Search

To recover some evolutionary aspects of the interaction between culicine larvae and dietary polyphenols of the vegetation surrounding mosquito breeding sites, we constructed a phylogeny of the most common French Aedesspecies, chosen as reference species. We also evaluated the differential resistance of these larval taxa to the polyphenols of leaf litter from the riparian vegetation used as a food source. Mitochondrial DNA sequence analysis was performed among 14 different taxa and ecotypes (Aedes aegypti, Ae. albopictus, Ae. cantans, Ae. caspius, Ae. cataphylla, Ae. cinereus, Ae. detritus, Ae. geniculatus, Ae. mariae, Ae. pullatus, Ae. punctor, Ae. rusticus, Ae. sticticus,and Ae. vexans) through direct sequencing of a 763-base segment of the cytochrome oxidase subunit I gene. Phylogenetic analysis, based on nucleotide and amino acid sequences, was conducted by means of parsimony and distance methods. The differential tolerance of larvae to vegetable leaf litter was comparatively tested by use of 10-month-old alder leaf litter as an experimental standard. The absence of correlation between resistance to polyphenols and molecular phylogeny suggests that larval adaptation to polyphenol-rich vegetable breeding sites is a labile character. The acquisition of such resistance appears not to be ancestrally inherited, but rather to be a dynamic adaptation to the environment. Molecular data also support the classical morphological classification within the Aedesgenus.

The toxicological characteristics of dietary decomposed alder leaf litter against mosquito larvae were further investigated through enzymatic and chemical purification of a phenoliclike cell-wall fraction isolated from crude litter. The toxicity of the subfractions obtained was controlled by standard bioassays on third instars of Aedes aegypti chosen as a reference target species. Enzymatic hydrolyses of the cell-wall fraction were performed with caylase, pectolyase, esterase, and β-glycosidase, in order to release, respectively, cellulose material and phenolic compounds bound to lignins. These treatments did not affect the larvicidal activity and the phenolic activity of the cell-wall fraction. Chemical alkaline and acid hydrolyses were carried out to break ester and glycosidic bonds of the cell-wall fraction. Comparison of HPLC profiles of the hydrolysates from both toxic and nontoxic fractions did not reveal differences between the phenolic acids released. Aluminum chloride, known for its phenolic complexing activity, counteracted the larvicidal activity of the cell-wall fraction. Altogether, these results suggest the involvement of ligninlike compounds in the toxicity of dietary alder leaf litter against larval mosquitoes. The toxicity of this fraction, which was very sensitive to drastic and smooth oxidations, seemed to be associated with a strong oxidative potential. These results are discussed in relation to a possible mode of action of lignins in the plant–mosquito interactions.

Decaying alder leaves in water from Alpine Aedesbreeding sites, particularly their toxicity to larval Culicidae, were investigated and characterized with comparative toxicological and chemical methods. Bioassays that used third-instar Aedes aegyptias a reference species indicated that the larvicidal effect of crude leaf litter varied with decaying age of the litter, while no toxicity was detected from leaching water of the mosquito breeding sites. Ten-month-old leaf litter was the most toxic. Comparison of the different soluble and insoluble fractions obtained after sequential extraction of decomposed litter allowed us to localize the toxicity factor to an insoluble cell-wall fraction. The toxicity seems to be linked to phenolic activity. It is higher than that found for tannic acid solutions used as a reference to mimic the larvicidal effects of the molecules naturally occurring in decaying litter. The pattern of establishing the larvicidal effect of alder leaf litter in water of Alpine Aedesbreeding sites is discussed.

The relative toxicity of leaf litter to nematocerous dipteran larvae characteristic of mosquito developmental sites was investigated. Culicidae, Chironomidae, and Simuliidae taxa originating from alpine hydrosystems were tested together with two laboratory nonindigenous culicid taxa. Bioassays indicate that ingestion of 10-month-old decaying leaves from Alnus glutinosa, Populus nigra,and Quercus roburby larvae is more deleterious for Aedes aegypti, A. albopictus, Culex pipiens, Simulium variegatum,and Chironomus annulariusthan for A. rusticus.Histopathological observations reveal that the midgut epithelium is the main target organ of the toxic effect of dietary leaf litter, which appears to be stronger than that of previously reported tannic acid. There is a general response of the nematocerous larval midgut epithelium to dietary tannins–phenolic compounds: clear cells of the anterior midgut showing symptoms of intoxication before dark cells of the posterior midgut.

The sensitivity of larval Culicidae to vegetable tannins was investigated in different taxa representative of the fauna from alpine hydrosystems (Aedes rusticus, Culex pipiens) and foreign noxious fauna (Aedes aegypti, A. albopictus). Bioassays reveal that tannic acid at concentrations of 0.1–6 mM is significantly more toxic for C. pipiensthan for Aedestaxa, and A. aegyptiis more sensitive than A. albopictusand A. rusticus.Comparison of the rank order of sensitivity among taxa with the associated levels of cytochrome P-450, esterase, and glutathione-S-transferase activities suggests that cytochrome P-450 and esterases may be involved in the detoxification of tannins. A possible involvement of these detoxifying enzymes is also revealed in vivo by the synergistic effects of S,S,S-tributyl phosphorotrithioate (esterase inhibitor) and piperonyl butoxide (P-450 inhibitor). The differential sensitivity to tannins among taxa is discussed in terms of ecological implications within mosquito communities from alpine hydrosystems, where the acquisition of tannins–detoxifying enzymatic systems may be considered as a key innovation.

The impact of tannins on larval Nematocera was investigated by an extensive survey of the relative toxicity of tannic acid in Diptera larvae representative of mosquito communities from alpine hydrosystems (Culicidae, Chaoboridae, Chironomidae, and Simuliidae) together with a nonindigenous vector competent Culicidae species. Bioassays indicate that exposure to tannic acid at concentrations from 0.25 to 4 mM is deleterious forCulex pipiens, Simulium variegatum,andChironomus annularius,but not forAedes, Anopheles, Culiseta,andChaoborusspecies. Histopathological observations reveal that, among the target organs of tannic acid, mainly the midgut epithelium is affected by treatment. However, the extent of degeneration varies according to the taxon, the duration of the treatment, and the concentrations assayed. The vulnerability of epithelial cells differs among cell types, clear cells of the anterior midgut showing symptoms of intoxication before dark cells of the posterior midgut. The toxic effects of tannic acid are discussed, particularly in comparison to those of insecticidal bacteria, in order to evaluate the potential for use of tannins in the regulation of larval populations of dipteran pests.