ABSTRACT
Glyphosate is the world’s most widely used herbicide. The commercial
success of this molecule is due to its non-selectivity and its action,
which would supposedly target specific biosynthetic pathways found
mainly in plants. Multiple studies have however provided evidence for
high sensitivity of many non-target species to glyphosate and/or to
formulations (glyphosate mixed with surfactants). This herbicide, found
at significant levels in aquatic systems through surface runoffs,
impacts life history traits and immune parameters of several aquatic
invertebrates’ species. Some of these species are vectors of diseases,
one of the most important of which is the mosquito. Mosquitoes, from
hatching to emergence, are exposed to aquatic chemical contaminants. In
this study, we first compared the toxicity of pure glyphosate to the
toxicity of glyphosate-based formulations for the main vector of avian
malaria in Europe, Culex pipiens mosquito. Then we evaluated, for
the first time, how field realistic dose of glyphosate interacts with
larval nutritional stress to alter mosquito life history traits and
susceptibility to avian malaria parasite infection. Our results show
that exposure of larvae to field-realistic doses of glyphosate, pure or
in formulation, did not affect larval survival rate, adult size and
female fecundity. One of our two experimental blocks showed, however,
that exposure to glyphosate decreased development time and reduced
mosquito infection probability by malaria parasite. Interestingly the
effect on malaria infection was lost when the larvae were also subjected
to a nutritional stress, probably due to a lower ingestion of
glyphosate.
Keywords: Diet; Glyphosate-based herbicides; Plasmodium
relictum ; vector
Introduction
Glyphosate is the world’s most widely used herbicide. More than 85 000
tons are spread every year (Benbrook, 2016). The commercial success of
glyphosate is due to its non-selectivity and its action, which would
supposedly target specific biosynthetic pathways found only in plants
and certain microorganisms, thus minimizing its toxicity to non-target
organisms. Many studies belie this assumption and provided evidence for
high sensitivity of many non-target species to glyphosate (Annett et
al., 2014; Gill et al., 2018). In addition, glyphosate is mixed with
surfactants to improve the performance of this foliar applied herbicide
(surfactants increase the surface area in contact with the vegetation).
Comparative toxicity assessments between formulations (glyphosate mixed
with surfactants) and pure glyphosate consistently show that
formulations are more toxic than glyphosate alone (Gill et al., 2018;
Nagy et al., 2019).
It has been estimated that less than 0.1% of glyphosate-based
herbicides applied to crops reach their specific targets (Nguyen et al.,
2016). Herbicide and their degradation products are therefore found at
significant levels in the environment (Bai and Ogbourne, 2016; Peruzzo
et al., 2008; Struger et al., 2008) and, in particular, in aquatic
systems through surface runoffs (Giesy et al., 2000; Morrissey et al.,
2015; Van Bruggen et al., 2018). Exposure to sublethal concentrations of
glyphosate, pure or in formulation, induces biochemical, physiological
and behavioral alterations in many fish and amphibian species (Daam et
al., 2019; Langiano and Martinez, 2008; Mann and Bidwell, 1999; Modesto
and Martinez, 2010). An increasing number of studies also showed
negative effects of glyphosate exposure on life history traits (Hansen
and Roslev, 2016; Janssens and Stoks, 2017; Kibuthu et al., 2016; Morris
et al., 2016) and immune parameters (Matozzo et al., 2019; Mohamed,
2011; Monte et al., 2019) of several aquatic invertebrates’ species.
Some of these species are vectors of diseases, one of the most important
of which is the mosquito.
The pre-imaginal stages of mosquitoes develop in freshwater environments
and, from hatching to emergence, larvae and pupae may therefore be
exposed to aquatic chemical contaminants. Glyphosate exposure may alter
mosquito behavior (Baglan et al., 2018), life history traits such as
larval survival rate or reproduction (Bara et al., 2014; Kibuthu et al.,
2016; Morris et al., 2016 but see Baglan et al., 2018), as well as the
expression of genes conferring resistance to insecticides (Riaz et al.,
2009). Glyphosate exposure has therefore the potential to influence the
vectorial capacity of mosquitoes. To date, no studies have however
tested whether exposure to glyphosate could directly affect mosquito
susceptibility to vector-borne parasite infections and whether this
potential impact is modulated by larvae nutritional stress. In the wild,
mosquito larvae are exposed simultaneously to multiple biotic
(competition, predation) and abiotic (temperature, bioactive
contaminants, food quality, (Beketov and Liess, 2007; Muturi et al.,
2011; Tripet et al., 2008)) stresses. All possible outcomes of stressor
combinations (i.e. additive, synergistic or antagonistic) may be
observed (Coors and Meester, 2008; Crain et al., 2008) and influence
vector-borne pathogen transmission directly through changes in vector
density or indirectly by changing vector immunocompetence, lifespan or
reproductive potential.
Our specific aims in this study were to answer three questions. First,
does exposure of larvae to glyphosate influence mosquito life history
traits and susceptibility to malaria parasite infections? Second, is the
potential effect of glyphosate-based herbicides (formulation) stronger
than glyphosate alone? Third, is there an additive, synergistic or
antagonistic effect between two different stressors, namely glyphosate
exposure and food limitation? Experiments were conducted with a natural
system consisting on the avian malaria parasite Plasmodium
relictum and its vector in the wild, the mosquito Culex pipiens(Pigeault et al., 2015).
Materials and methods