Effects of organochlorines on microbial diversity and community structure in Phragmites australis rhizosphere.

This study investigated the impacts of an organochlorine (OC, γ-hexachlorocyclohexane and chlorobenzenes) mixture on microbial communities associated to Phragmites australis rhizosphere. Seventy-eight distinct colony morphotypes were isolated, cultivated and analysed by 16S rDNA sequence analysis. Toxicity tests confirmed sensitivity (e.g. Hevizibacter, Acidovorax) or tolerance (e.g. Bacillus, Aeromonas, Pseudomonas, Sphingomonas) of isolates. Rhizosphere analysis by pyrosequencing showed the microbial adaptation induced by OC exposure. Among the most abundant molecular operational taxonomic units, 80 % appeared to be tolerant (55 % opportunist, 25 % unaffected) and 20 % sensitive. P. australis rhizosphere exposed to OCs was dominated by phylotypes related to α-, ß- and γ-Proteobacteria. Specific genera were identified which were previously described as chlorinated organic pollutant degraders: Sphingomonas sp., Pseudomonas sp., Devosia sp. and Sphingobium sp. P. australis could be suitable plants to maintain their rhizosphere active microbial population which can tolerate OCs and potentially improve the OC remediation process in part by biodegradation.

Biological responses of maize (Zea mays) plants exposed to chlorobenzenes. Case study of monochloro-, 1,4-dichloro- and 1,2,4-trichloro-benzenes.

A 7-day-exposure time experiment was designed to investigate the phytotoxicity of chlorobenzenes (CBs) on Zea mays seedlings, focusing on the growth and generation of oxidative stress. Significant growth inhibition (based on biomass gain) was observed for exposure to monochlorobenzene (MCB), dichlorobenzene (DCB) and trichlorobenzene (TCB) concentrations higher than 10 mg l(-1). It would seem that CBs inhibit cell division, since the mitotic index decreased for roots exposed to DCB at 80 mg l(-1) dose (8%) and to all the TCB concentrations tested (20% inhibition). CBs exposure resulting in an increase in the oxidative stress response in maize seedlings [reactive oxygen species like H(2)O(2), antioxidant enzymes (POD, GR), lipid peroxidation] correlated to the compound's degree of chlorination, where damage increasing with the number of chlorine atoms (MCB < DCB < TCB). This biological response was also dependent on the dose-exposure. Z. mays exposed to CBs at concentrations <10 mg l(-1) did not induce sufficient oxidative damage to cause root cell death. Therefore, CBs at current environmental concentrations are unlikely to produce evident phytotoxic effects on Z. mays seedlings.

How can the fipronil insecticide access phloem?

Seeds of sunflower plants coated with the fipronil (14)C-insecticide were allowed to grow in the greenhouse. The distribution of the (14)C-compounds was studied in each part of the plant after three months. After 83 days of culture small amounts of (14)C-compounds were found in the inflorescence (0.6 per thousand of the seed deposit) which were fipronil itself or its lipophilic or hydrophilic metabolites. The (14)C-compounds were found in each part of the inflorescence (bracts, ray and disk florets containing pollen, akenes). The (14)C-concentration in the xylem sap evaluated at this stage was much too low to explain the accumulated amount in the inflorescence. Under controlled conditions in a culture chamber, it was then demonstrated that a net phloem transfer of (14)C-fipronil occurred from developed leaves to growing organs. This allowed us to suppose that a similar (14)C-fipronil phloem transfer could occur toward the inflorescence during its formation. A quantitative evaluation suggests that most of the labeled compounds at this stage were not coming from the leaves but from the roots and stem where storage compounds were hydrolyzed for sustaining inflorescence development.

Interactions between immune and biotransformation systems in fish: a review.

In animals biotransformation and immune system are not totally independent, there are numerous functional interrelationships between these two systems. They are both implicated in the capacity of organisms to resist to a wide variety of environmental components such as viruses, bacteria and xenobiotics. It is known for a long time that the immune system functions as a physiologic system and interacts with all the other components of the organism including nervous or endocrine ones. In the same manner, the biotransformation system (especially the cytochrome P450 monooxygenases) is involved in the regulation of numerous hormone productions. In this way, many studies in mammals have revealed the possible interaction between immune and biotransformation systems. Among these interactions, the capacity of the activation of host defense mechanisms to down-regulate microsomal cytochrome P450 and the role of biotransformation system in the xenobiotic-mediated immunotoxicity have been underlined. Advances in the basic knowledge of fish immune and biotransformation systems should lead to a better understanding of the possible interactions between both systems and should improve fish health monitoring which is a crucial ecotoxicological goal.

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.

A PIXE and ICP-MS analysis of metallic atmospheric contaminants in tree bark tissues, a basis for biomonitoring uses.

The qualitative and quantitative metallic content of tree barks of Argania spinosa (L.) Skeels were studied. Argania spinosa is an endemic species in Morocco. This tree is adapted to semi-arid climates and exposed to specific conditions of relative humidity, temperature, wind, and particle transport. Three sites were sampled in Morocco: the large town of Rabat, the harbor of Agadir, and Aït Baha, a countryside location exposed to continuous desert wind. The methodologies included (1) in situ microanalysis with proton-induced X-ray emission (PIXE) and (2) trace element determinations by mass spectrometry with inductively coupled plasma (ICP-MS) associated with extraction procedures. Both methods allowed detection of elements coming from different bark compartments. The profile of airborne contaminants in the barks was typical of the sampling sites. The level of lead in barks sampled in Rabat reached 100 ng cm(-2), or higher, while it varied between 3 and 35 ng cm(-2) in Aït Baha. The in situ study of the microscopic structure of the bark provided the location of major and minor elements at various depths inside the bark. A differential between free deposit on the bark surface and penetrated content was found for the major and trace elements. The free deposit on the bark surface was suspected to be mostly the result of recent contamination. Part of the contaminants spread out on the surface penetrated the superficial suber. This long-term accumulation affected mostly Pb. In deeper levels, airborne elements at low concentrations and elements resulting from root uptake were concurrently present and resulted in a complex situation, as noted for zinc.

Soil distribution of fipronil and its metabolites originating from a seed-coated formulation.

Seed-coating with the insecticide fipronil has been intensively used in sunflower cultivation to control soil pests such as wireworms. A research project was undertaken to determine the soil distribution of fipronil and of its main phenylpyrazole metabolites. Under agronomic conditions, the quantity of fipronil in the seed-coat (437 microg/seed) decreased continuously during the cultivation period (3.9 microg day(-1) during the first two months; 0.3 microg day(-1) during the next four months). At the end of the cultivation period, 42% of all phenylpyrazole compounds remained in the seed-coat. Fipro nil was poorly mobile in soil, and at the end of the cultivation period it was mostly concentrated in the soil layer close to the seed (3240 microg kg(-1) soil). Starting from the seed-coating, a fipronil concentration gradient was measured in the soil, up to a distance of 11 cm from the seed. Degradation in the soil occurred at a moderate rate, probably due to the fact that water solubilization of the solid active ingredient present in the seed coating was rate limiting. Indeed, after 6 months of cultivation, only 51% of the fipronil seed-coating was found in the soil, about 7% having been absorbed by the sunflower plant, and 42% remaining in the seed coat. The predominant metabolites produced in the soil were sulfone-fipronil, sulfide-fipronil and amide-fipronil, which were produced at average rates of 5 microg kg(-1) soil day(-1), 3 microg kg(-1) soil day(-1), and 0.4 microg kg(-1) soil day(-1), respectively. In contrast, the photoproduct, desulfinyl-fipronil, was barely detected. All phenylpyrazole compounds were poorly mobile, except for the amide derivative, which is devoid of insecticidal activity in marked contrast to the other metabolites. Furthermore, detectable soil contamination was limited to a zone of about 11 cm around the seed.

Differential sensitivity to Bacillus thuringiensis var. israelensis and temephos in field mosquito populations of Ochlerotatus cataphylla (Diptera: Culicidae): toward resistance?

In the present study, four populations of the same field mosquito species, Ochlerotatus cataphylla, were sampled over the Rh6ne-Alpes region (France), and their respective sensitivity to the organophosphate temephos and the bacterio-insecticide Bacillus thuringiensis var. israelensis (Bti) was measured. The results obtained in toxicological tests showed significant differences in the larval sensitivities of the four populations for both insecticides. These differences appeared to be related to the activity of the three main families of detoxifying enzymes: Cytochrome P450 monooxygenases, glutathione-S-transferases (GSTs), and esterases. All three enzyme families were significantly overexpressed in the less susceptible larval population, and after multiple regressions, GSTs and esterases came out as the most explicative variables of the larval sensitivity. Considering these results and the chemical history of the sites in terms of insecticide treatments, the hypothesis of cross-effects of insecticides leading to resistance acquisition to Bti in field organisms emerges.

Uptake and xylem transport of fipronil in sunflower.

The phenylpyrazole insecticide, fipronil, is used in seed coating against Agriotes larvae, which infest mainly corn and sunflower. Coating the seeds of the cultivated plants with fipronil has proven its effectiveness against Agriotes populations. In the case of sunflower or even corn, the possible root uptake of this insecticide may lead to a toxic effect against pollinators such as honeybees. In the present report, the uptake and transport of fipronil inside the sunflower seedling was studied in the laboratory. In a first study, sunflower was cultivated on an aqueous medium containing fipronil. An intense root uptake of fipronil occurred, leading to a transport into leaves depending upon transpiration. In a second study, plants were cultivated on a soil in which fipronil was uniformly distributed. Under our soil conditions (20% organic carbon), the partition coefficient between soil and water (K(d)) was found to be equal to 386 +/- 30. The average rate of fipronil transfer from soil water to seedlings was from 2 to 2.6 times lower than water transfer. During the 3 week experiment, 55% of recovered labeled compounds was in the parent form and 35% had been converted to lipophilic metabolites, with either a 4-CF(3)-SO(2) or 4-CF(3)-S substituant, which are also very potent lipophilic insecticides. This paper suggests that the possible uptake of fipronil by sunflower seedlings under agronomic conditions is mainly controlled by the physicochemical characteristics of the seed-coating mixture.

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).

Response of Aedes aegypti (Diptera: Culicidae) larvae to three xenobiotic exposures: larval tolerance and detoxifying enzyme activities.

The ability of mosquito larvae to tolerate toxic compounds (temephos, Bacillus thuringiensis var. israelensis, toxic vegetable leaf litter) was examined on a laboratory larval strain of Aedes aegypti L. Bioassays and detoxifying enzyme activity measurements were performed to compare tolerance/resistance capacities. The possibility of a functional plasticity of detoxifying equipment was investigated through experimental determination of the inductive effect of each xenobiotic within a given generation. In the same way, the selective effect of a toxic leaf litter was also investigated along successive generations. Results revealed that differential cytochrome P450 monooxygenase, esterase, and glutathione S-transferase activity levels correlated with the bioassay results. Both induction and selection increased larval tolerance to the xenobiotics used and increased the levels of larval detoxifying enzyme activities.

The possible role of hydroxylation in the detoxification of atrazine in mature vetiver (Chrysopogon zizanioides Nash) grown in hydroponics.

The resistance mechanism of vetiver (Chrysopogon zizanioides) to atrazine was investigated to evaluate its potential for phytoremediation of environment contaminated with the herbicide. Plants known to metabolise atrazine rely on hydroxylation mediated by benzoxazinones, conjugation catalyzed by glutathione-S-transferases and dealkylation probably mediated by cytochromes P450. All three possibilities were explored in mature vetiver grown in hydroponics during this research project. Here we report on the chemical role of benzoxazinones in the transformation of atrazine. Fresh vetiver roots and leaves were cut to extract and study their content in benzoxazinones known to hydroxylate atrazine, such as 2,4-dihydroxy-2H-1,4-benzoxazin-3(4H)-one (DIBOA), 2,4-dihydroxy-7-methoxy-2H-1,4-benzoxazin-3(4H)-one (DIMBOA) and their mono- and di-glucosylated forms. Identification of benzoxazinones was performed by thin layer chromatography (TLC) and comparison of retention factors (Rf) and UV spectra with standards: although some products exhibited the same Rf as standards, UV spectra were different. Furthermore, in vitro hydroxylation of atrazine could not be detected in the presence of vetiver extracts. Finally, vetiver organs exposed to [14C]-atrazine did not produce any significant amount of hydroxylated products, such as hydroxyatrazine (HATR), hydroxy-deethylatrazine (HDEA), and hydroxy-deisopropylatrazine (HDIA). Altogether, these metabolic features suggest that hydroxylation was not a major metabolic pathway of atrazine in vetiver.

Fipronil metabolism and dissipation in a simplified aquatic ecosystem.

Several phenylpyrazole derivatives are selective inhibitors of chloride channel activities in insects. In this chemical family, fipronil is a powerful insecticide now widely used for several purposes. The dissipation of this molecule in a simplified aquatic ecosystem has been studied for 3 months, using (14)C-labeled fipronil. The main features of the complex process leading to fipronil transformation in this system were the following. The fipronil aqueous solution was submitted to two chemical transformations: the photodependent desulfuration of the side chain bound to the 4-position of the heterocyclic ring and the chemical hydrolysis of the nitrile function bound to the 3-position. Fipronil, rapidly transferred from the water solution to the organic matter, was protected from the previously mentioned chemical transformations but evolved to give two main metabolites, which were either reduced or oxidized in the side chain on the 4-position. These derivatives were powerful insecticides as shown by LC(50) measurements on Aedes aegypti larvae (LC(50) for CF(3)-S-R and CF(3)-SO(2)-R = 8.8 nM). During the course of this experiment, nitrile hydrolysis took place slowly, originating either from the chemical hydrolysis in the aqueous solution or from enzymatic hydrolysis inside the microbial biomass. The fipronil-amide (3-NH(2)-CO-R') derivative, although much more polar than fipronil itself, was mostly bound to the organic matter. Other more polar derivatives were also detected but in very small amounts. No (14)CO emission was observed during the experiment.

A comparative study on the uptake and translocation of organochlorines by Phragmites australis.

Organochlorines (OCs) are persistent chemicals found in various environmental compartments. The differences in the uptake of (14)C-labeled 1,4-dichlorobenzene (DCB), 1,2,4-trichlorobenzene (TCB) and γ-hexachlorocyclohexane (γHCH) by Phragmites australis were investigated under hydroponic conditions. The first step in sorption appears to be correlated with the hydrophobic nature of the compounds, since log-linear correlations were obtained between root concentration factor and partition coefficient (LogK(ow)). After 7 days of exposure, plant uptake of DCB, TCB, γHCH was significant with bioconcentration factors reaching 14, 19 and 15, respectively. Afterwards, uptake and translocation were seen to be more complex, with a loss of the simple relationship between uptake and LogK(ow). Linear correlations between the bioconcentration/translocation factors and the physico-chemical properties of OCs were shown, demonstrating that translocation from roots to shoots increases with solubility and volatility of the OCs. This suggests that OC-translocation inside plants might result from the combination of two processes, xylem sap flow and vapor fluxes. (14)C-phytovolatilization was measured and was correlated with the volatility of the compounds; the more volatile OCs being most the likely to be phytovolatilized from foliar surfaces (p=0.0008). Thus, OC-uptake/translocation appears to proceed at a rate that depends mostly on the OCs hydrophobicity, solubility and volatility.

Response of phase II detoxification enzymes in Phragmites australis plants exposed to organochlorines.

Mixed pollution is a characteristic of many industrial sites and constructed wetlands. Plants possessing an enzymatic detoxifying system that is able to handle xenobiotics seems to be a viable option for the removal of mixed persistent contaminants such organochlorines (OCs: monochlorobenzene (MCB), 1,4-dichlorobenzene (DCB), 1,2,4-trichlorobenzene (TCB), γ-hexachlorocyclohexane (HCH)). In this study, Phragmites australis plants were exposed to sub-lethal concentrations of OCs (7 days), in single-exposure (0.8 to 10 mg l(-1)) and in mixture of OCs (0.2 mg l(-1) MCB + 0.2 mg l(-1) DCB + 2.5 mg l(-1) TCB + 0.175 mg l(-1) HCH). Studies were conducted on the detoxification phase II enzymes; glutathione S-transferases (GST), and glucosyltransferases (UGT). Measurements of GST and UGT activities revealed that OCs may be buffered by glutathione and glucose conjugation. There appeared to be a correlation between the effects on phase II enzymes and the degree of chlorination of the benzene ring with, for example, the greatest effects being obtained for HCH exposure. In the case of mixed pollution, the induction of some GST isoenzymes (CDNB, 35 % non-significant) and UGT (118 %) in leaves and the inhibition of phase II enzymes in the other organs were measured. UGTs appear to be key enzymes in the detoxification of OCs.

Concentration responses to organochlorines in Phragmites australis.

Phragmites australis shows potential for the phytoremediation of chlorinated chemicals. Also there has been some attempt to determine the phytotoxic effects of organochlorines (OC). This study reports for lindane (HCH), monochlorobenzene (MCB), 1,4-dichlorobenzene (DCB) and 1,2,4-trichlorobenzene (TCB), a no-observed-effect-concentration (NOEC(7d)) that was 1000-300,000 times higher than environmental concentrations. Nevertheless, the combined OC mixture (NOEC(7d) level of each congener) induced a synergistic toxic effect, causing a severe drop (70%) in chlorophyll concentration. The mixture 0.2 mg L(-1) MCB+0.2 mg L(-1) DCB+2.5 mg L(-1) TCB+0.175 mg L(-1) HCH, that was 15 times more concentrated than environmental OC mixture, did not cause phytotoxicity during 21 days. Antioxidant enzymes were affected immediately after the start of exposure (3 days), but the plants showed no signs of stress thereafter. These data suggest that environmental OC mixtures do not pose a significant risk to P. australis.

The inclusion of atmospheric particles into the bark suber of ash trees.

A slow deposition of atmospheric components occurs on tree barks. One part of them can be incorporated into bark tissues. This work demonstrates that mineral particles are present inside the suber tissue in four-year-old ash tree stems. Most of these particles are smaller than 2 microm. Scanning electronic microscope studies - using energy dispersive X-ray spectroscopy (SEM-EDX) - on stem cuttings show that they are located either inside the dead suber cells or between these cells. Numerous particles are composed of clay, quartz, feldspar or mica. Others, clearly of anthropogenic origin, are metallic fragments composed of Fe, Ni, Cr, Pb, etc. Spherical fly ashes were found, composed of Si, Al or Fe, and demonstrating an anthropogenic formation. Such particles were isolated and purified from suber ashes obtained at 550 degrees C, after aqueous and acidic treatments, and their composition was established through SEM-EDX. These results reinforce the idea that the suber of tree barks alone can be considered as archives for atmospheric deposition.

Long lasting persistence of Bacillus thuringiensis Subsp. israelensis (Bti) in mosquito natural habitats.

BACKGROUND: The detrimental effects of chemical insecticides on the environment and human health have lead to the call for biological alternatives. Today, one of the most promising solutions is the use of spray formulations based on Bacillus thuringiensis subsp. israelensis (Bti) in insect control programs. As a result, the amounts of Bti spread in the environment are expected to increase worldwide, whilst the common belief that commercial Bti is easily cleared from the ecosystem has not yet been clearly established. METHODOLOGY/MAIN FINDINGS: In this study, we aimed to determine the nature and origin of the high toxicity toward mosquito larvae found in decaying leaf litter collected in several natural mosquito breeding sites in the Rhône-Alpes region. From the toxic fraction of the leaf litter, we isolated B. cereus-like bacteria that were further characterized as B. thuringiensis subsp. israelensis using PCR amplification of specific toxin genes. Immunological analysis of these Bti strains showed that they belong to the H14 group. We finally used amplified length polymorphism (AFLP) markers to show that the strains isolated from the leaf litter were closely related to those present in the commercial insecticide used for field application, and differed from natural worldwide genotypes. CONCLUSIONS/SIGNIFICANCE: Our results raise the issue of the persistence, potential proliferation and environmental accumulation of human-spread Bti in natural mosquito habitats. Such Bti environmental persistence may lengthen the exposure time of insects to this bio-insecticide, thereby increasing the risk of resistance acquisition in target insects, and of a negative impact on non-target insects.

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.

Phototransformation of the insecticide fipronil: identification of novel photoproducts and evidence for an alternative pathway of photodegradation.

Fipronil is a recently discovered insecticide of the phenylpyrazole series. It has a highly selective biochemical mode of action, which has led to its use in a large number of important agronomical, household, and veterinary applications. Previous studies have shown that, during exposure to light, fipronil is converted into a desulfurated derivative (desulfinyl-fipronil), which has slightly reduced insecticidal activity. In this study, the photodegradation of fipronil was studied in solution at low light intensities (sunlight or UV lamp). In addition to desulfinyl-fipronil, a large number of minor photoproducts were observed, including diversely substituted phenylpyrazole derivatives and aniline derivatives that had lost the pyrazole ring. Desulfinylfipronil itself was shown to be relatively stable under both UV light and sunlight, with only limited changes occurring in the substitution of the aromatic ring. Since this compound accumulated to levels corresponding to only 30-55% of the amount of fipronil degraded, it was concluded that one or more alternative pathways of photodegradation must be operating. On the basis of the structurally identified photoproducts, it is proposed that fipronil photodegradation occurs via at least two distinct pathways, one of which involves desulfuration at the 4-position of the pyrazole ring giving the desulfinyl derivative and the other of which involves a different modification of the 4-substituent, leading to cleavage of the pyrazole ring and the formation of aniline derivatives. The latter compounds do not accumulate to high levels and may, therefore, be degraded further. The ecological significance of these results is discussed, particularly with regard to the insecticidal activity of the photoproducts.