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DOI 10.7717/peerj.7931
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OPEN ACCESS
Combined effect of the entomopathogenic
fungus Metarhizium robertsii and
avermectins on the survival and immune
response of Aedes aegypti larvae
Yuriy A. Noskov1,2, Olga V. Polenogova1, Olga N. Yaroslavtseva1,
Olga E. Belevich1, Yuriy A. Yurchenko1, Ekaterina A. Chertkova1,
Natalya A. Kryukova1, Vadim Yu Kryukov1and Viktor V. Glupov1
1Institute of Systematics and Ecology of Animals, Siberian Branch of Russian Academy of Sciences,
Novosibirsk, Russia
2Tomsk State University, Tomsk, Russia
ABSTRACT
Combination of insect pathogenic fungi and microbial metabolites is a prospective
method for mosquito control. The effect of the entomopathogenic fungus Metarhizium
robertsii J.F. Bischoff, S.A. Rehner & Humber and avermectins on the survival and phys-
iological parameters of Aedes aegypti (Linnaeus, 1762) larvae (dopamine concentration,
glutathione S-transferase (GST), nonspecific esterases (EST), acid proteases, lysozyme-
like, phenoloxidase (PO) activities) was studied. It is shown that the combination of
these agents leads to a synergistic effect on mosquito mortality. Colonization of Ae.
aegypti larvae by hyphal bodies following water inoculation with conidia is shown
for the first time. The larvae affected by fungi are characterized by a decrease in PO
and dopamine levels. In the initial stages of toxicosis and/or fungal infection (12
h posttreatment), increases in the activity of insect detoxifying enzymes (GST and
EST) and acid proteases are observed after monotreatments, and these increases are
suppressed after combined treatment with the fungus and avermectins. Lysozyme-
like activity is also most strongly suppressed under combined treatment with the
fungus and avermectins in the early stages posttreatment (12 h). Forty-eight hours
posttreatment, we observe increases in GST, EST, acid proteases, and lysozyme-like
activities under the influence of the fungus and/or avermectins. The larvae affected
by avermectins accumulate lower levels of conidia than avermectin-free larvae. On
the other hand, a burst of bacterial CFUs is observed under treatment with both
the fungus and avermectins. We suggest that disturbance of the responses of the
immune and detoxifying systems under the combined treatment and the development
of opportunistic bacteria may be among the causes of the synergistic effect.
Subjects Ecology, Entomology, Parasitology, Toxicology
Keywords Synergism, Lysozyme-like activity, Mosquito, Acid proteases, Phenoloxidase,
Dopamine, Fungal colonization, Detoxifying enzymes, Biocontrol
INTRODUCTION
Mosquitoes are obligate intermediate hosts for a variety of pathogens that cause human
mortality and morbidity worldwide. Aedes aegypti is considered to be an important vector
How to cite this article Noskov YA, Polenogova OV, Yaroslavtseva ON, Belevich OE, Yurchenko YA, Chertkova EA, Kryukova NA,
Kryukov VY, Glupov VV. 2019. Combined effect of the entomopathogenic fungus Metarhizium robertsii and avermectins on the survival and
immune response of Aedes aegypti larvae. PeerJ 7:e7931 http://doi.org/10.7717/peerj.7931
of human diseases such as dengue and yellow fever, chikungunya, and Zika infections
(Tolle, 2009;Bhatt et al., 2013), and its control is therefore an objective to prevent the
transmission of these diseases. Chemical insecticides are still the most important element
in mosquito control programs, despite direct and indirect toxic effects on nontarget
organisms, including humans. In addition, chemicals induce resistance in a number of
vector species (Vontas, Ranson & Alphey, 2010;Ranson & Lissenden, 2016;Smith, Kasai
& Scott, 2016). Therefore, there is a need for alternative nonchemical vector control
approaches. Classical biological control based on using various microorganisms, such as
entomopathogenic fungi and bacteria, is a frequent tool for addressing this issue.
Among the biological agents employed for mosquito larvae control, bacteria from the
genus Bacillus are the most widely used. In addition, products of the entomophatogenic
fungi Metarhizium anisopliae s.l., and Beauveria bassiana s.l. are actively being developed
for use against mosquito adults and larvae (Butt et al., 2013;Greenfield et al., 2015;Ortiz-
Urquiza, Luo & Keyhani, 2015). It should be noted that mosquitoes and other insects
can develop resistance to the Bacillus thuringiensis Berliner biological larvicide (Tilquin
et al., 2008;Paris et al., 2011;Boyer et al., 2012). However, the resistance of insects to
entomopathogenic fungi develops very slowly (Dubovskiy et al., 2013). Various species
of mosquito larvae present different susceptibilities to Metarhizium, among which Ae.
aegypti is the least susceptible (Greenfield et al., 2015;Garrido-Jurado et al., 2016). Thus,
a concentration of conidia that is effective for Ae. aegypti control would affect a range of
nontarget aquatic invertebrates. Recent studies have found that some nontarget aquatic
species are more sensitive to fungal metabolites (Garrido-Jurado et al., 2016) and conidia
(Belevich et al., 2017) than target mosquito species. To reduce toxic effects on the aquatic
environment and increase efficacy against mosquitoes, entomopathogenic fungi may be
combined with other biocontrol agents or low doses of natural insecticides. For example,
combined treatment with Metarhizium and mosquito predator species (Toxorhynchites) has
shown additive or synergistic effects on the mortality of Ae. aegypti (Alkhaibari et al., 2018).
However, few studies have been carried out to determine the effect of combined treatment
with entomopathogenic fungi and other insecticides or plant or microbial metabolites
as a potential tool for improving mosquito larvae control. Synergistic effects between
entomopathogenic fungi and some chemical insecticides (temephos, spinosad) (Shoukat
et al., 2018;Vivekanandhan et al., 2018) or biological agents (Azadirachta indica) A. Juss
(Badiane et al., 2017) on the mortality of mosquito larvae have been found. However, the
physiological and biochemical aspects of this synergism were not considered.
One type of promising insecticide that can be effectively used for mosquito vector control
is the avermectins. Avermectins are a class of macrocyclic lactones isolated from the soil
actinomycete Streptomices avermitilis (ex Burg et al.) Kim and Goodfellow (Drinyaev et
al., 1999) and include several commercial derivatives (ivermectin, abamectin, doramectin
and eprinomectin) with the same mode of action—activation of glutamate-gated chloride
channels, followed by uncontrolled influx of chloride ions into the cells, which leads to
paralysis and death of the organism (Campbell et al., 1983). At the same time, avermectins
are relatively safe for humans (Crump & Omura, 2011). Previous studies have shown that
avermectins are efficient for the control of Culex quinquefasciatus Say (Freitas et al., 1996;
Noskov et al. (2019), PeerJ, DOI 10.7717/peerj.7931 2/23
Alves et al., 2004), Anopheles albimanus, Wiedemann An. stephensi Liston (Dreyer, Morin &
Vaughan, 2018), and An. gambiae Giles (Alout et al., 2014;Chaccour et al., 2017). However,
most of these studies have been carried out with adult mosquitoes feeding on blood
containing ivermectin. Avermectins exhibit a relatively short half-life period, which limits
their ability to kill mosquitoes but may be compensated by the application of multiple
treatments or use of higher concentrations. However, these approaches may contribute
to the development of mosquito larvae resistance to avermectins (Su et al., 2017). We
hypothesize that the interaction of entomopathogenic fungi with avermectins can have a
stable insecticidal effect at relatively low concentrations and is a promising combination
for safe and effective mosquito control.
It is important that entomopathogenic fungi such as Metarhizium are adapted to
terrestrial hosts and that in mosquito larvae, the fungi do not adhere to the cuticle
surface and do not germinate through integuments into the hemocoel. Conidia ingested
by mosquito larvae do not penetrate the gut wall (Butt et al., 2013). Thus, a ‘‘classic’’
host-pathogen interaction does not occur, and larval mortality is associated with stress
induced by spore-bound proteases on the surface of ingested conidia (Butt et al., 2013).
These authors suggest that fungal proteases cause an increase in the activity of caspases
in mosquitoes, which leads to apoptosis, autolysis of tissues and death of the larvae. The
activation of detoxifying enzymes and antimicrobial peptides (AMPs) occurs in larvae
infected with the fungus but is not sufficient to protect the larvae from death. As a rule,
mosquito larvae die showing symptoms of bacterial decomposition after treatment with
Metarhizium and Beauveria (Scholte et al., 2004). Therefore, this pathogenesis can be
considered mixed (both bacterial and fungal).
During this process, particular superfamilies of enzymes such as glutathione-S-
transferases (GST) and nonspecific esterases (EST) are usually involved in the biochemical
transformation of xenobiotics (Li, Schuler & Berenbaum, 2007). Various hormones such
as biogenic amines are involved in insect stress reactions. Among them, the role of the
neurotransmitter dopamine (which serves as a neurohormone as well) in this process
remain poorly understood. It is known that dopamine mediates phagocytosis and is
involved in the activation of the pro-phenoloxidase (proPO) cascade, thus playing an
important role in fungal and bacterial pathogenesis as well as in the development of
toxicoses caused by insecticides (Delpuech, Frey & Carton, 1996;Gorman, An & Kanost,
2007;Wu et al., 2015). In addition, both pathogens and toxicants can lead to changes in the
antimicrobial activity of insects and the bacterial load that can affect the susceptibility of
insects to pathogenic fungi (Wei et al., 2017;Ramirez et al., 2018;Polenogova et al., 2019).
It should be noted that the above mentioned physiological reactions in mosquito larvae
under the combined action of entomopathogenic fungi and insecticides have not yet been
studied.
The aims of this study were (1) to determine the susceptibility of Aedes aegypti larvae
to combined treatment with avermectins and Metarhizium robertsii and (2) estimate their
immune and detoxificative responses to M. robertsii and avermectins either alone or their
combination.
Noskov et al. (2019), PeerJ, DOI 10.7717/peerj.7931 3/23
MATERIALS & METHODS
Insecticides and fungi
The entomopathogenic fungus Metarhizium robertsii (strain MB-1) from the collection of
microorganisms of the Institute of Systematics and Ecology of Animals SB RAS was used in
this work. The conidia of the fungus were grown on autoclaved millet for 10 days at 26 ◦C
in the dark, followed by drying and sifting (Belevich et al., 2017). The industrial product
‘‘Phytoverm’’ 0.2% (SPC ‘‘Pharmbiomed’’, Russia) was used in these experiments and
includes a complex of natural avermectins (A1a (9%), A2a (18%), B1a (46%), B2a (27%))
produced by Streptomyces avermitilis.
Insect maintenance and toxicity tests
Aedes aegypti larvae from the collection of the Institute of Systematics and Ecology of
Animals SB RAS were maintained in tap water in the laboratory at 24 ◦C (±1◦C) under a
natural photoperiod (approximately 16:8 light:dark). The larvae were fed Tetramin Junior
fish food (Tetra, Germany). The susceptibility of Ae. aegypti to both avermectins and
the conidia of M. robertsii was tested in 200 ml plastic containers containing 100 ml of
water with 15 larvae. Third-4th-instar larvae were used in the experiment. The experiment
involved four treatments: control, fungus, avermectins, and fungus +avermectins. The
fungal conidia and avermectins were suspended in distilled water, vortexed and applied
separately or together to the containers with mosquito larvae at a volume of 2 ml per
container. The final conidial concentration for infection was 1×106conidia/ml. The final
concentration of avermectins was 0.00001%/ml. The control was treated with the same
amount of distilled water. Mortality was assessed daily for 6 days. Ten replicates with 15
larvae were performed for each treatment.
Light microscopy and colonization assessment
Forty-eight hours posttreatment (pt), mosquito larvae (n=3 for each treatment) were
collected in 2% glutaraldehyde containing 0.1 M Na-cacodylate buffer (pH 7.2) and were
maintained at 4 ◦C for 1–24 h. Semi-thin sections were stained with crystal violet and basic
fuchsin and were observed with a phase-contrast microscope (Axioskop 40, Carl Zeiss,
Germany).
To assess fungal colonization, 48 h pt and newly dead larvae (4–6 days pt) were cut
open, and their internal contents were squeezed onto a glass side. The contents were
examined for the presence/absence of hyphal bodies using light microscopy (n=30 for
each treatment). Newly dead larvae were placed on moistened filter paper in Petri dishes
(n=30) to determine the germination and surface sporulation of Metarhizium.
Total larval body supernatant
Mosquito larvae bodies of individual 4th-instar larvae of Ae. aegypti were collected in
50 µl of cool (+4◦C) 0.01 M PBS (50 mM, pH 7.4, 150 mM NaCl) with 0.1 mM N-
phenylthiourea (PTU) to measure GST, EST and acid protease activities or without PTU to
measure phenoloxidase (PO) activity. Then, the samples were sonicated in an ice bath with
three 10 s bursts using a Bandelin Sonopuls sonicator. The sample solution was centrifuged
Noskov et al. (2019), PeerJ, DOI 10.7717/peerj.7931 4/23
at 20.000 g for 5 min at +4◦C. The obtained supernatant was directly used to determine
enzyme activities.
Detection of phenoloxidase, glutathione-S-transferase and esterase
activity
The activities of PO, GST and EST were measured at 12 and 48 h after exposure (n=20
per treatment for each enzyme).
PO activity was assayed by using a method modified from that described by
Ashida & Söderhäll (1984). The PO activity of the larval homogenates was determined
spectrophotometrically on the basis of the formation of dopachrome at a wavelength of
490 nm. Aliquots of the samples (10 µl) were added to microplate wells containing 200 µl
of 10 mM 3.4-dihydroxyphenylalanine and incubated at 28 ◦C in the dark for 45 min. The
PO activity was measured kinetically every 5 min and the time point was chosen according
to Michaelis constant.
The activity of EST was measured using the method of Prabhakaran & Kamble (1995)
with some modifications. Aliquots of the samples (3 µl) were added to microplate wells
containing 200 µl of 0.01% p-nitrophenylacetate and incubated for 10 min at 28 ◦C. The
activity of EST was determined spectrophotometrically at a wavelength of 410 nm on the
basis of the formation of nitrophenyl. The EST activity was measured kinetically every
2 min and the time point was chosen according to Michaelis constant.
The measurement of GST activity was carried out according to the method of Habig,
Pabst & Jakoby (1974) with some modifications. Aliquots of the samples (7 µl) were
added to microplate wells containing 200 µl of 1 mM glutathione and 5 µl of 1 mM
2.4-Dinitrochlorobenzene and incubated at 28◦C for 12 min. The activity of GST was
determined spectrophotometrically on the basis of the formation of 5-(2.4-dinitrophenyl)-
glutathione at a 340 nm wavelength. The GST activity was measured kinetically every 3 min
and the time point was chosen according to Michaelis constant.
Enzymes activity was measured in units of the transmission density (1A) of the
incubation mixture during the reaction per 1 min and 1mg of protein. The protein
concentration in the samples was determined by the method of Bradford (1976). To
generate the calibration curve, bovine serum albumin was used.
Dopamine concentration measurements
Dopamine concentrations were measured at the 12 and 48 h pt in individual larval bodies
(n=10 per treatment). Mosquito larvae were homogenized in 30 µl of phosphate buffer
and incubated in a Biosan TS 100 Thermoshaker for 10 min at 28 ◦C and 600 rpm, then
incubated at room temperature for 20 min and centrifuged at 4 ◦C and 10.000 g for 10 min.
The supernatants were transferred to clean tubes and centrifuged with the same settings
for 5 min. Before transfer to the chromatograph, the samples were filtered.
Dopamine concentrations were measured by an external standard method using an
Agilent 1,260 Infinity high-performance liquid chromatograph with an EsaCoulochem
III electrochemical detector (cell model 5010A, potential 300 mV) according to the
method of Gruntenko et al. (2005) with some modifications. Dopamine hydrochloride
Noskov et al. (2019), PeerJ, DOI 10.7717/peerj.7931 5/23
(Sigma-Aldrich) was used as a standard. Separation was performed in a ZorbaxSB-C18
column (4.6–250 mm, particles 5 µm) in isocratic mode. Mobile phase: 90% buffer (200
mg/l 1-OctaneSulfonicAcid (Sigma-Aldrich), 3.5 g/l KH2PO4) and 10% acetonitrile. The
flow rate was 1 ml/min. Chromatogram processing was performed using ChemStation
software, and the amount of dopamine was determined by comparing the peak areas of
the standard and the sample.
Acid proteases
Acid protease activity was measured using method described by Anson (1938) with
modifications. Fifty µl of the homogenate supernatant was added to 250 µl of 0.1 M
acetate buffer (pH 4.6) containing 0.3% hemoglobin (Sigma, CAS number 9008-02-0).
The samples were incubated for 60 min at 27 ◦C, and the reaction was stopped by adding
500 µl of 5% TCA and cooling on ice for 10 min at 4 ◦C. The samples were centrifuged at
14,000 g for 5 min at 4 ◦C, and the enzyme activity was determined spectrophotometrically
at a wavelength of 280 nm in a 96-well plate reader.
Lysozyme-like activity
Lysozyme-like activity in the mosquito homogenate was determined through analysis of
the lytic zone by diffusion into agar. Ten milliliters of Nutrient Agar (NA) (HiMedia,
India) and Micrococcus lysodeikticus bacteria (1×107cells/ml) were added to Petri dishes.
The agar was perforated to create 2 mm-diameter wells, which were then filled with 3 µl
of full-body homogenate, followed by incubation at 37 ◦C for 24 h. Series of dilutions
of chicken egg white lysozyme (EWL) (Sigma) (0.5 mg/ml, 0.2 mg/ml, 0.1 mg/ml, 0.005
mg/ml, 0.001 mg/ml) were added to each dish, allowing us to obtain a calibration curve
based on these standards. Lytic activity was determined by measuring the diameter of the
clear zone around each well and expressed as the equivalent of EWL (mg/ml) (Mohrig &
Messner, 1968).
CFU counts of Metarhizium and cultivated bacteria in infected larvae
Homogenates of the mosquitoes (3 larvae per sample) were suspended in 1 ml of sterile
aqueous Tween-20 (0.03%), and the suspensions were then diluted 50-fold. Next, 100 µl
aliquots were inoculated onto the surface of modified Sabouraud agar (10 g peptone, 40 g
D-glucose anhydrous, 20 g agar, 1 g yeast extract) supplemented with an antibiotic cocktail
(acetyltrimethyl ammonium bromide 0.35 g/L; cycloheximide 0.05 g/L; tetracycline 0.05
g/L; streptomycin 0.6 g/L; PanReacAppliChem, Germany) for the inhibition of bacteria
and saprotrophic fungi. The Petri dishes were maintained at 28 ◦C in the dark. The colonies
were then counted after 7 days.
For the estimation of cultivated bacterial CFU counts, homogenates of larvae (3 larvae
per sample) were suspended in 1 ml of 0.1 M phosphate buffer. Then, the suspension was
diluted to 10−2, 10−3and 10−4. Aliquots of 100 µl of the larval dilutions were inoculated
onto the surface of blood agar media (HiMedia, Mumbai, India). The Petri dishes were
maintained at 28 ◦C. The colonies were counted after 48 h. Three samples of each treatment
were used in the analysis.
Noskov et al. (2019), PeerJ, DOI 10.7717/peerj.7931 6/23
Statistical analysis
Data were analyzed using GraphPad Prism v.4.0 (GraphPad Software Inc., USA), Statistica 8
(StatSoft Inc., USA), PAST 3 (Hammer, Harper & Ryan, 2001) and AtteStat 12.5 (Gaidyshev,
2004). Differences between synergistic and additive effects were determined by comparing
the expected and observed insect mortality using the χ2criterion (Robertson & Preisler,
1992). The expected mortality from dual treatment was calculated by the formula PE
=P0+(1 −P0)×(P1)+(1 −P0)×(1 −P1)×(P2), where PEis the expected
mortality after combined treatment with fungus and avermectins, P0is mortality in the
control groups, P1is the mortality posttreatment with M. robertsii, P2is the mortality
posttreatment with avermectins. The χ2values were calculated by the formula χ2=(L0
−LE)2/LE+(D0−DE)2/DE, where L0is the observed number of survived larvae, LEis
the expected number of surviving larvae, D0is the observed number of dead larvae, and
DEis the expected number of dead larvae. This formula was used to test the hypothesis
of independence (1 df: P=0.05). Additive effect was indicated if χ2< 3.84. A synergistic
effect was indicated if χ2> 3.84 and observed mortality greater than the expected one. A
value of 3.84 corresponds to P<0.05 with a degree of freedom =1. The Kaplan–Meier
test was used to calculate the median lethal time (presented as LT50 ±SE). A log-rank
test was used to quantify differences in mortality dynamics. As the distribution of the
physiological parameters except for the dopamine concentration deviated from a normal
distribution (Shapiro–Wilk test, P<0.05), we used the nonparametric equivalent of a
two-way ANOVA: the Scheirer-Ray-Hare test (Scheirer, Ray & Hare, 1976), followed by
Dunn’s post hoc test. The data on dopamine concentrations passed the normality test
(Shapiro–Wilk test, P>0.05) and were analyzed by two-way ANOVA followed by Tukey’s
post hoc test. Differences between Metarhizium CFU counts were compared by t-tests.
RESULTS
Synergy between avermectins and the fungus
Significant differences in the dynamics of larval mortality between the treatments were
observed (log-rank test: χ2=397.3, df =3, P<0.0001; Fig. 1). Treatment with avermectins
or conidia of M. robertsii led to 57 and 55% mortality, respectively, whereas combined
treatment led to 99% mortality at the 6th day pt. Mortality in the control treatment did not
exceed 1%. The median lethal time post-combined treatment (3 ±0.1 d) occurred twice
as fast as under treatment with avermectins (6 ±0.3 d) or the fungus (6 d ±inf.) alone
(χ2>113.7, df =1, P<0.0001).
From the 2nd to the 6th day pt, the avermectins and fungus interacted synergistically
(χ2>18.5, df =1, P<0.001, ESM Table S1). These effects were consistently observed in
four independent experiments.
Colonization assay
At 48 h pt, we observed mass accumulation of Metarhizium conidia in the gut lumen
(Fig. 2A). Germinated conidia were not detected in larvae at 48 h pt (n=12). However, in
one sample (combined treatment), hyphal bodies were detected in the hemocoel (Fig. 2A).
In the newly dead larvae after the fungal and combined treatments (4–6 days), we detected
Noskov et al. (2019), PeerJ, DOI 10.7717/peerj.7931 7/23
Days posttreatment
Mortality, %
1 2 3 4 5 6
0
20
40
60
80
100 Control
Avermectins
Fungus
Avermectins+Fungus
*
*
***
Figure 1 Mortality dynamics of Ae. aegypti larvae after treatment with M. robertsii (1 ×106conidi-
a/ml), avermectins (0.00001%) and their combination. The control was treated with distilled water. The
asterisks (*) indicate a synergistic effect (χ2>18.5, df =1, P<0.001, see Table S1).
Full-size DOI: 10.7717/peerj.7931/fig-1
colonization of the hemocoel with hyphal bodies (Fig. 2B). Under the combined treatment,
83% hypha-positive larvae were found, while in the fungal treatment, 90% hypha-positive
larvae were recorded. No significant differences between these treatments were observed
(χ2=0.58, df =1, P=0.45, n=30 larvae per treatment). No hyphal bodies were detected
in the fungus-free treatments. A total of 70% and 60% of larvae were overgrown with
Metarhizium under incubation in moist chambers (Fig. 2C) after treatment with the
fungus or the mixture (avermectins +fungus), respectively. Only nongerminated conidia,
but no hyphal bodies, were detected in the water in which treated larvae were maintained
(Fig. 2D).
Phenoloxidase activity
At 12 h pt, we registered a significant decrease in PO activity under the influence of fungal
infection (Scheirer-Ray-Hare test, effect of fungus: H1.52 =12.6, P=0.00038; Fig. 3).
Avermectins did not significantly change PO activity (H1.52 =0.3, P=0.54). A stronger
decrease in enzyme activity was observed after combined treatment, but a significant factor
interaction was not revealed (H1.52 =1.9, P=0.16). At 48 h pt, we detected a significant
increase in PO activity under the influence of avermectins (H1.32 =5.33, P=0.02). The
effect of the fungus was not significant (H1.32 =0.7, P=0.39), but a tendency toward
an interaction between the factors was revealed (H1.32 =3.2, P=0.07). This is explained
by the inhibition of PO activity by the fungus alone (Dunn’s test, P=0.01, P=0.04,
compared to the control and avermectin treatments, respectively) and by the tendency of
increased enzyme activity after combined treatment.
Dopamine concentration
The effects of the fungus or avermectins on the dopamine concentration at 12 h pt were not
significant (F1.31 =1.2, P=0.27; Fig. 4), although a trend toward a factor interaction was
revealed (F1.31 =3.3, P=0.07). This was due to a clear tendency to decrease the dopamine
concentration after treatment with the fungus alone (HSD Tukey test, P=0.07 compared
to fungus-free treatments) but not with the combination of the fungus and avermectins
Noskov et al. (2019), PeerJ, DOI 10.7717/peerj.7931 8/23
Figure 2 The colonization of Ae. aegypti by M. robertsii.(A) Accumulation of conidia in the gut and
colonization of the hemocoel by hyphal bodies. (B) Colonization of the fat body. (C–E) Mosquito larvae
with surface conidiation of Metarhizium in a moist chamber. (F) Nongerminated conidia in a sample of
water in which infected larvae were maintained. Scale bar: 20 µm.
Full-size DOI: 10.7717/peerj.7931/fig-2
0.0
0.2
0.4
0.6 Control
Avermectins
Fungus
Avermectins+Fungus
A490 / min / mg protein
12
ab
c
a
bc
48
Hours posttr eatment
a
b
a
ab
Figure 3 Activity of PO in the whole-body homogenates of Ae. aegypti larvae after treatment with M.
robertsii, avermectins and their combination. In the control treatment, equal amounts of water were
added. Error bars represent the standard error of the mean. Significant differences are indicated with dif-
ferent letters within one time point (Dunn’s test, P<0.05).
Full-size DOI: 10.7717/peerj.7931/fig-3
(P=0.47 compared to fungus-free treatments). At 48 h pt, we observed a significant
decrease in the dopamine concentration under the influence of the fungus (F1.31 =4.62,
P=0.03); however, there were no significant differences between the treatments (HSD
Noskov et al. (2019), PeerJ, DOI 10.7717/peerj.7931 9/23
ng / 10 ml
0
10
20
30
ab ab
a
ba
a
aa
12 48
Hours posttr eatment
Control
Avermectins
Fungus
Avermectins+Fungus
Figure 4 Dopamine concentration in whole-body homogenates of Ae. aegypti larvae after treatment
with M. robertsii, avermectins and their combination. In the control treatment, equal amounts of water
were added. Error bars represent the standard error of the mean. Significant differences are indicated by
different letters within one time point (HSD Tukey test, P<0.05).
Full-size DOI: 10.7717/peerj.7931/fig-4
Tukey test, p=0.1). No significant interaction effects between the factors on the dopamine
concentration at 48 h pt were detected (F1.31 =0.0, P=1.0).
Detoxifying enzymes
At 12 h pt, an interaction effect between the two factors (avermectins and the fungus)
on GST activity was observed (H1.44 =8.1, P=0.0043, Fig. 5A). Avermectins and the
fungus alone significantly (1.5–2-fold) increased GST activity compared to untreated
larvae (Dunn’s test, P=0.007, P=0.001, respectively), but after combined treatment,
the enzyme activity did not significantly differ from that in the control. Similar patterns
were registered for EST activity at 12 h pt (Fig. 5B). In this case, EST was activated under
the influence of avermectins alone (Dunn’s test, P=0.002, compared to control), but
fungal infection inhibited this activation. In particular, EST activity in the fungal and
combined treatments did not differ from that in the control (Dunn’s test, P=0.28,
P=0.34, respectively).
At 48 h pt, we observed a significant increase in GST activity under the influence of
avermectins (H1.32 =4.2, P=0.03). The effect of the fungus as well as the interaction
between the factors on GST activity at this time point was not significant (H1.32 =2.5,
P=0.1 and H1.32 =0.61, P=0.43, respectively). EST activity nonsignificantly increased
under the influence of avermectins (H1.32 =1.85, P=0.17). The effect of the fungus on
enzyme activity was not significant (H1.32 =0.001, P=0.96), and no significant interactions
between the factors were detected (H1.32 =0.49, P=0.48).
Acid protease activity
At 12 h pt, a significant interaction between avermectins and the fungus on acid protease
activity was observed (H1.48 =14.8; P=0.00011; Fig. 6). In particular, protease activity was
increased after fungal treatment alone (3-fold compared to control, P=0.0003) but not
after combined treatment. At 48 h pt, protease activity was strongly increased under the
influence of avermectins (H1.43 =27.4, P=0.000016), and a trend toward an increase in
Noskov et al. (2019), PeerJ, DOI 10.7717/peerj.7931 10/23
A340 / min / mg pr otein
0.0
0.5
1.0
1.5
a
ab
b
b
a
bb
ab
12 48
Control
Avermectins
Fungus
Avermectins+Fungus
Hours posttr eatment
A412 / min / mg protein
0
2
4
6
a
b
aa
a
ab
ab
b
12 48
A
B
Figure 5 GST (A) and EST (B) activity in whole-body homogenates of Ae. aegypti larvae after treat-
ment with M. robertsii, avermectins and their combination. In the control treatment, equal amounts
of water were added. Error bars represent the standard error of the mean. Significant differences are indi-
cated by different letters within one time point (Dunn’s test, P<0.05).
Full-size DOI: 10.7717/peerj.7931/fig-5
enzyme activity was registered under the influence of the fungus (H1.43 =3.18, P=0.07).
No significant interaction between the factors was revealed, although a trend toward the
highest increase in protease activity was registered after the combined treatment.
Lysozyme-like activity
At 12 h pt, we recorded a decrease in lysozyme-like activity under the influence of both
fungal infection and avermectins (effect of fungus: H1.56 =6.46, P=0.011; effect of
avermectins: H1.56 =14.04, P=0.00017; Fig. 7). The greatest decrease was observed after
the combined treatment (Dunn’s test, P<0.001, compared with the other treatments). At
48 h pt, a sharp (1.7–2.0-fold) increase in lysozyme-like activity was recorded under the
influence of avermectins (H1.116 =68.6, P<0.0000001). The effect of the fungus on the
level of the enzyme at 48 h pt was not significant. No significant interaction effect between
the factors on the level of lysozyme was observed at 12 and 48 h pt (H=0.23, P=0.63).
Fungal and bacterial CFUs
The plating of mosquito larval homogenates on modified Sabouraud agar showed
significant differences in the Metarhizium CFUs between treatment with the fungus
either alone or combined with avermectins (Fig. 8A). The Metarhizium CFU count in the
Noskov et al. (2019), PeerJ, DOI 10.7717/peerj.7931 11/23
A280 / min / mg protein
0
1
2
3
4
5
a
b
a
a
Control
Avermectins
Fungus
Avermectins+Fungus
a
b
b
a
12 48
Hours posttreatment
Figure 6 Acid protease activity in whole-body homogenates of Ae. aegypti larvae after treatment with
M. robertsii, avermectins and their combination. In the control treatment, equal amounts of water were
added. Error bars represent the standard error of the mean. Significant differences are indicated by differ-
ent letters within one time point (Dunn’s test, P<0.05).
Full-size DOI: 10.7717/peerj.7931/fig-6
EWL equivalent mg / ml
0.000
0.001
0.002
0.003
0.004
a
b
a
a
aa
bb
12 48
Hours posttr eatment
Control
Avermectins
Fungus
Avermectins+Fungus
Figure 7 Lysozyme-like activity in whole-body homogenates of Ae. aegypti larvae after treatment with
M. robertsii, avermectins and their combination. In the control treatment, equal amounts of water were
added. Error bars represent the standard error of the mean. Significant differences are indicated by differ-
ent letters within one time point (Dunn’s test, P<0.05).
Full-size DOI: 10.7717/peerj.7931/fig-7
fungal treatment was twice as high as that in the combined treatment (t=6.4, df =18,
P=0.001). Homogenates of the larvae from the fungus-free treatments (avermectin alone
and control) did not form any fungal colonies.
The plating of larval homogenates on blood agar showed a significant (17–75-fold)
increase in bacterial CFUs after treatment with the fungus and avermectins. A significant
effect was registered for avermectins (H1.19 =4.8, P=0.03; Fig. 8B) but not for the
fungus (H1.19 =1.9, P=0.17). However, a clear tendency toward an increase in CFUs
was registered after treatment with the fungus alone (Dunn’s test, P=0.054, compared to
control). No significant interaction effect between the fungus and avermectins on bacterial
CFUs was revealed (H1.19 =1.9, P=0.17).
Noskov et al. (2019), PeerJ, DOI 10.7717/peerj.7931 12/23
CFUs per one larvae
Fungus Avermectins+Fungus
0
50000
100000
150000
CFUs per one larvae
Control Avermectins Fungus Av.+Fungus
100
102
104
106
108
101 0
A B
b
aa
bbab
Figure 8 Colony forming units of M. robertsii (A) and cultivable bacteria (B) in whole-body
homogenates of Ae. aegypti larvae after treatment with M. robertsii, avermectins and their
combination. Error bars show min and max values. Significant differences are indicated by different
letters (t-test, P<0.001, for fungal CFU, and Dunn’s test, P<0.05, for bacterial CFU).
Full-size DOI: 10.7717/peerj.7931/fig-8
DISCUSSION
We showed a synergistic effect between avermectins and Metarhizium fungi on aquatic
invertebrates for the first time. A similar effect was shown previously only in terrestrial
insects (Colorado potato beetle, cotton moth) (Anderson et al., 1989;Asi et al., 2010;
Tomilova et al., 2016), which are characterized by a completely different mode of fungal
penetration (through the exo-skeleton). The accumulation of conidia of the fungus mainly
in the gut lumen of mosquitoes coincides with studies of other researchers (Butt et al.,
2013). However, we report the first observation of colonization of Ae. aegypti larvae after
inoculation with Metarhizium conidia. It was previously suggested that only blastospores
(and not conidia) are able to germinate from the gut lumen into the hemocoel of mosquito
larvae (Alkhaibari et al., 2016;Alkhaibari et al., 2018). Interestingly, the larvae treated with
avermectins accumulated a lower amount of conidia, but this dose was sufficient for a
synergistic effect on mortality. It is likely that reduced accumulation of conidia was due
to disturbance of feeding. For example, decrease in quantity of consumed food under the
influence of avermectins was shown for terrestrial insects (Akhanaev et al., 2017).
We observed a decrease in PO activity and dopamine levels under the influence of the
fungus, whereas in terrestrial arthropods, these enzymes are activated during mycoses
(Ling & Yu, 2005;Yassine, Kamareddine & Osta, 2012;Yaroslavtseva et al., 2017;Chertkova,
Grizanova & Dubovskiy, 2018). It has been suggested that dopamine release is associated
with the general stress reactions related to the insect’s responses to pathogens (Hirashima,
Sukhanova & Rauschenbach, 2000;Chertkova, Grizanova & Dubovskiy, 2018). In addition,
dopamine is involved in the modulation of energetic metabolism and general defense
mechanisms such as phagocytosis (Wu et al., 2015). PO is involved in the inactivation of
fungal propagules in the cuticle and hemocoel (Butt et al., 2016). Dopamine is involved in
the PO cascade (Andersen, 2010); however, synchronous and unidirectional changes in the
levels of PO and dopamine are not always observed during infections (E Chertkova, 2016,
personal observations). Since we observed differentiation of fungal infection structures,
Noskov et al. (2019), PeerJ, DOI 10.7717/peerj.7931 13/23
we suggest that some fungal metabolites inhibit the PO cascade of Ae. aegypti larvae. It was
shown on terrestrial insects that Metarhizium secondary metabolites (e.g., destruxins) may
reduce the number of PO-positive hemocytes (Huxham, Lackie & McCorkindale, 1989)
and these metabolites may upregulate serine protease inhibitors, which inhibit proPO
cascade (Pal, Leger & Wu, 2007). Alkhaibari et al. (2018) noted a short-term increase in PO
activity in the whole-body homogenates of Culex quinquefasciatus larvae after infection
with conidia or blastospores of M. brunneum Petch (4–6 h pt). It is possible that this
reaction depends on species of mosquitoes as well as strain of the pathogen. Especially,
inhibition of hemolymph melanization under M. robertsii infections was dependent from
the production of secondary metabolites by different strains (Wang et al., 2012).
We observed activation of detoxifying enzymes (GST, EST) in Ae. aegypti larvae at the
early stages of toxicosis and infection (12 h pt) under mono-treatments with the fungus
and avermectins. However, the combined treatment led to inhibition of the activation of
GST and EST. A similar effect was observed at 12 h pt for antibacterial (lysozyme-like)
and acid protease activities. Combined treatment leads to either inhibition or containment
of the activation of these enzymes. GST and EST are used by insects to inactivate toxic
products formed by insecticide-induced toxicoses (DeSilva et al., 1997;Boyer et al., 2006;
Aponte et al., 2013) as well as under mycosis (Dubovskiy et al., 2012). Especially Tang et al.
(2019) showed that up-regulation of GSTz2 decreased the susceptibility of tephritid fruit fly
Bactrocera dorsalis (Hendel) to abamectin. Moreover, GST may participate in inactivation
of fungal secondary metabolites (Loutelier, Cherton & Lange, 1994) and reactive oxygen
species (Sherratt & Hayes, 2002). Lysozyme inhibits the reproduction of Gram-positive
bacteria (Abdou et al., 2007;Gandhe, Janardhan & Nagaraju, 2007;Chapelle et al., 2009),
which (e.g., Microbacteriaceae) are among the dominant bacteria in Ae. aegypti larvae (Coon
et al., 2014). It should also be noted that at 12 h pt of Ae. aegypti with blastospores of M.
brunneum, a decrease in the expression levels of genes encoding defensins and cecropins
(Alkhaibari et al., 2016), which inhibit the growth of both Gram-positive and Gram-
negative bacteria and fungi, was observed (Jozefiak & Engberg, 2017). The inhibition of acid
protease activity under combined treatment may indicate disorders in food consumption
and absorption. Disruption of food absorption and starvation can increase mortality
from both fungi and insecticides (Furlong & Groden, 2003). Thus, we assume that the
physiological causes of the observed synergism lie in the initial stages of the development
of infection and toxicosis.
In the later stages (48 h pt), we mainly observed activation of the enzymes (PO, GST, EST,
acid proteases and lysozyme-like activity), which apparently indicates destructive processes
in tissues and organs under the action of both avermectins and fungi. The increase in
PO activity on the second day after treatment with avermectins was probably due to the
destruction of hemocytes and the release of intracellular proPO components. We have
previously shown the cytotoxic effect of avermectins on hemocytes, leading to their death
(Tomilova et al., 2016). Additionally, the cytostatic and cytotoxic effects of the avermectins
complex on various cells of warm-blooded animals are well known (Sivkov, Yakovlev &
Chashov, 1998;Kokoz et al., 1999;Korystov et al., 1999;Maioli et al., 2013). Increase in PO
activity under the influence of avermectins could also be symptom linked with proliferation
Noskov et al. (2019), PeerJ, DOI 10.7717/peerj.7931 14/23
of bacteria (Fig. 8). The enhancement of PO is observed under development of various
bacterioses and caused by damages of insect’s tissues as well as by recognition of bacterial
cell wall compounds, formation of hemocyte nodules and their melanization (Bidla et al.,
2009;Tokura et al., 2014;Dubovskiy et al., 2016). An increase in GST under mycoses usually
correlates with the severity of the infectious process (Dubovskiy et al., 2012;Tomilova et al.,
2019) and confirms the results obtained by Butt et al. (2013) when studying the pathogenesis
of M. brunneum in Ae. aegypti larvae. An increase in lysozyme-like activity under the action
of avermectins could have occurred due to tissue destruction accompanied by the release
of lysosome contents containing lysozyme (Zachary & Hoffmann, 1984). The activation of
proteases at 48 h pt under the influence of avermectins is correlated with the increase in
PO and lysozyme-like activity, which also indicates destructive changes in the tissues.
We observed an increase in the number of cultivated bacteria in the larvae when treated
with both avermectins and fungi. This effect may be associated with impaired intestinal
peristalsis as well as changes in the level of PO and antibacterial activity in the initial stages
of toxicosis and fungal infection. Similar effects have been observed in terrestrial insects
following topical infection by fungi (Wei et al., 2017;Ramirez et al., 2018;Polenogova et al.,
2019) and are associated with the redistribution of immune responses between the cuticle
and the gut. In mosquito larvae, the fungus comes into direct contact with gut microbiota,
which may exhibit fungistatic properties (Sivakumar et al., 2017;Zhang et al., 2018, etc.)
or, alternatively, may act as synergists of fungi, as shown by Wei et al. (2017) in the adults
of the mosquito Anopheles stephensi. It is possible that conflicting data on the colonization
of mosquito larvae by Metarhizium fungi are associated with differences in bacterial
communities, which requires further research. In any case, fungi cannot successfully
complete colonization in an aquatic environment, and bacterial decomposition is observed
in mosquito larvae, whereas surface conidiation occurs in the air environment.
CONCLUSIONS
In conclusion, this is the first study of the survival and physiological reactions of mosquito
larvae under the combined action of avermectins and entomopathogenic fungi. The
synergism observed under the combined action of these agents appears to be associated
with physiological changes in the early stages of toxicosis and infection. In particular,
inhibition of the activity of a number of enzymes is observed under the combined treatment
associated with the detoxifying and immune systems.
Colonization of Aedes aegypti larvae by the fungus Metarhizium robertsii is shown for
the first time in this study. Further investigations may be focused on studying the role of
endosymbiotic mosquito bacteria in the development of toxicoses and mycoses as well
as the development of preparative forms based on fungi and avermectins for mosquito
control in natural conditions.
ACKNOWLEDGEMENTS
The authors are grateful to Dr. V.A. Shilo (Karasuk Station of the ISEA) for help in
organizing the experiments, to Dr. AA Alekseev for determining the exact ratio of
Noskov et al. (2019), PeerJ, DOI 10.7717/peerj.7931 15/23
avermectins and their isomers in an industrial product ‘‘Phytoverm’’ using HPLC, to
Dr. AA Miller for preparation of semi-thin sections of the mosquito larvae.
ADDITIONAL INFORMATION AND DECLARATIONS
Funding
This work was supported by the Russian Science Foundation (project No. 18-74-00090).
The funders had no role in study design, data collection and analysis, decision to publish,
or preparation of the manuscript.
Grant Disclosures
The following grant information was disclosed by the authors:
Russian Science Foundation: 18-74-00090.
Competing Interests
The authors declare there are no competing interests.
Author Contributions
•Yuriy A. Noskov conceived and designed the experiments, performed the experiments,
analyzed the data, prepared figures and/or tables, approved the final draft.
•Olga V. Polenogova performed the experiments, authored or reviewed drafts of the
paper, approved the final draft.
•Olga N. Yaroslavtseva, Olga E. Belevich, Yuriy A. Yurchenko and Ekaterina A. Chertkova
performed the experiments, authored or reviewed drafts of the paper, approved the final
draft.
•Natalya A. Kryukova analyzed the data, authored or reviewed drafts of the paper,
approved the final draft.
•Vadim Yu Kryukov conceived and designed the experiments, analyzed the data, prepared
figures and/or tables, authored or reviewed drafts of the paper, approved the final draft.
•Viktor V. Glupov conceived and designed the experiments, contributed reagents/-
materials/analysis tools, authored or reviewed drafts of the paper, approved the final
draft.
Data Availability
The following information was supplied regarding data availability:
The raw data of the survival, dopamine concentration, glutathione S-transferase,
nonspecific esterases, acid proteases, lysozyme-like and phenoloxidase activities are
available in the Supplemental Files.
Supplemental Information
Supplemental information for this article can be found online at http://dx.doi.org/10.7717/
peerj.7931#supplemental-information.
Noskov et al. (2019), PeerJ, DOI 10.7717/peerj.7931 16/23
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