Effects of inorganic fertilizer on larval development, adult longevity and insecticide susceptibility in the malaria vector Anopheles arabiensis (Diptera: Culicidae)

The effect of larval exposure to acids and detergents on the life history of the major malaria vector Anopheles arabiensis Patton (Diptera: Culicidae)

BACKGROUND

Anopheles arabiensis, a highly adaptable member of the Anopheles gambiae complex, poses a challenge for control efforts due to its outdoor biting and resting behaviour. Consequently, indoor insecticide-based control methods are ineffective against An. arabiensis. Furthermore, An. arabiensis are adapting to breeding in polluted waters, and may be contributing to residual malaria and malaria in urban areas. There have been some advances in understanding the effect of rural pollutants on Anopheles mosquitoes, but the effect of urban pollutants is poorly understood. Thus, in this study, the effect of acidic pollutants [nitric acid (HNO3) and hydrochloric acid (HCl)] and alkaline pollutants (phosphate-free and phosphate-containing detergent) on two laboratory-reared An. arabiensis strains - an insecticide susceptible strain (SENN) and an insecticide-resistant strain selected from SENN (SENN-DDT) - were determined.

RESULTS

The median lethal concentration (LC50) and larval exposure on larval development, adult longevity and insecticide tolerance were evaluated. Nitric acid and phosphate-containing detergent were found to be more toxic than HCl and phosphate-free detergent in terms of LC50 values. Detergent exposure (both phosphate-containing and phosphate-free) increased adult longevity of both strains. Nitric acid reduced larval development time in both SENN and SENN-DDT, whereas HCl reduced larval development time in SENN only. By contrast, both phosphate-containing and phosphate-free detergents increased larval development time of both strains. Furthermore, HNO3 and phosphate-containing detergent increased insecticide tolerance the most.

CONCLUSION

The two An. arabiensis strains responded to urban pollutants differently. Thus, this study provides insight into the adaptation of An. arabiensis to acidic and alkaline urban pollutants. © 2024 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

The influence of manure-based organic fertilisers on the oviposition behaviour of Anopheles arabiensis

The rice agroecosystem provides suitable breeding habitat for many malaria vector species, and rice-adjacent communities are consequently exposed to a greater malaria transmission risk than non-rice-associated communities. As part of efforts to expand rice production in Africa, sustainable and climate-adapted practices such as the System of Rice Intensification (SRI) are being promoted. SRI encourages the use of organic fertilisers (OFs) such as cow and chicken dung, as opposed to inorganic industrially produced fertilisers, due to their lower resource cost, apparent benefit to the rice agroecosystem and as a means to reduce the greenhouse gas emissions associated with the production of industrial fertilisers. However, the impact of OFs on mosquito fauna is not well documented and may have knock-on consequences on malaria transmission risk. Here, we demonstrate, using dual choice egg count assays, that both cow and chicken dung modulate the oviposition behaviour of Anopheles arabiensis, a major malaria vector in Sub-Saharan Africa. A significantly reduced proportion of eggs were laid in water treated with either cow or chicken dung compared to untreated water, with higher dung concentrations resulting in further reduced proportions. When presented in competition, significantly fewer eggs were laid in water treated with chicken dung than with cow dung. Moreover, there was no evidence of egg retention in any experiment, including in no-choice experiments where only dung-containing dishes were available. These results suggest both cow and chicken dung may act as oviposition deterrents to malaria vector species and that the application of manure-based OFs in rice agriculture may modulate the oviposition behaviour of An. gambiae s.l. within agroecosystems. Quantification of the ammonia present in dung-infused water showed higher concentrations were present in the chicken dung infusion, which may be one contributing factor to the difference in observed deterrence between the two dung types. Deterrence of mosquito oviposition in OF-treated farms may potentially affect the overall production of malaria vectors within rice fields and their contribution to local malaria transmission.

Marked Effects of Larval Salt Exposure on the Life History and Gut Microbiota of the Malaria Vector Anopheles merus (Diptera: Culicidae)

Anopheles merus can breed in a range of saltwater concentrations. The consequences of this ability on the life history of adult An. merus are poorly understood. This study examined the effects of exposure to 0, 2.1875, 4.375, 8.75, and 17.5 g/L of sodium chloride on An. merus. The effects on larval development, adult longevity, fertility, and fecundity, as well as deltamethrin tolerance were examined. The effect of larval salt exposure on the expression of defensin-1 in adults was examined by quantitative Real-Time PCR. Finally, the effect of the larval salt concentration on microbial dynamics was assessed by 16S Next Generation Sequencing. High concentrations of saltwater increased larval development time and number of eggs laid, as well as deltamethrin tolerance. Larval exposure to salt also reduced the expression of defensin-1. The exposure also had a significant effect on microbial diversity in larvae and adults. The diversity of larvae decreased once adults emerged. Salt-tolerant bacterial genera predominated in larvae but were absent in adults. High salt concentrations resulted in greater abundance of Plasmodium-protective genera in adults. Although this study was conducted on a laboratory strain of An. merus, these data suggest that osmoregulation has a significant effect on the life history of the species with potential epidemiological consequences.

Manure and mosquitoes: life history traits of two malaria vector species enhanced by larval exposure to cow dung, whilst chicken dung has a strong negative effect

BACKGROUND

Malaria vectors have a strong ecological association with rice agroecosystems, which can provide abundant aquatic habitats for larval development. Climate-adapted rice cultivation practices, such as the System of Rice Intensification (SRI), are gaining popularity in malaria-endemic countries seeking to expand rice production; however, the potential impact of these practices on vector populations has not been well characterised. In particular, SRI encourages the use of organic fertilisers (OFs), such as animal manures, as low-cost and environmentally friendly alternatives to industrially produced inorganic fertilisers. We therefore set out to understand the effects of two common manure-based OFs on the life history traits of two major African malaria vectors, Anopheles arabiensis and Anopheles gambiae sensu stricto (s.s.).

METHODS

Larvae of An. arabiensis and An. gambiae s.s. were reared from first instar to emergence in water containing either cow or chicken dung at one of four concentrations (0.25, 0.5, 0.75, and 1.0 g/100 ml), or in a clean water control. Their life history traits were recorded, including survival, development rate, adult production, and adult wing length.

RESULTS

Exposure to cow dung significantly increased the development rate of An. gambiae s.s. independent of concentration, but did not affect the overall survival and adult production of either species. Chicken dung, however, significantly reduced survival and adult production in both species, with a greater effect as concentration increased. Interestingly, An. arabiensis exhibited a relative tolerance to the lowest chicken dung concentration, in that survival was unaffected and adult production was not reduced to the same extent as in An. gambiae s.s. The effects of chicken dung on development rate were less clear in both species owing to high larval mortality overall, though there was some indication that it may reduce development rate. Adult wing lengths in males and females increased with higher concentrations of both cow and chicken dung.

CONCLUSIONS

Our findings suggest that manure-based OFs significantly alter the life history traits of An. gambiae s.s. and An. arabiensis. In both species, exposure to cow dung may improve fitness, whereas exposure to chicken dung may reduce it. These findings have implications for understanding vector population dynamics in rice agroecosystems and may inform the use of OFs in SRI, and rice agriculture more widely, to avoid their adverse effects in enhancing vector fitness.

High nitrogen in maize enriches gut microbiota conferring insecticide tolerance in lepidopteran pest Spodoptera litura

Abuse of chemical fertilizers and insecticides has created many environmental and human health hazards. We hypothesized that high nitrogen (N) in crops changes insect gut microbiota leading to enhanced insecticide tolerance. We investigated the effect of high N in maize on gut microbiota and insecticide tolerance of the polyphagous pest Spodoptera litura. Bioassays showed that high N applied in both maize plants and artificial diets significantly enhanced larval growth but reduced larval sensitivity to the insecticide methomyl. High N promoted the gut bacterial abundance in the genus Enterococcus. Inoculation with two strains (E. mundtii and E. casseliflavus) isolated from the larval guts increased larval tolerance to methomyl. Incorporation of antibiotics in a high-N diet increased the larval sensitivity to methomyl. These findings suggest that excessive application of N fertilizer to crops can increase insecticide tolerance of insect pests via changing gut microbiota, leading to increased use of insecticides worldwide.

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High N applied in maize plants enhances insect tolerance to the insecticide methomyl

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High N promotes the gut bacterial proliferation in the genus Enterococcus

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Two gut bacterial strains (E. mundtii and E. casseliflavus) degrade methomyl

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Depleting the gut microbiota in S. litura increased larval sensitivity to methomyl

Transcriptome profiling reveals the developmental regulation of NaCl-treated Forcipomyia taiwana eggs

Background

The biting midge, Forcipomyia taiwana, is one of the most annoying blood-sucking pests in Taiwan. Current chemical control methods only target the adult, not the immature stages (egg to pupa), of F. taiwana. Discovering new or alternative tactics to enhance or replace existing methods are urgently needed to improve the effectiveness of F. taiwana control. The egg is the least understood life stage in this pest species but may offer a novel point of control as addition of NaCl to the egg environment inhibits development. Thus, the objective of this study was to use RNA profiling to better understand the developmental differences between wild-type melanized (black) and NaCl-induced un-melanized (pink), infertile F. taiwana eggs.

Results

After de novo assembly with Trinity, 87,415 non-redundant transcripts (Ft-nr) with an N50 of 1099 were obtained. Of these, 26,247 (30%) transcripts were predicted to have long open reading frames (ORFs, defined here as ≥300 nt) and 15,270 (17.5%) transcripts have at least one predicted functional domain. A comparison between two biological replicates each of black and pink egg samples, although limited in sample size, revealed 5898 differentially expressed genes (DEGs; 40.9% of the transcripts with long ORFs) with ≥2-fold difference. Of these, 2030 were annotated to a Gene Ontology biological process and along with gene expression patterns can be separated into 5 clusters. KEGG pathway analysis revealed that 1589 transcripts could be assigned to 18 significantly enriched pathways in 2 main categories (metabolism and environmental information processing). As expected, most (88.32%) of these DEGs were down-regulated in the pink eggs. Surprisingly, the majority of genes associated with the pigmentation GO term were up-regulated in the pink egg samples. However, the two key terminal genes of the melanin synthesis pathway, laccase2 and DCE/yellow, were significantly down-regulated, and further verified by qRT-PCR.

Conclusion

We have assembled and annotated the first egg transcriptome for F. taiwana, a biting midge. Our results suggest that down-regulation of the laccase2 and DCE/yellow genes might be the mechanism responsible for the NaCl-induced inhibition of melanization of F. taiwana eggs.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08096-x.

Human practices promote presence and abundance of disease-transmitting mosquito species

Humans alter the environment at unprecedented rates through habitat destruction, nutrient pollution and the application of agrochemicals. This has recently been proposed to act as a potentially significant driver of pathogen-carrying mosquito species (disease vectors) that pose a health risk to humans and livestock. Here, we use a unique set of locations along a large geographical gradient to show that landscapes disturbed by a variety of anthropogenic stressors are consistently associated with vector-dominated mosquito communities for a wide range of human and livestock infections. This strongly suggests that human alterations to the environment promote the presence and abundance of disease vectors across large spatial extents. As such, it warrants further studies aimed at unravelling mechanisms underlying vector prevalence in mosquito communities, and opens up new opportunities for preventative action and predictive modelling of vector borne disease risks in relation to degradation of natural ecosystems.