The invasive aquatic plants They have become one of the most common headaches in lakes, reservoirs, and canals worldwide. When these species find calm water, high nutrient concentrations, and an absence of natural predators, they can completely cover the surface, block sunlight, and disrupt the functioning of ecosystems.
This problem, which has already been very clearly documented in urban lagoons of South AmericaThis is increasingly relevant for wetland managers and technicians in Europe, including Spain. The experience accumulated in scientific projects focused on the control of water cabbage (Pistia stratiotes) It offers clues on how to address, with greater rigor and less environmental impact, the expansion of these invasive floating macrophytes.
What are invasive aquatic plants and why are they a concern?
With the term invasive aquatic plants This refers to species that, outside their natural range or in altered conditions, exhibit a very rapid capacity for expansion and generate significant ecological, economic, or social impacts. Those that form dense floating mats, capable of covering a large portion of the water's surface, are particularly problematic.
When an invasive macrophyte covers the surface, the light entering the waterThis limits photosynthesis in submerged plants and disrupts the oxygen balance. This leads to waters with less dissolved oxygenmortality of sensitive fish and macroinvertebrates, and a general impoverishment of aquatic biodiversity.
In addition to the ecological damage, these invasions have a clear urban and water management dimension. In areas with high rainfall, the lagoons and ponds they act like heavy rain buffersretaining some of the excess water and preventing downstream flooding. If the available volume becomes filled with plant biomass, this function is seriously compromised.
Clogging by floating plants also hinders the recreational and landscaping use of urban wetlands, complicates maintenance tasks, and can encourage the appearance of bad smells and proliferation of mosquitoes by altering water circulation. It is therefore not surprising that these species are considered a priority for environmental management in both Latin America and various regions of Europe.
Water cabbage: an example of an invasive floating macrophyte
Among the various invasive macrophytes, the species Pistia stratiotesWater lettuce, commonly known as water cabbage or water lettuce, perfectly illustrates the type of problems these plants can cause in warm and temperate climates. It is a free-floating plant with thick leaves arranged in a rosette, capable of forming very dense mats.
Su reproductive capacity is extraordinaryIn nutrient-rich waters, the plant multiplies rapidly via stolons, colonizing large areas in a matter of weeks. Where it finds favorable conditions, it can cover a significant portion of the water's surface, easily exceeding 30 or 40% of the surface area in some lagoons.
When this happens, the chain reaction is clear: light decreases, oxygen levels drop, and the environment degrades. water quality and the ecosystem is impoverished. The accumulation of plant biomass also promotes sedimentation and accelerates silting, reducing the usable storage volume and flood control capacity.
Many municipalities try to curb its expansion through mechanical controlWhether by manual harvesting or with specialized machinery, removing tons of plants from the water on a regular basis is costly, labor-intensive, and logistically demanding, and it doesn't address the root of the problem: as soon as any fragments remain, the population regenerates rapidly.
How biological control of invasive macrophytes is being tested
Given the limitations of mechanical control, in different parts of the world, the following are being explored: biologic control As a medium- and long-term alternative for managing invasive aquatic plants, this strategy is based on the use of insects or other organisms that feed almost exclusively on the target weed, reducing its vigor and ability to spread.
In the case of water cabbage, the research has focused on highly specific phytophagous insectsThese plants are capable of damaging leaves and plant tissues without feeding on other native species of ecological value. The idea is not to completely eradicate the plant, but to keep it at much lower levels, compatible with the functioning of the ecosystem and water management.
Before considering any release into the environment, research teams conduct detailed studies in controlled environmentsIn large experimental ponds or pools, conditions similar to those found in eutrophic lagoons are reproduced: abundant water lettuce, high concentration of nutrients, and absence of significant herbivory.
Different things are introduced into these systems densities of biocontrol insects to evaluate how the plant and the aquatic community respond. Data are recorded on water lettuce growth, foliar damage, insect reproduction, changes in the structure of the vegetation cover, and variations in the physicochemical parameters of the water.
Previous trials have shown that, with adequate densities of biocontrol agents, a notable decrease in size and vigor of invasive plants. The water surface coverage is reduced, the layer is fragmented, and the macrophyte's ability to monopolize the ecosystem is clearly limited.
Augmentative rearing of biocontrol insects: a key piece
For augmentative biological control to be viable, it is essential to have stable insect colonies in the laboratory and using reliable mass rearing protocols. It is not enough to know that a species of weevil or leafhopper feeds on the invasive plant: it is necessary to be able to produce thousands of healthy specimens, with good reproductive capacity, and at the right time for their release.
Specialized laboratories work with experimental ponds where the invasive plant is cultivated under controlled conditions. Insect populations are maintained on this living substrate, with careful attention paid to aspects such as water quality, temperature, nutrient levels, and regular plant replacement to ensure a constant food supply.
In these systems, the collection, identification, and separation of the biocontrol agents of interest are carried out. Individuals are counted, potential unwanted predators are discarded, and their presence is continuously monitored. feeding and reproduction of the colonies. The equipment records life cycles, oviposition rates, larval and adult survival, and damage capacity on the target plant.
The design of these breeding systems requires meticulous planning, because the synchronization between production and release This is crucial: if the colonies are not at their optimal point when the intervention window opens in the field (for example, before the peak rainy season), effectiveness is lost and the entire program is delayed.
Furthermore, special attention is paid to ensuring that the selected insects are natives of the region or at least be fully adapted to the local climatic and ecological conditions. This avoids introducing new exotic species and reduces the risk of unforeseen impacts on other components of the ecosystem.
What can Spain and Europe learn from these experiences?
The problems linked to the invasive aquatic plants These problems are not exclusive to Latin America. In different regions of Spain and other European countries, the spread of floating and submerged macrophytes—some originating from the ornamental trade or their use in aquariums and ponds—has generated very similar situations: blocked canals, loss of habitats, and increased maintenance costs.
In this context, the trials of augmentative biological control The projects being developed in warm wetlands in other parts of the world provide an interesting testing ground for European management. The aim is not simply to transfer the same biocontrol species, but to leverage methodological expertise: early diagnosis, impact studies, rigorous selection of beneficial organisms, and evaluation under controlled conditions before implementing any interventions on the ground.
Accumulated experience shows that the first step is to address the problem comprehensively: controlling the nutrient sources that feed the growth of invasive plants (spills, urban runoff, illegal connections to the sanitation network), while studying alternatives to constant mechanical removals, which are usually expensive and unsustainable.
In Spain, where interest in wetland restoration and nature-based solutions is on the rise, integrating biological control tools In the management options box, this can help reduce dependence on herbicides and heavy machinery. However, any initiative of this kind would have to undergo a rigorous risk assessment, pilot trials, and close coordination between government agencies, research centers, and local stakeholders.
The case of water lettuce, with detailed studies on optimal insect densities, plant response, and changes in lagoon functioning, serves as an example of the importance of basing the management of aquatic invasions on solid scientific evidence and not only in specific emergency situations.
In light of recent scientific findings, managing invasive aquatic plants involves combining several approaches: reducing the nutrient input that triggers their growth, maintaining a certain level of mechanical control where necessary, and incorporating, when supported by studies, biological control programs based on specific and well-evaluated insects. This mix of measures, supported by research teams, training of young specialists and institutional collaborationIt offers a more realistic horizon for restoring the balance of lagoons and wetlands, both in warm regions of South America and in the aquatic ecosystems that are to be preserved and restored in Spain and the rest of Europe.