Ubiquitous water-soluble molecules in aquatic plant exudates determine specific insect attraction.

Plants produce semio-chemicals that directly influence insect attraction and/or repulsion. Generally, this attraction is closely associated with herbivory and has been studied mainly under atmospheric conditions. On the other hand, the relationship between aquatic plants and insects has been little studied. To determine whether the roots of aquatic macrophytes release attractive chemical mixtures into the water, we studied the behaviour of mosquito larvae using olfactory experiments with root exudates. After testing the attraction on Culex and Aedes mosquito larvae, we chose to work with Coquillettidia species, which have a complex behaviour in nature and need to be attached to plant roots in order to obtain oxygen. This relationship is non-destructive and can be described as commensal behaviour. Commonly found compounds seemed to be involved in insect attraction since root exudates from different plants were all attractive. Moreover, chemical analysis allowed us to identify a certain number of commonly found, highly water-soluble, low-molecular-weight compounds, several of which (glycerol, uracil, thymine, uridine, thymidine) were able to induce attraction when tested individually but at concentrations substantially higher than those found in nature. However, our principal findings demonstrated that these compounds appeared to act synergistically, since a mixture of these five compounds attracted larvae at natural concentrations (0.7 nM glycerol, <0.5 nM uracil, 0.6 nM thymine, 2.8 nM uridine, 86 nM thymidine), much lower than those found for each compound tested individually. These results provide strong evidence that a mixture of polyols (glycerol), pyrimidines (uracil, thymine), and nucleosides (uridine, thymidine) functions as an efficient attractive signal in nature for Coquillettidia larvae. We therefore show for the first time, that such commonly found compounds may play an important role in plant-insect relationships in aquatic eco-systems.

How human practices have affected vector-borne diseases in the past: a study of malaria transmission in Alpine valleys.

BACKGROUND: Malaria was endemic in the Rhône-Alpes area of eastern France in the 19th century and life expectancy was particularly shortened in Alpine valleys. This study was designed to determine how the disease affected people in the area and to identify the factors influencing malaria transmission. METHODS: Demographic data of the 19th century were collected from death registers of eight villages of the flood-plain of the river Isère. Correlations were performed between these demographic data and reconstructed meteorological data. Archive documents from medical practitioners gave information on symptoms of ill people. Engineer reports provided information on the hydraulic project developments in the Isère valley. RESULTS: Description of fevers was highly suggestive of endemic malaria transmission in the parishes neighbouring the river Isère. The current status of anopheline mosquitoes in the area supports this hypothesis. Mean temperature and precipitation were poorly correlated with demographic data, whereas the chronology of hydrological events correlated with fluctuations in death rates in the parishes. CONCLUSION: Nowadays, most of the river development projects involve the creation of wet areas, enabling controlled flooding events. Flood-flow risk and the re-emergence of vector-borne diseases would probably be influenced by the climate change. The message is not to forget that human disturbance of any functioning hydrosystem has often been linked to malaria transmission in the past.

Do herbicide treatments reduce the sensitivity of mosquito larvae to insecticides?

Invasive mosquitoes are economic and sanitary concerns especially in Europe and America. Most work has emphasized the role of resistance [Berrada, S., Fournier, D., Cuany, A., Nguyen, T.X., 1994. Identification of resistance mechanisms in a selected laboratory strain of Cacopsylla pyri (Homoptera: Psyllidae): altered acetylcholinesterases and detoxifying oxidases. Pesticide Biochemistry and Physiology 48, 41-47; Hemingway, J., Hawkes, N.J., McCarroll, L., Ranson, H., 2004. The molecular basis of insecticide resistance in mosquitoes. Insect Biochemistry and Molecular Biology 34, 653-665] to insecticides. Compounds acting on larval sensitivity to insecticides are not well studied and their action remains poorly understood. Among several residual chemicals in ecosystems, particularly in wetlands, we identified a possible interaction of an herbicide on larval resistance to an insecticide. Our work contributes to the global control of mosquito populations by identifying possible pathways of resistance to insecticides of these vectors. Resistance or tolerance to insecticide treatments might contribute to successful invasion by mosquitoes. Here we report an ecotoxicological approach to test the hypothesis of an indirect effect of atrazine on mortality of an invasive vector. A brief contact (48h) between Aedes aegypti mosquito larvae and atrazine led to a modification of larval sensitivity to an insecticide: using atrazine as an inducer led to a decrease in the mortality of larvae treated with Bacillus thuringiensis var. israelensis (Bti).