasing. In their article, they express that the introduction of Neochetina (weevils) was the primary factor in the limitation of the hyacinth. The article does acknowledge the presence of the 1998 El Niño weather pattern however, and does recognize that the increase in waves and water levels played a role (albeit small) in controlling the hyacinth. And although they also do say high cloud levels could also have decreased hyacinth levels, Wilson stands firm that El Niño only propagated the effects of the weevils.

On the other hand, Williams et al. claimed the weevils assisted El Niño. This article believes that if El Niño had not occurred, the weevils never would have controlled hyacinth levels. Williams affirms that while biocontrol through weevils “is an integral part of the future management of Lake Victoria”, lake wide reduction in hyacinth populations resulted from El Niño. Williams also supports this claim with more in-depth studies of the factors mentioned above (cloud level, wave, and water lever) to prove that El Niño nearly wiped out the hyacinth.

While in hindsight we do know that Williams et al. hypothesized correctly (once El Niño subsided hyacinth populations increased once again), I do have to  say that Williams did provide a more convincing argument from the start. Williams provided and researched numerous points that Wilson seemed to look at from the wrong angle, such as water levels and waves. Although both researchers do claim that one would not have been as effective without the other, Williams is successful in convincing me that El Niño played a larger, more important role the reduction of hyacinth levels in the late 90s.

References:

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.

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.

Sep
13
Filed Under (SW3) by Kyle Rand on 13-09-2010

Eichhornia crassipes, better known as water hyacinth, is an invasive specie first introduced to the waters of Lake Victoria in 1897, and by 1998, had grown to cover 77 square miles of Lake Victoria. These weeds form thick mats by growing daughter plants from runners at extremely fast paces.  These mats are devastating to the lake’s ecosystem, as they deprive native species of light, and deplete the oxygen in the water.

In 1995, scientists attempted to halt their growth by introducing biological control through the Neochitina weevil.  In 1998, there was a decrease in the growth of water hyacinth, but it coincided with the weather pattern El Niño, which questions the effectiveness of weevils as a means of biological control.  Wilson et al. (2007) argues that the weevils have been the main factor of the decline on Lake Victoria.  The Neochetina eichhorniae are known to feed on the plants by ripping through the petioles and the root-stock, which creates a breeding ground for bacteria to grow inside of the plants and further damage them.  As shown in previous studies in other countries, large populations of weevils are capable of destroying large mats of water hyacinth.  Wilson et al. (2007) demonstrates that the drop off in hyacinth population occurred a few years after the introduction of the weevils, and argues the drop in 2000 was due to the persistence of the weevils.

Although there is a correlation between the introduction of weevils and the decrease in water hyacinth growth, the presence of El Niño weather pattern mystifies the true explanation of the decrease.  Williams et al. (2007) admits that while the weevils played a large part in the decrease, the effects of El Niño were much more drastic, and changed the environment of Lake Victoria and created stormy weather, water currents, and other conditions that hampered the water hyacinth’s survival.  Williams et al. (2007) explains the sudden drop in hyacinth in the first half of 1998, using similar data as Wilson, but evidencing that the drop coincides with El Niño.

Both Wilson et al. (2007) and Williams et al. (2007) present valid arguments, but recent satellite images demonstrate that a resurgence occurred in 2006.  This suggests that El Niño caused more of a reduction than weevils.  However, Williams et al. (2007) presents a valid point, that it is hard to manage a lake wide means of biological control, so perhaps there is room for future development to make the weevils more consistent in killing the hyacinth.  Whatever is done, it must occur soon before the fisherman and other native species suffer further.

References:

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.

Sep
13
Filed Under (SW3) by Drew Van Orden on 13-09-2010

Lake Victoria, a freshwater body of water, has long been used as a primary source of food for the peoples of Africa. However, beginning in the late 1980’s, the water was invaded by an alien plant called the water hyacinth. The conditions of the lake were ideal for the rapid spread of the alien species and as a result, indigenous species quickly began to decline in number. It was obvious that something had to be done to exterminate the unwanted plant, so the spread of a specific weevil was implemented as a form of biocontrol in 1995. By the end of the decade, a substantial improvement had occurred and the water hyacinth had receded. While to some it seemed like the weevils directly caused this recession, other scientists questioned whether other forces had an impact.

Two scientific articles, Wilson et. al (2007) and Williams et. al. (2007), were published with contrasting conclusions to why the plant had stopped spreading. Wilson et. al. (2007) attributed the recession of the plant mostly to the use of the weevils as biocontrol, while admitting that the global weather pattern El Niño could have catalyzed the extermination of water hyacinth. Williams et. al. (2007), on the other hand, stated that while weevils did have some effect the powerful weather conditions directly caused water hyacinth to die off, backing it up with data showing the effect the El Niño had on the sunlight of the area, the water levels, the humidity, the temperature, and other variables that greatly accelerated the removal of the plant.

In my opinion, based on the recent repopulation of water hyacinth in Lake Victoria, the Williams et. al. (2007) article was more convincing. Wilson et. al. (2007) greatly attributed the decline of the plant to the weevils while not giving much credit to the weather, which at the time the article was written would have been a convincing argument, but since the plants have returned it seems like weevils were unable to have a lasting effect. Williams et al. (2007), however, stated how both the weevils and the El Niño contributed. The Wilson article indirectly argues that biocontrol alone can be greatly effective against species, and that does not seem to be the case. Hopefully more studies will look into the true cause of the decline of water hyacinth so humans can be more effective in removing the invasive species.

References:

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.

Sep
12
Filed Under (SW3) by Jania Arcia-Ramos on 12-09-2010

Eichhornia crassipes, commonly known as water hyacinth, is an aquatic plant that has lily-pad-shaped leaves and beautiful lavender flowers. Despite its aesthetic beauty, water hyacinth is an extremely invasive species. It originated in the tropics of South America and has spread to inhabit every continent except Europe. In Lake Victoria, for instance, water hyacinth is so abundant that it blocks the passage of fisherman to fishing waters, and blocks pipes necessary to bring drinking water into cities and villages. In addition, since the plant covers the surface of the water, it inhibits the sun from reaching native species deeper down, causing those plants and fish to suffer (NASA Earth Observatory, 2007).

Since its first appearance on the lake in 1989, scientists have been trying to find ways to eliminate this plant. One of the mechanisms that they employed is biological control, or introducing a biological species to help reduce the invasive specie.  To help exterminate the water hyacinth problem in Lake Victoria, Neochetina, or weevils, were introduced in 1995. Since these animals have an appetite for water hyacinth but don’t affect native species, Wilson et al. published an article in Aquatic Botany in which they argue that this method was extremely effective in reducing the abundance of the invasive plant on Lake Victoria  (Wilson et al., 2007).  However, in 1997-1998 there was also an occurrence of El Niño in the area which caused weather patterns that were detrimental to water hyacinths. Therefore, Williams et al. published another article in the same journal in which they attributed the reduction of the invasive species to this event, and not to biological control. They argue that the prolonged lack of light caused by El Niño reduced the growth and reproduction rates of the plant, making it more susceptible to other factors such as change in water levels and wave action brought about by El Niño (Williams et al., 2007). Although El Niño was responsible for the water hyacinth reduction that happened immediately after El Niño occurred, the weevils were responsible for the major reductions of the plant which didn’t occur until 1999-2000.

Although their opinions differ on what caused the decline of water hyacinth in 1999-2000, both Williams and Wilson agree that further research on the issue is necessary (Williams et al., 2007). This is even more crucial now that there has been a resurgence of water hyacinth in the lake (NASA Earth Observatory, 2007).

 References:

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.

Sep
11
Filed Under (SW3) by Cole Arora on 11-09-2010

When water hyacinth first became rampant on Lake Victoria in 1989 (Wilson et al., 2007), the general consensus among the scientific community involved the need to eradicate the invader by any means necessary.  To expunge the weed, biological control agents Neochetina bruchi and Neochetina eichhorniae (weevils) were set loose into the water hyacinth populations – what had already become a vast mass of floating vegetation – in 1995; from 1997 to 1998, however, the strongest El Niño in a century (Williams et al., 2007) took place.  The current debate centers on the comparative impacts of Neochetina and the El Niño phenomenon on the water hyacinth population.

One argument claims that the reduction in water hyacinth was effected predominantly by the biocontrol of the Neochetina colonies (Wilson et al., 2007).  The hungry Neochetina larvae “tunnel the petioles and the root-stock” of the water hyacinth, thereby allowing damaging bacteria and fungi to invade the plant; these tunnels, due to their obvious hollowness, then become flooded, reducing the plant’s buoyancy and causing it to sink (Wilson et al., 2007).  Also, because similar weevils species have led to clear repressions in plant population densities in other tropical locales, effective weevil biocontrol would be an appropriate assumption (Wilson et al., 2007).

A counter argument claims that El Niño played the major part by accelerating water hyacinth decline through direct – wind/wave action – and indirect – reduced incident light – effects (Williams et al., 2007).  The  flooding, in addition to breaking up mats, would have submersed the water hyacinth, thereby reducing the amount of light they receive (and food they produce). According to this view, El Niño was responsible for the sharp initial decrease in hyacinth, and the weevils were simply responsible for sustaining these low levels.

Wilson et al. presents the more compelling argument, in that, while the El Niño weather would account for the temporary 1998 decrease, it is illogical to claim a correlation between it and the decrease that took place in 1999-2000. The reason for the hyacinth population rebound after the 1998 crash is most likely because the El Niño floods would have destroyed a significant portion of the weevil population as well, slowing their effectiveness. The past successes of weevil biocontrol agents strengthen the case of Wilson et al., and the fact that water hyacinth growth was suppressed from 1999 to 2005 indicates that the weevil populations were able to find a niche in the lake area (Williams specifically states that “weevil populations although present are likely unstable”).  Unstable biocontrol agents are not effective ones.  The general and consistent decline in water hyacinth density on Lake Victoria after El Niño, which, according to MODIS satellite images, spanned from the year 2000 to the year 2005, is in no way attributable to El Niño phenomenon, due to the gap in time; this proves that the weevils were effective at hyacinth repression (NASA Earth Observatory 2007). But with no food readily available, most of the weevils would have perished.  Therefore seeds left behind by the dying water hyacinth would have slowly germinated and flourished under a condition devoid of the bugs but rich in nutrients from both the decomposing sunken water hyacinth mats and agriculturalrunoff due to heavy rains.  The result is a renewed blooming of the weed.

As stated by Wilson et al., one of the basic principles of biological control of plants is that it is “sustainable through population regulation.”  But because insect populations change in response to changes in the host plant population, effective biocontrol is a mutually-destructive process.  This is the main reason why greater human regulation of biocontrol is necessary – to ensure there are always enough weevils to do the job. However, if the weevil colonies establish themselves again like the water hyacinth, the successful precedent they set in 1999-2000 will repeat itself.