Christian Kjær

The impact of phenology, exposure and instar susceptibility on insecticide effects on a chrysomelid beetle population

Direct topical impact of an insecticide spray on a population of a non-target leaf-eating beetle, Gastrophysa polygoni, was studied, and the relative importance of phenology, instar susceptibility and instar specific exposure was evaluated. Two insecticides, cypermethrin and dimethoate, were investigated. In the laboratory, topical toxicity to eggs, second-instar larvae and adults was recorded in dose-response experiments. The spatial distribution of larvae and eggs were measured in the field. Deposition of insecticide onto eggs, second-instar larvae and adult specimens was measured at different positions within the crop canopy by use of a dye tracer technique. A temperature-driven population model was constructed to simulate population development of all life stages in the field. The model was based on laboratory measures of growth and development at various temperatures. Mortality due to direct insecticide exposure was calculated as a function of population demography, spatial distribution of individuals, spatial deposition of the insecticide, and stage-specific susceptibility. Cypermethrin had the greatest impact, reducing population size by 19-32%. The life stages most sensitive to cypermethrin were the larval instars. As the population developed from eggs to larvae and imagines, the impact of one spraying first increased and then decreased according to the proportion of larvae in the population. Dimethoate had less effect on the population, i.e. 1.9-7.6% reduction. Dimethoate was most toxic to the egg stage, and consequently the effect on the population decreased as the proportion of eggs decreased due to hatching. The direct effect of insecticide spraying was significantly affected by all three factors investigated, i.e. phenology, life stage susceptibility and stage-specific exposure. The latter factor is composed of both spray flux at various spatial positions in the canopy and the ability of different life stages to retain spray droplets.

Temperature—dependent, time–dose–effect model for pesticide effects on growing, herbivorous arthropods: Bioassays with dimethoate and cypermethrin

A toxicokinetics-based, temperature-dependent survival model for growing animals with oral uptake of toxicants is used to analyze the results of two pesticide bioassays. With this approach, we aim at simultaneously addressing two complementary goals of pesticide bioassays, namely to assess species sensitivity and to elucidate underlying mechanisms of toxic effects. As test organisms, newly hatched larvae of the chrysomelid beetle Gastrophysa polygoni L., dwelling on the underside of leaves of black bindweed Fallopia convolvulus (L.), kept at 12, 17, or 25°C were used. Plants with larvae were sprayed with dimethoate and cypermethrin at five dosages and a control. Survival was assessed during the following 6 d. The pesticide deposition pattern on the plants and the overall concentrations in the plants were determined. For dimethoate, which is mainly taken up orally, observed survival curves could be simulated successfully by the model. In the case of cypermethrin, which acts as a feeding deterrent, the model showed poor correspondence to the data. The significance of the results is discussed in relation to the test conditions and to toxicokinetics.

Are we analysing knockdown in the right way? how independence of the knockdown‐recovery process from mortality may affect measures for behavioural effects in pesticide bioassays

In pesticide bioassays, especially those with neurotoxic agents, effects on animals are typically grouped into classes according to behaviour, such as normal and affected behaviour, which may range from unstable walking behaviour, to unable to move, to mortality. Generally, recovery is observed in all these effect classes, except the last. Mortality, however, disturbs the analysis of the recovery processes because it decreases the number of animals that otherwise could have shown a reversible effect. We consider that this interaction between mortality and other, reversible, effects is a conceptual problem, and give arguments in favour of analysing changes in behaviour and mortality as two independent, simultaneously occurring neurotoxic syndromes. As an illustration, two data sets are analysed in both ways and these show that marked differences may exist between conclusions reached by the two viewpoints. The consequences thereof are discussed in relation to toxico-kinetic explanations for neurotoxicant effects on behaviour and mortality.

Assessment of effects of Bt-oilseed rape on large white butterfly (Pieris brassicae) in natural habitats

Much effort has been allocated to the definition of risk, relevant for the assessment of genetically modified plants. However, few studies have emphasised the limitations in testing methods. In this study, tests for and effects on non‐target herbivores were exemplified and evaluated for Pieris brassicae (L.) (Lepidoptera: Pieridae) and a genetically modified Brassica napus L. (Brassicaceae) expressing the Bacillus thuringiensis (Bt)‐toxin Cry1Ac. It was established that this herbivore recognises and accepts the transgenic plant as a host. It was found that ovipositing females of P. brassicae preferred the transgenic variety for egg‐laying. Therefore, effects of the transgenic host plant on the herbivore were determined. Larvae feeding on the Bt‐plants experienced 100% mortality for all larval stages. Based on these observations, a population model was established. The model showed that larval survival is increased with amount of food (number of plants) and reduced with the frequency of transgenic specimens, number of host plants needed for completing larval development, and number of egg‐laying butterflies. Such models may both aid the design of further tests for effects and support the assessment whether population effects are likely to occur due to the presence of insect‐resistant plants outside the agricultural area.

COMPARISON OF THE TOXICITY OF DIMETHOATE AND CYPERMETHRIN IN THE LABORATORY AND THE FIELD WHEN APPLYING THE SAME BIOASSAY

The present paper studies the relationship between laboratory- and field-based survival data of beetle larvae sprayed with either dimethoate or cypermethrin. The comparison for dimethoate was based on an earlier constructed pesticide-effect model derived from greenhouse data. To predict field effects, the model was supplied with field data for temperatures, the insecticide concentrations on the plants, and the control mortality rate of the beetles. The same model structure was unable to predict greenhouse mortality from cypermethrin treatment, for which reason the differences between laboratory and field for this compound are discussed on the basis of direct observations. For best comparison, the same bioassay method was used in both the greenhouse and the field in the present study. Tests were based on Gastrophysa polygoni larvae dwelling on the undersides of leaves of Fallopia convolvulus plants. Dietary uptake was regarded as the main source of exposure. The simulation results for dimethoate underestimated the field mortality even after accounting for the higher control mortality in the field in the model. It was shown that an additional increase of the toxicant-induced death rate by more than a factor of five was required for the model to fit the field data. This indicated a relatively severe impact of dimethoate under field conditions. For cypermethrin, the effects in the field increased with time, pointing at increasing exposure of and/or effects on the larvae. Contributions of the present results toward the bridging of the gap between laboratory and field are discussed.

Herbivory and plant community dynamics: Competitive interactions between an insect-resistant and an insect-susceptible Arabidopsis thaliana genotype

Abstract Resistance towards herbivory is expected to influence the competitive ability and ecological success of the resistant plant, but it is unclear how this general knowledge should be incorporated into long-term ecological predictions of plant community dynamics. In order to answer such questions, the long-term ecological effects of density, competition, herbivory and their compound interactions were investigated in a model system of a transgenic herbivore-resistant Arabidopsis thaliana genotype and the isogenic herbivore-sensitive A. thaliana genotype. It was concluded that herbivory had a significant effect on the fecundity of the susceptible genotype at high plant density. The most likely long-term scenario was that the susceptible genotype outcompeted the resistant genotype. But it was also shown that herbivory could down-regulate the equilibrium density of the susceptible genotype and, when the two genotypes were coexisting, up-regulate the equilibrium density of the resistant genotype.