Physical conditions affecting pyrethroid toxicity in arthropods

The aim of this thesis was to obtain mechanistic information about how the toxicity of pesticides in the field is affected by physical factors, pesticide bioavailability and arthropod behaviour. The pyrethroid insecticide deltamethrin and linyphiid spiders were selected as pesticide-effect model. In Part I of this thesis the relationships between the toxic action of deltamethrin, physical factors and spider behaviour were studied in field experiments, and attention was focused on the contribution of walking behaviour to pesticide exposure. in Part 11 laboratory experiments were carried out where special attention was payed to residual bioavailability of deltamethrin and the effects of this compound on the behaviour and hygrothermal physiology of female O. apicatus .Part I: Field studiesField studies have shown that high walking activity in spiders after deltamethrin application leads to a strong reduction of the trapping success (Chapter 6). This supports the hypothesis that walking activity is an important means of exposure to residues of deltamethrin for spiders on the soil surface. The testing of the above hypothesis was made possible by the spraying of the pesticide under conditions of low and high spider walking activity, the use of small plots that still allowed for independent replication, and the use of a low level of toxicant application rate. The low application rate was chosen so that trapping success would then depend both on physical factors and the effect of the toxicant. The prediction of daily spider walking activity in relation to physical factors was based on observations of the effects of physical factors and internal activity rhythms on trapping success. Factors showing a very high positive or negative relationship with trapping success were identified in an experiment in which the number of spiders trapped daily in 30 pitfall traps for a period of 72 days was correlated with a number of easily measurable physical factors using multivariate analysis (Chapter 2). Another experiment demonstrated that spiders posess highly predictable, species dependent nicthemeral activity rhythms which are little affected by varying physical conditions, the latter implying an internal mechanism, i.e. some physiological factor (Chapter 3). It was shown that the independence of treatments for at least one week could be assured when traps in a certain field are surrounded by a similarly treated boundery of 7 meters, as spider migration into a sprayed plot was found to be relatively slow; however, the spatial distribution of spider activity showed rapid changes in relation to microclimatic conditions in the crop (Chapter 4). An appropriate low dose rate for deltamethrin was determined to be between 0.2 and 0.5 g ai ha -1deltamethrin (Chapter 5).Part II: Laboratory studiesLaboratory experiments were used to study the bioavailability of deltamethrin in substrate, and the effect of deltamethrin on the hygrothermal physiology and behaviour of 0. apicatus.Using [ 14C]deltamethrin, it was shown that residual bioavailability was more than 100 times higher for moss or fungi covering the soil, than for the clay soil used in the experiments. Bioavailability showed an exponential decline over time with a halflife of 157 min and was raised by increasing water content of the soil only at 63% (dwt), at which humidity the soil was water- logged. The rate of pesticide uptake by 0. apicatusdecreased with distance walked (Chapter 7).Experiments on the hygrothermal physiology of female 0. apicatus,indicated that water loss is probably the main cause of mortality following deltamethrin poisoning. Water loss in 0. apicatusisthe sum of evaporation and deltamethrin induced water excretion. Evaporation was related linearly to the vapour pressure deficit of the air, and showed an exponential increase with temperature in relation to water conductivity of the cuticle. Deltamethrin induced water excretion was independent of the vapour pressure deficit, and showed an increase with dose and temperature (Chapter 8).As long as spiders are able to walk, they can compensate for water loss by drinking. Immobile spiders loose the capacity to drink free water and are therefore vulnerable to lethal water loss. In this respect it should be noted that the results in chapter 8 indicated that the immobilisation rate of spiders poisoned by deltamethrin depends on the humidity of the air. A simulation model combining quantitative measurements of the independent effects of deltamethrin on immobilisation and water excretion is presented in Chapter 9. The model simulates mortality in a laboratory population of female 0. apicatusat different air humidities and temperatures.