Measuring pesticide ecological and health risks in West African agriculture to establish an enabling environment for sustainable intensification.

We outline an approach to pesticide risk assessment that is based upon surveys of pesticide use throughout West Africa. We have developed and used new risk assessment models to provide, to our knowledge, the first detailed, geographically extensive, scientifically based analysis of pesticide risks for this region. Human health risks from dermal exposure to adults and children are severe enough in many crops to require long periods of up to three weeks when entry to fields should be restricted. This is impractical in terms of crop management, and regulatory action is needed to remove these pesticides from the marketplace. We also found widespread risks to terrestrial and aquatic wildlife throughout the region, and if these results were extrapolated to all similar irrigated perimeters in the Senegal and Niger River Basins, they suggest that pesticides could pose a significant threat to regional biodiversity. Our analyses are presented at the regional, national and village levels to promote regulatory advances but also local risk communication and management. Without progress in pesticide risk management, supported by participatory farmer education, West African agriculture provides a weak context for the sustainable intensification of agricultural production or for the adoption of new crop technologies.

A test system to evaluate the susceptibility of Oregon, USA, native stream invertebrataes to triclopyr and carbaryl.

The susceptibility of six indigenous macroinvertebrate species representative of U.S. Pacific Northwest streams (Ameletus sp., Brachycentrus americanus, Calineuria californica, Cinygma sp., Lepidostoma unicolor, Psychoglypha sp. early and late instar) to formulated triclopyr ester (herbicide) and carbaryl (insecticide) was determined using laboratory bioassays. Acute toxicity was expressed as the lethal concentration to 50% (LC50) and 1% (LC1) of the test population based on a 96-h exposure duration. Carbaryl was found to be 1,000 times more toxic than triclopyr for all the organisms tested. The LCI values (7.5, 28.8, 9.0, 3.0, 9.5, 14.8, 33.8 microg/L, respectively, for carbaryl and 1.8, 3.9, 4.0, 4.2, 29.0, 16.1 mg/L, respectively, for triclopyr) were used in the calculation of hazardous concentration to 5% of the stream macroinvertebrate community (HC5) based on the lower 95% confidence limit (HC5/95). The hazardous concentration (HC5/95) for triclopyr was 0.11 mg/L and for carbaryl ranged from 0.43 to 0.66 microg/L, respectively. Triclopyr and carbaryl symptomology were analyzed for two organisms, C. californica and Cinygma sp. Carbaryl symptomology included knockdown and moribund states with severity and time of appearance being a function of dose. In triclopyr poisoning, death occurred suddenly with little or no symptomology. Time to 50% mortality (LT50) values were consistently higher for C. californica than for Cinygma sp. exposed to both chemicals at similar concentrations.

Effect of varying pesticide exposure duration and concentration on the toxicity of carbaryl to two field-collected stream invertebrates, Calineuria californica (Plecoptera: Perlidae) and Cinygma sp (Ephemeroptera: Heptageniidae).

The effect of exposure duration on the toxicity of a forest insecticide (carbaryl) was assessed under environmentally realistic exposure regimes against two stream invertebrates indigenous to the United States Pacific Northwest, Calineuria californica (Plecoptera: Perlidae) and Cinygma sp. (Ephemeroptera: Heptageniidae). Laboratory bioassays were conducted to evaluate the relationship between pulsed exposures of 15, 30, and 60 min and toxicity for a range of chemical concentrations (10.2-1,730 microg/L). For Cinygma sp., the 50% lethal concentration (LC50) values were calculated as 848 microg/L (15 min), 220 microg/L (30 min), and 165 microg/L (60 min). The C. californica consistently had lower mortality at a given concentration compared with Cinygma sp. Fifteen- and 30-min exposures did not elicit 50% mortality with C. californica, and it had a 60-min LC50 of 1,139 microg/L. Time to 50% mortality over 96 h after a 15-, 30-, or 60-min exposure, with the rest of the test period in freshwater (PLT50), was a function of exposure duration and concentration. Analysis of symptomology throughout the test period for C. californica gave evidence of recovery from the knockdown and moribund states, but this was not the case for Cinygma sp. The pulse duration resulting in 50% mortality was calculated as 43 min for Cinygma sp. exposed at 204 microg/L and 16 min at 408 microg/L. A three-dimensional probit plane model [Y = -10.86 + 4.83(ln C) + 3.0(ln T)], where Y is probit mortality, C is concentration in microg/L and T is time in hours, was used to explain the interaction between concentration (microg/L) and duration of exposure (hours) for Cinygma sp.

Leaf surfaces and the bioavailability of pesticide residues.

Laboratory bioassays were carried out to determine the toxicity to Folsomia candida Willem (Collembola: Isotomidae) of residues of a pyrethroid insecticide, deltamethrin, and an organophosphorus insecticide, dimethoate, on different leaf surfaces. The test leaves included a range of species and leaves of different ages. Dose-response relationships were estimated for F candida walking over the various treated leaf substrates. Probit analysis was used to estimate the means and standard deviations of the associated tolerance distributions expressed as gAIha-1. Parallelism tests were undertaken to compare the susceptibilities of F candida to the two compounds applied to the different leaf surfaces. On deltamethrin-treated leaf surfaces, the LD50 values for F candida varied from 6.36 to 77.14 gAIha-1. F candida was least susceptible to deltamethrin residues when applied to leaves of dwarf bean (Phaseolus vulgarus L) and the highest susceptibility was observed following application to leaves of seedlings of barley (Hordeum vulgare L). In contrast, the LD50 values observed for dimethoate treatments did not differ significantly between leaf types, ranging from 1.35 to 8.69 gAIha-1. The laboratory data on susceptibility of F candida on different leaf types for different pesticides can be used to investigate the role of leaf surface properties in modifying the toxicity of applied pesticides to exposed invertebrates.

The risks posed by deltamethrin drift to hedgerow butterflies.

The susceptibilities of Pieris rapae and P. brassicae (Lepidoptera: Pieridae) to topical treatment with deltamethrin were determined. LD(50)s continued to fall for an extended period after treatment and did not reach a clear end-point for P. rapae. This indicated a failure to excrete or metabolise the active ingredient, which may have continued to exert toxic effects throughout the insect's life cycle. Weight loss by P. brassicae larvae was detected at sublethal rates of exposure and treated insects exhibited feeding inhibition and produced smaller pupae and adults. Residual exposure bioassays over 72 h for P. brassicae exposed to deltamethrin deposits on cabbage leaves, detected toxic effects at rates as low as 1/520th of field application rate. LD(50)s again fell for an extended period after exposure. A model predicting mortality at given levels of drift indicated high levels of short-term risk to larvae exposed to rates of drift deposition recorded in the field. The potential of buffer zones to reduce the toxicological impact of spray drift in the field is discussed.

Do seasonal changes in numbers of aerially dispersing spiders reflect population density on the ground or variation in ballooning motivation?

Groups of linyphiid spiders (Erigone spp.) (Araneae, Linyphiidae), collected at intervals from arable land, were tested in laboratory bioassays to determine the proportion of individuals that exhibited ballooning behaviour on each field sampling occasion. There was no significant variation in the proportions of spiders in each test group ballooning in the laboratory over a year. Investigations of ground density and aerial dispersal, in a grass field and a winter-wheat field, confirmed that peaks in numbers of spiders observed ballooning in the field generally coincided with population peaks in the summer and autumn. Significant correlations between ground populations and aerial catches were found for total spiders, immature spiders, and Bathyphantes gracilis (Blackwall) in both fields, adult spiders in the grass field, Erigone spp. in the grass, and Meioneta rurestris (C.L. Koch) in the wheat. The other groups analysed, Lepthyphantes tenuis (Blackwall) in both fields, Erigone spp. in wheat, and M. rurestris in grass, showed similar but non-significant trends. No significant difference was found between overall ground-to-air ratios for males compared to females, but adult spiders were more likely to balloon than immatures.

The effect of food supply on the colonisation of barley by aerially dispersing spiders (Araneae).

A field experiment was carried out to determine whether different levels of food availability affected the retention rate of ballooning spiders landing in trays of seedling barley plants, half of which were infested with aphids from laboratory cultures. The trays were placed within bases in the field, then collected sequentially and spider numbers assessed in each tray. Deposition trays, containing trapping fluid only, were used to measure ballooning activity throughout the experimental period. The experiment was repeated four times. Overall, ballooning spiders were more likely to be retained in trays where aphid prey were present, with a total of 340 spiders found in the infested trays and 251 in the aphid-free trays, over the four experiments. Most of the spiders found were of the family Linyphiidae. In the second and fourth experiments the increased retention of spiders in the aphid-infested trays was statistically significant. Immature linyphiids alone also showed significantly higher retention in the infested trays in those two experiments and in the fourth experiment were largely responsible for the higher numbers found in the infested trays. There was also a statistically significant trend for a higher retention rate of female spiders, compared to males, in the barley trays than would have been expected from the ratios of females to males caught in the deposition trapping trays alone.