Cadmium exposure disrupts the olfactory sensitivity of fire ants to semiochemicals

The exposure risk of heavy metals to insect pests and their impact on pests occurrence and cross-tolerance to insecticides: A review

Heavy metal (HM) pollution is a severe global environmental issue. HMs in the environment can transfer along the food chain, which aggravates their ecotoxicological effect and exposes the insects to heavy metal stress. In addition to their growth-toxic effects, HMs have been reported as abiotic environmental factors that influence the implementation of integrated pest management strategies, including microbial control, enemy insect control, and chemical control. This will bring new challenges to pest control and further highlight the ecotoxicological impact of HM pollution. In this review, the relationship between HM pollution and insecticide tolerance in pests was analyzed. Our focus is on the risks of HM exposure to pests, pests tolerance to insecticides under HM exposure, and the mechanisms underlying the effect of HM exposure on pests tolerance to insecticides. We infer that HM exposure, as an initial stressor, induces cross-tolerance in pests to subsequent insecticide stress. Additionally, the priming effect of HM exposure on enzymes associated with insecticide metabolism underlies cross-tolerance formation. This is a new interdisciplinary field between pollution ecology and pest control, with an important guidance value for optimizing pest control strategies in HM polluted areas.

Semiochemicals Produced by Microbes in Mealybug Honeydew Attract Fire Ants

Semiochemicals produced by microbes in hemipteran honeydew play an important role in mediating the ant-hemipteran interaction. However, the specific volatile compounds and their origins remain unclear. Here, we found that honeydew produced by Phenacoccus solenopsis exhibited strong attractiveness to fire ant workers, but sterilized honeydew was much less attractive. Four compounds were identified from the honeydew volatiles, including 1-octen-3-ol, limonene, phenylethyl alcohol, and 2,4-ditert-butylphenol. All the identified compounds triggered strong electroantennography response of fire ants and showed repellence at relatively high concentrations while attractiveness at low concentrations to fire ant workers. Furthermore, six bacterial isolates were identified based on 16S rRNA sequencing, namely, Bacillus, Brachybacterium, Kocuria, Microbacterium, Pseudomonas, and Staphylococcus. Fire ants exhibited a strong preference for Brachybacterium, Kocuria, and Microbacterium, suggesting that these bacterial isolates are involved in the attracting effect of honeydew. Both limonene and phenylethyl alcohol were detected from Brachybacterium, while limonene was only detected from Kocuria, and phenylethyl alcohol was exclusively detected from Microbacterium. Reinoculation of these bacteria restored the attractiveness of honeydew to fire ants, and the active compounds, limonene and phenylethyl alcohol, were detectable in bacteria-reinoculated honeydew. Collectively, our results reveal the active compounds in hemipteran honeydew and their association with honeydew bacteria. The findings will contribute to the development of novel attractants for efficient monitoring of fire ants.

Effects of Cadmium Stress on Bacterial and Fungal Communities in the Whitefly Bemisia tabaci

Heavy metal contamination is among the most prominent environmental problems in China, posing serious threats to both ecosystem and human health. Among the diverse heavy metal contaminants, cadmium is the most serious. The whitefly Bemisia tabaci is a cosmopolitan pest capable of causing severe damage to a broad range of agricultural crops, especially vegetables. At present, little is known about the effects of cadmium stress on B. tabaci, including on its bacterial and fungal communities. In the current study, we investigated the effects of cadmium on bacterial and fungal communities in whiteflies. Meta-barcode sequencing of the 16S rRNA gene revealed that the whitefly bacterial community contained 264 operational taxonomic units (OTUs) belonging to 201 known genera and 245 known species. The top five most frequent bacterial genera were Rickettsia, Rhodococcus, Candidatus Portiera, Candidatus Hamiltonella, and Achromobacter. Meta-barcode sequencing of the fungal ITS locus revealed that the whitefly fungal community contained 357 OTUs belonging to 187 known genera and 248 known species. The top five most frequent fungal genera were Wallemia, unclassified_f_Dipodascaceae, Apiotrichum, Penicillium, and unclassified_o_Saccharomycetales. Cadmium exposure reduced the fungal OTU richness but increased the bacterial Shannon and Simpson diversity indices in whiteflies. In addition, upon exposure to cadmium, the microbial community composition in whiteflies changed significantly, with increased prevalence of the bacterial genera Rhodococcus and Exiguobacterium and fungal genus Wallemia. Our results indicate that the whitefly microbiota likely contributed to their adaptation and resistance to cadmium and suggested that whiteflies may contain microbes that could help remediate cadmium contamination in natural environments and agricultural fields.

Impact of Red Imported Fire Ant Nest-Building on Soil Properties and Bacterial Communities in Different Habitats

The red imported fire ant (Solenopsis invicta Buren) is a highly adaptable invasive species that can nest and reproduce in different habitat soils. We aimed to explore the adaptability of red imported fire ants in different habitats by analyzing changes in the physicochemical properties of nest soils and bacterial communities. Five habitat types (forest, tea plantation, rice field, lawn, and brassica field) were selected. The results showed that the pH of the nest soils increased significantly in all five habitats compared to the control soils of the same habitat. A significant increase in nitrogen content was detected in the nests. The Cr, Pb, Cu, and Ni levels were significantly reduced in the soils of the five habitats, due to nesting activities. Analysis of the composition and diversity of the soil microbial community showed that, although the richness and diversity of bacteria in the nest soils of red imported fire ants in the five habitats varied, the relative abundance of Actinobacteria significantly increased and it emerged as the dominant bacterial group. These results indicate that red imported fire ants modify the physicochemical properties of nest soils and bacterial communities to create a suitable habitat for survival and reproduction.

A pair of cadmium-exposed zebrafish affect social behavior of the un-exposed majority

To account for global contamination events, we must identify direct and indirect pollutant effects. Although pollutants can have direct effects on individuals, it is unknown how a few contaminated individuals affect groups, a widespread social organization. We show environmentally relevant levels of cadmium (Cd) can have indirect social effects revealed in the social context of a larger group. Cd-contaminated individuals had poor vision and more aggressive responses, but no other behavioral effects. The presence of experienced Cd-exposed pairs in the groups had an indirect effect on the un-exposed individual's social interactions leading to the shoal becoming bolder and moving closer to a novel object than control groups. Because a few directly affected individuals could indirectly affect social behavior of the un-exposed majority, we believe that such acute but potentially important heavy metal toxicity could inform reliable predictions about the consequences of their use in a changing world.

Sublethal Exposure to Cadmium Induces Chemosensory Dysfunction in Fire Ants

Ants easily accumulate cadmium (Cd) from the food web in terrestrial ecosystems. Cd contamination may cause olfactory dysfunction and consequently disorders in the social behavior of ants. To explore the molecular mechanism underlying the effect of Cd exposure on the chemosensory process of ants, we characterized the Cd-induced variations in the expression of genes involved in chemoreception and electrophysiological and behavioral sensitivity to semiochemicals by using the red imported fire ant, Solenopsis invicta, as a model system. As a result, Cd exposure increased Cd accumulation and decreased the survival rate of S. invicta. Cd exposure altered the expression profiles of odor binding protein genes of S. invicta (SiOBPs). Specifically, SiOBP15 protein expression was upregulated upon Cd exposure. Both SiOBP7 and SiOBP15 exhibited high binding affinities to limonene, nonanal, and 2,4,6-trimethylpyridine. S. invicta exposed to Cd showed less sensitive electrophysiological and behavioral response to the three chemicals but exhibited sensitive perception to undecane. Silencing of SiOBP7 and SiOBP15 abolished the behavioral response of S. invicta to nonanal and undecane, respectively, suggesting that SiOBP7 and SiOBP15 play essential roles in the chemoreception of S. invicta. In general, our results suggest that Cd contamination may interfere with olfactory signal transduction by altering the expression of SiOBPs, consequently evoking chemosensory dysfunction in fire ants.