Insects against invasive plants: what can bioagents mean in practice?
Imagine: a plant you'd rather not have suddenly appears along a ditch on your farm. You remove it, but a few weeks later it's back. Or a ditch keeps getting more overgrown, so water can no longer drain properly. For farmers and land managers, invasive plants aren't just a nature problem. They can also mean extra work, costs, and water management issues.
One example is Japanese knotweed. This plant can form dense growth along ditches, banks, and field edges, crowding out other plants. Its strong root systems also make it difficult to remove. Even small pieces of root or stem can grow into a new plant. On dikes and slopes, the species can also displace existing vegetation, affecting the stability of the growth there.
Floating pennywort and Australian swamp stonecrop can cause problems too. These water plants can clog ditches and waterways, hindering the supply and drainage of water. For a farm where good water management matters, that can become a very concrete problem. At the moment, it's mainly water boards that have a lot of experience with these species and with controlling them.
Nature as an ally
Many invasive plants originally come from another part of the world. In their native range, they are eaten by insects and mites that have adapted to the plant over the course of evolution. When such a plant ends up here, it leaves part of its natural enemies behind. That can contribute to the success with which some non-native species spread.
Biological control attempts to bring back part of that ecological relationship. This doesn't happen just like that. A potential bioagent has to be highly specific to its target plant. Before a species may be released in the Netherlands, extensive research is done into whether the organism could also damage other, native plants. For the biological control agents within the Dutch BioCops research, such safety tests have been carried out and permits have since been granted.
From plant lice to weevils
Within BioCops, three natural enemies are currently being studied. For Asian knotweeds, including Japanese knotweed, the Japanese knotweed psyllid Aphalara itadori is being researched. The psyllid feeds on young shoots and leaves and can slow the plant's growth.
For floating pennywort, a specialized weevil, Listronotus elongatus, is being used. The weevil feeds on the plant and is meant to slow its growth and spread. Against Australian swamp stonecrop, a specialized gall mite is being deployed. It's so small that it isn't released on its own — the mites are brought to the trial site on plants, after which they can spread to other swamp stonecrop plants.
The first field trials with the weevil and the gall mite started in 2025. Among other things, the trials should clarify whether these species can establish, reproduce, and spread in the Netherlands, and whether they actually have an effect on their target plant.
What's in it for farmers?
Perhaps the most important point is that a bioagent doesn't have to be a miracle cure. The goal usually isn't to completely clear a location of an invasive plant within one season. It's about weakening the plant over the long term.
That can be useful, for example, for a ditch or field edge where Japanese knotweed comes back every year. Removing it again and again costs time and doesn't always prevent the plant from resprouting. If a natural enemy structurally weakens the plant, that can help other measures work better.
In that sense, biological control fits within a broader approach. First you prevent spread and map out where the plant occurs. Then mechanical measures can be applied. A bioagent can eventually add an extra layer by reducing the plant's growth vigor.
For Japanese knotweed, for instance, researchers are looking into whether the psyllid could help with aftercare once plants have already been mechanically removed or weakened. The idea isn't that the insect takes over all the work, but that it helps keep regrowth under control.
Can you release insects yourself?
No, not for now. The bioagents from the BioCops research aren't pest control products you can order as a farmer and release yourself in a ditch or field. Releasing these organisms into nature requires a permit and takes place within controlled research and monitoring programs. The current permits are tied to the research into the relevant species.
That doesn't mean there's nothing a farmer can do. Do you have Japanese knotweed on your farm, for instance, or a ditch clogging up with floating pennywort or Australian swamp stonecrop? Then it can be worthwhile to first determine which species it is and what measures are already being applied.
For information about the current Dutch research, you can contact STOWA. That's where the BioCops project is based, in collaboration with, among others, CABI, Koppert, and Leiden University. The contact person for the project is Tessa van der Wijngaart.
A concrete first step, then, is this: take a photo of the plant, note where it's growing and roughly how large the area is. Then discuss this with your water board or with STOWA. They can indicate whether the problem fits within ongoing research or whether other suitable measures are available.
No miracle cure, but an extra tool
For farmers, a bioagent won't make the difference tomorrow between having invasive plants on the farm or not. Current applications are still very much in the research phase and are mainly focused on species that cause a lot of problems for water boards and land managers.
Still, the development is interesting. Where control mostly consists of removing plants again and again, a natural enemy can eventually help structurally weaken an invasive plant.
The strength probably doesn't lie in a single insect solving the problem, but in combining different measures. Prevention, monitoring, mechanical control, and, where possible, natural enemies can together lead to an approach in which invasive plants don't necessarily disappear entirely, but do become manageable.
For the Centre for Sustainable Agriculture Transition, this development fits within a broader search for farming methods that are less burdensome for the environment. Bioagents are not a replacement for existing measures, but a possible addition that could eventually contribute to a more sustainable way of dealing with invasive plants on and around the farm.
This article was written by Joost Besijn, senior practice researcher at CDLT. Before joining CDLT, he spent eight years working on a program aimed at controlling invasive species, in which the use of bioagents was one of the methods studied.