Feeding inhibition and mortality in Reticulitermes flavipes (Isoptera: Rhinotermitidae) after exposure to imidacloprid-treated soils

Time-Cumulative Toxicity of Neonicotinoids: Experimental Evidence and Implications for Environmental Risk Assessments

Our mechanistic understanding of the toxicity of chemicals that target biochemical and/or physiological pathways, such as pesticides and medical drugs is that they do so by binding to specific molecules. The nature of the latter molecules (e.g., enzymes, receptors, DNA, proteins, etc.) and the strength of the binding to such chemicals elicit a toxic effect in organisms, which magnitude depends on the doses exposed to within a given timeframe. While dose and time of exposure are critical factors determining the toxicity of pesticides, different types of chemicals behave differently. Experimental evidence demonstrates that the toxicity of neonicotinoids increases with exposure time as much as with the dose, and therefore it has been described as time-cumulative toxicity. Examples for aquatic and terrestrial organisms are shown here. This pattern of toxicity, also found among carcinogenic compounds and other toxicants, has been ignored in ecotoxicology and risk assessments for a long time. The implications of the time-cumulative toxicity of neonicotinoids on non-target organisms of aquatic and terrestrial environments are far reaching. Firstly, neonicotinoids are incompatible with integrated pest management (IPM) approaches and secondly regulatory assessments for this class of compounds cannot be based solely on exposure doses but need also to take into consideration the time factor.

Sublethal Effect of Imidacloprid on Solenopsis invicta (Hymenoptera: Formicidae) Feeding, Digging, and Foraging Behavior

There is increasing evidence that exposure to neonicotinoid insecticides at sublethal levels impairs colonies of honeybees and other pollinators. Recently, it was found that sublethal contamination with neonicotinoids also affect growth and behavior of ants. In this study, we exposed red imported fire ants, Solenopsis invicta Buren, to sublethal dosages of dietary imidacloprid and investigated its effect on ant feeding, digging, and foraging behavior. S. invicta consumed significantly more sugar water containing 0.01 μg/ml imidacloprid than untreated sugar water. Ants fed with 0.01 μg/ml imidacloprid also showed significantly increased digging activity than ants fed with untreated sugar water. However, imidacloprid at ≥ 0.25 μg/ml significantly suppressed sugar water consumption, digging, and foraging behavior. These results indicate that imidacloprid at sublethal concentrations may have a significant and complicated effect on S. invicta.

Delayed and time-cumulative toxicity of imidacloprid in bees, ants and termites

Imidacloprid, one of the most commonly used insecticides, is highly toxic to bees and other beneficial insects. The regulatory challenge to determine safe levels of residual pesticides can benefit from information about the time-dependent toxicity of this chemical. Using published toxicity data for imidacloprid for several insect species, we construct time-to-lethal-effect toxicity plots and fit temporal power-law scaling curves to the data. The level of toxic exposure that results in 50% mortality after time t is found to scale as t^1.7 for ants, from t^1.6 to t⁵ for honeybees, and from t^1.46 to t^2.9 for termites. We present a simple toxicological model that can explain t² scaling. Extrapolating the toxicity scaling for honeybees to the lifespan of winter bees suggests that imidacloprid in honey at 0.25 μg/kg would be lethal to a large proportion of bees nearing the end of their life.

The insecticide imidacloprid causes mortality of the freshwater amphipod Gammarus pulex by interfering with feeding behavior

If an organism does not feed, it dies of starvation. Even though some insecticides which are used to control pests in agriculture can interfere with feeding behavior of insects and other invertebrates, the link from chemical exposure via affected feeding activity to impaired life history traits, such as survival, has not received much attention in ecotoxicology. One of these insecticides is the neonicotinoid imidacloprid, a neurotoxic substance acting specifically on the insect nervous system. We show that imidacloprid has the potential to indirectly cause lethality in aquatic invertebrate populations at low, sublethal concentrations by impairing movements and thus feeding. We investigated feeding activity, lipid content, immobility, and survival of the aquatic arthropod Gammarus pulex under exposure to imidacloprid. We performed experiments with 14 and 21 days duration, both including two treatments with two high, one day pulses of imidacloprid and one treatment with a low, constant concentration. Feeding of G. pulex as well as lipid content were significantly reduced under exposure to the low, constant imidacloprid concentration (15 µg/L). Organisms were not able to move and feed--and this caused high mortality after 14 days of constant exposure. In contrast, feeding and lipid content were not affected by repeated imidacloprid pulses. In these treatments, animals were mostly immobilized during the chemical pulses but did recover relatively fast after transfer to clean water. We also performed a starvation experiment without exposure to imidacloprid which showed that starvation alone does not explain the mortality in the constant imidacloprid exposure. Using a multiple stressor toxicokinetic-toxicodynamic modeling approach, we showed that both starvation and other toxic effects of imidacloprid play a role for determining mortality in constant exposure to the insecticide.

Soil microbial degradation of neonicotinoid insecticides imidacloprid, acetamiprid, thiacloprid and imidaclothiz and its effect on the persistence of bioefficacy against horsebean aphid Aphis craccivora Koch after soil application

BACKGROUND

The neonicotinoids imidacloprid, imidaclothiz, acetamiprid and thiacloprid consist of similar structural substituents but differ considerably with respect to soil use. Therefore, the effects of soil microbial activity on the degradation and bioefficacy persistence of the four neonicotinoids were evaluated.

RESULTS

In unsterilised soils, 94.0% of acetamiprid and 98.8% of thiacloprid were degraded within 15 days, while only 22.5% of imidacloprid and 25.1% of imidaclothiz were degraded over a longer period of 25 days. In contrast, in sterilised soils, the degradation rates of acetamiprid and thiacloprid were respectively only 21.4% and 27.6%, whereas the degradation rates of imidaclothiz and imidacloprid were respectively 9.0% and almost 0% within 25 days. The degradation products of imidacloprid and imidaclothiz were identified as olefin, nitroso or guanidine metabolites, the degradation product of thiacloprid was identified as an amide metabolite and no degradation product of acetamiprid was detected. A bioefficacy assay revealed that the bioefficacy and persistence of imidacloprid, imidaclothiz, acetamiprid and thiacloprid against horsebean aphid A. craccivora were related to their degradation rate and the bioefficacy of their degradation products in soil.

CONCLUSION

Soil microbial activity played a key role in the bioefficacy persistence of neonicotinoid insecticides and therefore significantly affected their technical profile after soil application.

Effect of vegetation on the longevity, mobility and activity of fipronil applied at the termiticidal rate in laboratory soil columns

BACKGROUND

Termiticides are applied at concentrations much higher than those used in agricultural settings. The longevity of fipronil has not yet been examined at the rates used for termite control, nor has the compound's movement in the soil been addressed.

RESULTS

Fipronil was detected in the eluates of treated soil cones, increasing initially and then decreasing to a steady concentration of about 1 microg mL(-1). In larger PVC pipe plots, fipronil in the top treated soil depth (0-7.5 cm) dissipated more rapidly (half-life of 11-13 months) than in treated soil at the next treated depth (7.5-15 cm; half-life of 20-29 months). The presence of vegetation had no significant effect on the mobility, longevity or movement into untreated depths. Treated soil remained toxic to termites throughout the duration of the study. Fipronil moved into the 15-22.5 cm soil depth in sufficient concentration to cause 100% mortality to eastern subterranean termites in 3 day bioassays.

CONCLUSION

Fipronil remains in treated soil at levels toxic to termites for at least 30 months. Movement of the active ingredient was observed in sufficient amounts to kill termites in non-treated soil directly below the treated soil.

Colony-level effects of imidacloprid in subterranean termites (Isoptera: Rhinotermitidae)

We determined the impact of imidacloprid (Premise) on colonies of Reticulitermes spp. (Isoptera: Rhinotermitidae) through soil applications in the field. We selected 11 houses in the Raleigh, NC, area with active termite infestations. In-ground monitoring stations (mean = 75.9 stations) were installed around each house, and samples of termites visiting the monitors, in mud tubes, as well as samples from wood debris in the yard, were collected monthly for up to 14 mo to determine the numbers and locations of colonies present before treatment. We used microsatellite genetic markers to identify individual colonies present on each property. All houses were treated with Premise 75 WSP by using an exterior perimeter/interior spot treatment. After treatment, termite samples were collected monthly for 3 mo and then quarterly for 2 yr to track the fate of colonies. Of the 12 treated colonies (those attacking structures), 75% disappeared within 90 d and were not detected again. In contrast, only 25% of 48 untreated colonies (located 2 m or further from the treatment zone) and 40% of the six likely treated colonies (located within 0.5 m of the treatment zone but not known to be attacking the structure) were not detected again during the study. Our findings are consistent with strong colony-level effects of soil treatments with imidacloprid, resulting in the suppression or elimination of Reticulitermes spp. colonies in many cases.

Bioavailability of xenobiotics in the soil environment

It is often presumed that all chemicals in soil are available to microorganisms, plant roots, and soil fauna via dermal exposure. Subsequent bioaccumulation through the food chain may then result in exposure to higher organisms. Using the presumption of total availability, national governments reduce environmental threshold levels of regulated chemicals by increasing guideline safety margins. However, evidence shows that chemical residues in the soil environment are not always bioavailable. Hence, actual chemical exposure levels of biota are much less than concentrations present in soil would suggest. Because "bioavailability" conveys meaning that combines implications of chemical sol persistency, efficacy, and toxicity, insights on the magnitude of a chemicals soil bioavailability is valuable. however, soil bioavailability of chemicals is a complex topic, and is affected by chemical properties, soil properties, species exposed, climate, and interaction processes. In this review, the state-of-art scientific basis for bioavailability is addressed. Key points covered include: definition, factors affecting bioavailability, equations governing key transport and distributive kinetics, and primary methods for estimating bioavailability. Primary transport mechanisms in living organisms, critical to an understanding of bioavailability, also presage the review. Transport of lipophilic chemicals occurs mainly by passive diffusion for all microorganisms, plants, and soil fauna. Therefore, the distribution of a chemical between organisms and soil (bioavailable proportion) follows partition equilibrium theory. However, a chemical's bioavailability does not always follow partition equilibrium theory because of other interactions with soil, such as soil sorption, hysteretic desorption, effects of surfactants in pore water, formation of "bound residue", etc. Bioassays for estimating chemical bioavailability have been introduced with several targeted endpoints: microbial degradation, uptake by higher plants and soil fauna, and toxicity to organisms. However, there bioassays are often time consuming and laborious. Thus, mild extraction methods have been employed to estimate bioavailability of chemicals. Mild methods include sequential extraction using alcohols, hexane/water, supercritical fluids (carbon dioxide), aqueous hydroxypropyl-beta-cyclodextrin extraction, polymeric TENAX beads extraction, and poly(dimethylsiloxane)-coated solid-phase microextraction. It should be noted that mild extraction methods may predict bioavailability at the moment when measurements are carried out, but not the changes in bioavailability that may occur over time. Simulation models are needed to estimate better bioavailability as a function of exposure time. In the past, models have progressed significantly by addressing each group of organisms separately: microbial degradation, plant uptake via evapotranspiration processes, and uptake of soil fauna in their habitat. This approach has been used primarily because of wide differences in the physiology and behaviors of such disparate organisms. However, improvement of models is badly needed, Particularly to describe uptake processes by plant and animals that impinge on bioavailability. Although models are required to describe all important factors that may affect chemical bioavailability to individual organisms over time (e.g., sorption/desorption to soil/sediment, volatilization, dissolution, aging, "bound residue" formation, biodegradation, etc.), these models should be simplified, when possible, to limit the number of parameters to the practical minimum. Although significant scientific progress has been made in understanding the complexities in specific methodologies dedicated to determining bioavailability, no method has yet emerged to characterized bioavailability across a wide range of chemicals, organisms, and soils/sediments. The primary aim in studying bioavailability is to define options for addressing bioremediation or environmental toxicity (risk assessment), and that is unlikely to change. Because of its importance in estimating research is needed to more comprehensively address the key environmental issue of "bioavailability of chemicals in soil/sediment."

CONTROLE QUIMICO DA COCHONILHA-FARINHENTA PLANOCOCCUS CITRI (RISSO, 1813) (HEMIPTERA: PSEUDOCOCCIDAE) EM DIFERENTES IDADES DA VIDEIRA

RESUMO O efeito de inseticidas neonicotinoides (acetamiprido, imidacloprido e tiametoxam) e reguladores de crescimento (buprofezina e piriproxifem) foram avaliados para o controle dePlanococcus citri na cultura da videira. Os experimentos foram conduzidos em casa-de-vegetação (plantas com um ano) e vinhedo comercial (plantas adultas-15 anos) utilizando a cultivar Cabernet Sauvignon. Os inseticidas acetamiprido (Mospilan, 0,6 g i.a./planta), imidacloprido (Confidor 700 GRDA, 0,7 g i.a./planta) e tiametoxam (Actara 250 WG, 0,75 g i.a./planta) foram aplicados via solo utilizando 200 e 1.000 mL de água por planta com um ano e 15 anos, respectivamente. Em pulverização foliar foram avaliados somente em plantas adultas os inseticidas acetamiprido (Mospilan, 6 g i.a./100L); buprofezina (Applaud 250, 25 g i.a./100 L); imidacloprido (Confidor 700 GRDA, 7 g i.a./100 L), piriproxifem (Tiger 100 CE, 10 g i.a./100 L), tiametoxam (Actara 250 WG, 7,5 g i.a./100 L) e um tratamento testemunha (somente água). Aos 3, 7, 14, 21, 28, 35 e 60 dias após a aplicação dos inseticidas (DAA) nos experimentos via solo e 1, 5 e 10 (DAA) no experimento em pulverização foliar as folhas foram retiradas das plantas e infestadas em laboratório com ninfas de primeiro instar provenientes de criação artificial. Em plantas novas de um ano de idade os inseticidas neonicotinoides acetamiprido, imidacloprido e tiametoxam nas doses avaliadas foram eficazes proporcionando uma mortalidade de 82, 94 e 82%, respectivamente, até 35 dias após a aplicação. Em plantas adultas, somente o inseticida imidacloprido reduziu a infestação em 63% até 7 dias após a aplicação. Os inseticidas aplicados via pulverização foliar, acetamiprido, buprofezina, imidacloprido, tiametoxam e piriproxifem não proporcionaram mortalidade significativa de P. citri na cultura da videira.

Imidacloprid mobility and longevity in soil columns at a termiticidal application rate

The mobility, longevity and termiticidal activity of imidacloprid (Premise 2 termiticide; Bayer Environmental Sciences) at the termiticidal labeled rate for perimeter treatment were tested in vegetated and non-vegetated soil columns in two tests: in cone plots and in polyvinyl chloride (PVC) pipes. Imidacloprid content in the cone plot eluate peaked at 1 month, declined rapidly by the second month and then entered a lagging phase. The concentration of imidacloprid in the cone plot soil declined from 84.5 microg g(-1) initially to 7.5 microg g(-1) (non-vegetated plots) and 8.1 microg g(-1) (vegetated plots) 6 months later. Neither eluate concentration nor soil concentration was affected by the presence of vegetation in the cone plots. In the PVC pipes, the top 15 cm of which was treated with Premise 2 at the perimeter labeled rate, imidacloprid half-life was estimated at 6-9 months for vegetated and non-vegetated soil. Extractable imidacloprid declined more rapidly in the first 15 months than afterwards. Mobility of imidacloprid into lower, untreated soil depths was higher in non-vegetated pipes, and was likely due to the effect of vegetation on soil moisture. The presence of vegetation had little effect on the termiticidal activity of treated soil in the PVC pipes.

Influence of Portland cement amendment on soil pH and residual soil termiticide performance

Soil adjacent to new brick veneer work is likely to have a higher pH owing to the mixture of cement with the soil. In the Gainesville, FL, area, soil samples taken from such locations had a range of pH values from 9.0 to 10.1; similar soils used in bioassays had a pH of 5.6 before the addition of cement. Addition of 15 mg of Portland cement to 33 g of soil increased the pH to 6, and addition of 291 mg of Portland cement increased the pH to 9. The pH of soil amended with cement was stable for the first 5 months. After 10 months, soil pH values decreased from alkaline to near neutral in all cases. Eastern subterranean termite workers, Reticulitermes flavipes (Kollar), were exposed to the treated soil at pH 6-9 for 24 h, and percentage mortality was recorded at 5 days, 5 months and 10 months. Termite mortality significantly decreased at higher soil pHs for bifenthrin, chlorpyrifos, fipronil and imidacloprid treatments at 5 months and similarly for bifenthrin, permethrin, chlorpyrifos, fipronil and imidacloprid treatments at 10 months. There was an inverse linear relationship between soil pH and mortality. Increased soil pH diminished residual activity of termiticide in the following order: imidacloprid > fipronil > chlorpyrifos = bifenthrin > permethrin > cypermethrin.

Experimental study on the toxicity of imidacloprid given in syrup to honey bee (Apis mellifera) colonies

Two groups of eight honey bee colonies were fed with two different concentrations of imidacloprid in saccharose syrup during summer (each colony was given 1 litre of saccharose syrup containing 0.5 microg litre(-1) or 5 microg litre(-1) of imidacloprid on 13 occasions). Their development and survival were followed in parallel with control hives (unfed or fed with saccharose syrup) until the end of the following winter. The parameters followed were: adult bee activity (number of bee entering the hive and pollen carrying activity), adult bee population level, capped brood area, frequency of parasitic and other diseases, mortality, number of frames with brood after wintering and a global score of colonies after wintering. The only parameters linked to feeding with imidacloprid-supplemented saccharose syrup when compared with feeding with non-supplemented syrup were: a statistically non-significant higher activity index of adult bees, a significantly higher frequency of pollen carrying during the feeding period and a larger number of capped brood cells. When imidacloprid was no longer applied, activity and pollen carrying were re-established at a similar level for all groups. Repeated feeding with syrup supplemented with imidacloprid did not provoke any immediate or any delayed mortality before, during or following the next winter, whereas such severe effects are described by several French bee keepers as a consequence of imidacloprid use for seed dressing in neighbouring cultures. In any case, during the whole study, mortality was very low in all groups, with no difference between imidacloprid-fed and control colonies. Further research should now address several hypotheses: the troubles described by bee keepers have causes other than imidacloprid; if such troubles are really due to this insecticide, they may only be observed either when bees consume contaminated pollen, when no other sources of food are available, in the presence of synergic factors (that still need to be identified), with some particular races of bees or when colonies are not strong and healthy.

Effect of imidacloprid tree treatments on the occurrence of Formosan subterranean termites, Coptotermes formosanus Shiraki (Isoptera: Rhinotermitidae), in independent monitors

Periodic sampling of 87 independent monitors, initially active with the Formosan subterranean termite, Coptotermes formosanus Shiraki, was conducted. Monitors, located in eight sectors adjacent to seven buildings, were various distances (1-46 m) from 57 trees treated with 0.1% imidacloprid foam. Termites collected from six of the eight sectors showed latent mortality attributed to imidacloprid intoxication at all monitor-tree distances. Approximately 6 mo after treatment, termite populations had recovered in these sectors. Another sector showed termite population suppression for approximately 15 mo, followed by recovery. Imidacloprid tree treatments did not control C. formosanus populations in independent monitors adjacent to the treatments.