Synthetic insecticides are no longer very popular when it comes to getting rid of unwanted guests in the environment. Among the tools available in terms of biological control are various strains of fungi, including Aspergillus. However, an in-depth understanding of the insecticidal effect of these microorganisms is necessary if we are to properly control their action under natural conditions. This is what Thomas Bawin set out to do in his doctoral thesis. The focal point?  Culex quinquefasciatus, a fearsome mosquito from tropical and subtropical regions.During his experiments, the researcher observed that Aspergillus spores ingested by the mosquito larvae become increasingly active as the hours go by, with the secretion of toxic components capable of damaging the larvae's digestive epithelium and muscle tissues.

The suggestion of using fungi to eliminate pathogenic insects would have been treated with indifference or sarcasm in the scientific and agricultural community 40 or 50 years ago, when synthetic organic insecticides (DDT, etc.) were all the rage. Today, biological pest control is no joke. We now know that there are 700 to 800 species of fungi that could infect and kill numerous insects and mites. These so-called "entomopathogenic" fungi are essentially divided into two groups: Zygomycota and Ascomycota.

Aspergillus belongs to the second category. These saprophytic fungi are composed of long filaments. They are found all over the world and in a wide variety of ecosystems, from decomposing organic matter in the wild to food that is left to rot in the bottom of our fridge. Owing to their close interactions with insects in the environment, some species of Aspergillus can develop pathogenic behaviour towards them. On a scientific level, this phenomenon is still widely undocumented. While a few strains have been isolated from various diseased insects and shown to be responsible for their condition, few studies have been carried out concerning how they cause the insect's death, the components involved and the spectrum of potentially infected hosts. This grey area means that it is very risky – and even dangerous – to use these fungi in the environment.

A very troublesome mosquito for man 

This is the context of the work (1) by Thomas Bawin, a doctoral student at Gembloux Agro-Bio Tech (University of Liège), conducted under the direction of professors Frédéric Francis (Functional and Evolutionary Entomology Laboratory) and Frank Delvigne (Microbial Processes and Interactions), the promotors of his thesis. "One of the major problems of biological control, is being able to isolate agents (for instance fungi) that are specific to the target (for instance, a mosquito), while spraying conditions can be highly variable (temperature, humidity, etc.) thus affecting their efficiency". In the work carried out by the Senegalese researcher Fawrou Seye, a former post-doctoral student at Gembloux and currently a lecturer and researcher at the Faculty of Health Sciences at Gaston Berger University in Saint Louis (Senegal), the young biologist took a particular interest in Aspergillus clavatus and secondly, Aspergillus flavus, two opportunistic entomopathogens.

The two experts chose a species well known in tropical and subtropical areas: Culex quinquefasciatus. This average-sized mosquito is essentially nocturnal. The particularity of the female is that it mainly bites birds, but when she can't find any, she loudly buzzes her way into people's homes and goes for humans instead. On a more serious note, Culex quinquefasciatus is a vector for dreaded human diseases such as lymphatic filariasis, caused by minuscule parasitic worms (nematodes) characterised by atrophied limbs. There are currently some 120 million people in tropical and subtropical regions who are affected by filariasis. The insect also has the specificity of producing large quantities of larvae, capable of entirely colonising the surface of polluted water: puddles, gutters, drains, septic tanks, etc.

"The first part of our work consisted of assessing the pathogenicity of Aspergillus clavatus and Aspergillus flavus compared with a well-known pathogen, Metarhizium anisopliae, some strains of which have already received trade approval for the biological control of other insects. Not simply to assess their insecticidal potential but also the ability to produce spores (the propagules responsible for infection) on two types of substrate that could be used for the culture of such organisms: white rice and wheat bran". First carried out on a small scale in incubation flasks then reproduced in bioreactors, the operation quickly turned out to be conclusive: it is easy and cheap to grow cultures of Aspergillus on these two substrates. What's more, the insecticidal potential of the spores on the mosquito larvae is, by and large, identical to that of Metarhizium spores.

(1) Histopathological effects of Aspergillus clavatus (Ascomycota: Trichocomaceae) on larvae of the southern house mosquito, Culex quinquefasciatus (Diptera: Culicidae), 2016, Fungal Biology.