Invasive apple snail in Kenyan irrigation scheme could potentially spread to more areas

Experts consider the invasive apple snail (Pomacea canaliculata)  one of the world’s worst invasive alien species. This species is native to South America and invades freshwater systems. It spreads rapidly and is very adaptable to stressful environmental conditions and changes. It also has high reproductive rates and strong defence mechanisms. Additionally, the areas it invades lack natural predators that can control its population. 

Adult laying eggs. Credit: CABI

In Kenya, the invasive apple snail has reduced rice yield by around 14% and net rice income by around 60%. This is because of the need to replant damaged crops and implement management practices. The invasive apple snail can also harm human health as a vector for parasites like rat lungworm. 

How the invasive apple snail made its way into Kenya is still unknown, and farmers have been managing the pest differently. They’ve used physical and cultural practices along with chemicals that may be illegal. Other strategies include crushing snail eggs and picking off the adults. All these proved to be ineffective. 

Irrigation schemes in Kenya

Irrigation schemes produce 80% of the rice in Kenya. The five major rice irrigation schemes in Kenya are Mwea, Bura, Hola, Ahero, and West Kano. Notably, the Mwea irrigation scheme, located in Kirinyaga County, produces 70% of the country’s rice.  

Scientists from CABI and the Kenya Plant Health Inspectorate Services (KEPHIS) studied the spread of the invasive apple snail in these irrigation schemes. Their findings were published in a research paper.   

The study aimed to map the range of the pest to establish the extent of the invasion. This would help them develop management and quarantine measures. The potential distribution of the invasive apple snail was also predicted. 

In the field to find the pest

With the help of scheme managers, extension officers, and lead farmers, the research team conducted field studies in December 2020 and February 2022. Researchers identified the potential points of invasion and used them as the starting point of the surveys.  

They looked for egg masses on the emergent aquatic vegetation while they searched for live snails among submerged vegetation, under stones, or buried. They counted and recorded these. Additionally, the team recorded information on rice crop variety and other infested plants. Individual farms surrounding the schemes were also surveyed. 

Pomacea canaliculata. Credit: CABI

Special measures were taken to ensure that the invasive apple snail would not spread due to the surveys. Researchers began in areas with low infestations and later progressed to areas with high infestations. Researchers considered drainage and irrigation canals as these could potentially spread the pests. 

Where is the snail, and where could it go?

Invasive apple snails were only found in the Mwea irrigation scheme. Researchers found no egg masses or adults in the Bura, Hola, Ahero, and West Kano schemes. The invasion boundary had expanded from the initial point of infestation in the Mwea rice scheme. Pests infested 80% of the scheme.  

When looking at the potential distribution of the invasive apple snail, they found that the southwest of Kenya, along the Tana River, and the coastal areas near Mombasa had good suitability for the pest. Although researchers found none in the Tana River and Kisumu counties, they identified a high potential for further spread in these areas. This is due to the high exchange of planting material and machinery between Mwea and these schemes. 

The invasive apple snail has a high level of suitability in countries such as Malawi, Madagascar, and Uganda. There is a high yet concentrated suitability in specific areas in Mozambique, Tanzania, and Ethiopia. Other countries, such as Sudan, South Sudan, Eritrea, Djibouti, and Somalia, were largely unsuitable for the pest. On the other hand, areas along the Nile in Sudan were found to be moderately suitable. Additionally, researchers found the Ethiopian highlands to be suitable, but they are bordered by regions and countries that are unsuitable for the invasive apple snail. 

The initial points of infestation show the highest density of concentration, with the spread following the direction of water flow. The interconnected water flow includes rice fields, rivers, and canals that have plants serving as hosts for the pest. The access to flowing water plus flooding events increased by climate change acts as pathways for the invasive apple snails. The pests use the downstream movement to both crawl and drift. Material transfer, such as rice seedlings and farm machinery, from one area to another also facilitates spread. 

Proposed measures for controlling invasive apple snail

Knowing which areas are more susceptible to pest invasion helps to establish boundaries for the spread of the pest. This means that robust quarantine strategies and resources can be implemented or allocated properly.  

Researchers have proposed several measures for controlling the invasive apple snail, such as using field screens to prevent snails from entering and continuing to handpick them off physically. Synchronizing farm activities, introducing changes to the cropping system and planting patterns, and managing water paddies are also some ways to control the pest. Plus, minimizing their habitats by distilling canals. 

Importantly, the national government should collaborate with stakeholders and enforce these measures to achieve the desired results. Area-wide control measures such as community-based and integrated pest management should be prioritized. The invasive apple snail’s spread can be combated by establishing these long-term control methods

Read the research paper in full with open access: Makale, F., Muvea, A.M., Mugambi, I. et al. Current and potential distribution of the invasive apple snail, Pomacea canaliculata in Eastern Africa: evidence from delimiting surveys and modelling studies. CABI Agric Biosci 5, 92 (2024). https://doi.org/10.1186/s43170-024-00301-7