At a time when scientists are raising the alarm over the accelerated disappearance of insects, it may seem astonishing to exhibit the results of research aimed at destroying them. But fighting certain species remains an economic and food-related obligation. Removing them in an eco-responsible way, for example by using essential oils, is…essential.
‘We began working on the protection of grain stocks a few years ago, in a collaboration with Cameroon’, recalls Prof. Thierry Hance, director of the UCLouvain Earth and Life Institute Biodiversity Division (ELIB) Ecology of interactions and biological control and a member of Louvain4Evolution. ‘There, the dry season can last six to eight months or longer, which makes it hard to make ends meet between harvests. To remedy this, farmers stored grain in earth silos but insects proliferated and destroyed much of the harvests.’
How to stop them? The project’s Cameroonian partners recalled that farmers had once used plants to eliminate insects, alternating layers of leaves and grains in silos. The tradition was lost after the arrival of synthetic insecticides. ‘It was done in African fashion, I dare say’, Prof. Hance says. ‘Farmers received small bags of insecticide without any information or instructions. Hence, of course, there were mishaps. So we had to find another way to keep insects away.’ . Elders were surveyed to identify the plants they had used. One in particular – Ethiopian black pepper – has shown promise. Researchers from UCLouvain and Gembloux Agro-Bio Tech (University of Liège) extracted the plant’s essential oils and studied the best ways to use them to protect crops, before giving way to Cameroonians.
In wallonia too
The Cameroonian experience was a success, which interested the Walloon Region and the Belgian company Biosix, which specialises in grain storage. They decided to co-finance research to develop a similar product for use in Wallonia: the Oilprotect project is at its halfway point. Prof. Hance’s team supervises the project, carried out in collaboration with the laboratories of ULB and Gembloux Agro-Bio Tech. Storing grain, which keeps an average of one year, is indeed a common practice in Wallonia. Grain silos become the den of fungi, mites and insects, primarily weevils (Sitophilus granarius), sawtoothed grain beetles (Oryzaephilus surinamensis) and other grain beetles (Rhyzopertha dominica). They’re small but cause major damage. Getting rid of them is an economic and food-related obligation; there are currently only two methods. ‘The ideal,’ explains Sébastien Demeter, the ELIB laboratory’s project manager, ‘is obviously to work on the type of storage. Today there are silos whose temperature is kept under 10° C, which prevents any insect activity or reproduction. But this is expensive. Almost all grain storage companies use highly toxic synthetic insecticides such as deltamethrin, which acts as a neurotoxic agent. A few drops (<40 ml) of this product per tonne of grain are enough to protect a crop for a year.’ But it’s in the European legislator’s sights because of its toxicity, so a we have to find a substitute that’s as effective but natural and, of course, non-toxic to humans. Why not an essential oil?le ?
Searching for plants that can play the role of bio-insecticide is a matter of course: during evolution, plants learned to defend against insects. As a result of mutations, some have probably stopped degrading secondary metabolites and store them in vacuoles. If these substances prove to be toxic or repellent to insects, the plants that developed this ‘anomaly’ are therefore likely to reproduce more effectively than those that didn’t. Researchers first worked on existing oil databases to select a dozen potentially cost-effective oils. Prof. Hance's team then began to investigate how essential oils, or blends of them, act, by focusing on the pathway of action on the insect’s octopamine, a neurotransmitter, that is, a molecule that ensures contact between two neurons for the transmission of nerve impulses. Interestingly, this neurotransmitter exists in insects but not in vertebrates and thus not in humans. Essential oils that block this transmission system therefore have no influence on our nervous system.
UCLouvain researchers are also interested in the effect of essential oils on bacteria that live in symbiosis with insects. François Renoz addressed this in his PhD.‘I studied the relationship between bacteria and aphids. Aphids harbour symbiotic “obligate” bacteria that manufacture amino acids essential to the insect’s survival but that the insect doesn’t find in its basic environment, in this case, in the plant sap the aphid feeds on. If the bacteria are removed, it can’t reproduce and will die.’ Further study of such bacteria was necessary. Dr Renoz used a transgene pathway: the aphids also harbour other types of symbiotic bacteria, termed ‘facultative’ as they aren’t essential to the insect’s survival but can confer benefits in specific ecological situations. He explains, ‘We were lucky to isolate one of these facultative bacteria, called Serratia symbiotica, in an artificial environment. Most of these bacteria aren’t culturable in an artificial environment. Thanks to this, I was able to study its impact on the aphid's immune system or the tools it has available to effectively infect a new host.’
Caption: view of an aphid with its symbiotic bacteria.
Green corresponds to the obligate symbiotic bacteria, Buchnera aphidicola (localised in specialised cells of aphids called bacteriocytes).
Red corresponds to the facultative Serratia symbiotica (located in the digestive tract of aphids).
Blue is aphid tissue.
The question is, in what way could they represent a weak link? Obligate symbionts, for example, are fragile because they are localised only in specific structures and have degenerated owing to their specialisation. Environmental stresses can therefore have a negative impact on them.
But back to our grain destroyers in the silos. ‘One of our hypotheses,’ Dr Renoz explains, ‘is that essential oils could be one of the stresses that impair this weak link in the targeted insects. I study the modes of action of these oils on the insect in general, its nervous and digestive system, but also on this partnership, which can be a weak link in the insect. For example, the weevil is associated with a obligate symbiont that has a nutritive role and could be impacted by oils. This would be an interesting mode of action to destroy the insect. For now, however, this is only a hypothesis to test.’
Finally, Prof. Hance’s team tackled another problem: destroying insects with essential oils not in silos but in orchards. An apple bought in a supermarket has been treated more or less 40 times (insecticides, fungicides, etc.), a repetition which is explained by the fact that insects have different life cycles. The team aims to inject the essential oil directly into the trees, as if intravenously. As a result, essential oils will be found in the leaves of trees and aphids (on apple trees) and psyllids (on pears) should be either repelled or killed. The difficulty is identifying essential oils that are effective but won’t damage trees, cause yield loss or change the fruit’s flavour.
After earning a bachelor’s degree in biology from the University of Namur, Thierry Hance obtained his master's degree in zoology, with a specialisation in ecology, at the University of Louvain (1982). At the same university he completed his PhD on the experimental comparison of two prey-predator systems (1988). Subsequently, he completed a postdoctorate at an agricultural research station in Quebec, where he worked on the cold resistance of beetle egg parasitoids.
Upon his return to Belgium, he first held a permanent position as a researcher at the Belgian National Fund for Scientific Research before becoming a professor at the University of Louvain in 1996. He has since developed a series of research topics focused on ecological interactions. He has also developed expertise in the surveillance of mosquito populations. More recently, he developed a new research topic on the relationship between endosymbiotic bacteria and plant-sucking insects. Passionate about popular science, Prof. Hance was the scientific advisor to the RTBF programme ‘Le Jardin extraordinaire’ from 1991 to 2014.
Sébastien Demeter was in Africa when he graduated with a master's degree in biology from UCLouvain in 2008. He was first in Kenya, then in Rwanda, where he worked in particular on seed dispersion and the extent to which Tasmanian devils participate in forest regeneration by dispersing plant seeds via their droppings. Back in Belgium in 2009, he wrote books on animal diversity and agri-environmental measures for the Walloon Region. He joined UCLouvain in 2014 as a research assistant; today he is responsible for the programme on the use of essential oils as an insecticide.
François Renoz earned his bachelor’s degree in biology at the University of Namur in 2007 and his master's degree in the same discipline at UCLouvain in 2009. A keen film and photography enthusiast, he took the time to do a master's degree in performing arts at ULB, specialising in film analysis and screenwriting (2011), before entering Prof. Hance’s laboratory to start his PhD on symbiotic relations between insects and bacteria, which he defended in December 2017. He has since been a researcher at the same laboratory under contract RW DGO6, as part of the Oilprotect project.