Evaluation of azadirachtin used to protect plants against arthropod pests

One alternative to synthetic pesticides for pest control are botanical insecticides such as neem (Azadirachta indica) products, which have the advantage of a low risk of bioaccumulation in the environment. Neem products based on extracts from seed kernels of the neem tree contain azadirachtin as the leading active substance. Aphids and spider mites are among the most difficult pests to control and are severe pests of horticultural crops such as rose plants. Considering previously published results for the control of aphids, effects of neem products can vary between aphid species. Only few results are available for the rose aphid Macrosiphum rosae and the two-spotted spider mite Tetranychus urticae. Furthermore, due to different and often not well characterized neem or pure azadirachtin formulations used, results are difficult to compare. The effects that were observed in laboratory experiments might not occur in the same manner under practical conditions in the greenhouse or field with today’s registered doses. The main objective of this thesis was to evaluate the potential of a commercial neem product (NeemAzal‑T/S) with a standardized concentration of azadirachtin for controlling M. rosae and T. urticae on rose plants with registered doses. The effects of the active ingredient as well as of the formulation were addressed. The efficacy was investigated in greenhouse trials and the mode of action was assessed. Furthermore, possible environmental factors were evaluated, which can influence the efficacy of the product. Greenhouse and field trials were carried out to complement laboratory trials to have realistic conditions.Efficacy trials were conducted in the greenhouse, where the azadirachtin product and a synthetic standard product (flupyradifurone) were applied to populations of M. rosae of mixed developmental stages. The neem product in the registered dose corresponding to 33 ppm azadirachtin A reached an efficacy of 40 % (corrected mortality) after two applications, while the synthetic standard reached 90 % with one application. Further experiments revealed a significant impact of NeemAzal-T/S on the survival of first to third instar nymphs and prolonged developmental times to the second and third stage of surviving nymphs. The reproduction of adult M. rosae was significantly reduced when they are exposed to azadirachtin-treated plants already as first instar nymphs. Furthermore, the population growth of M. rosae starting with one adult female was tested and revealed a significant, dose-dependent impact of NeemAzal-T/S. The moderate efficacy and the population-growth inhibiting effect of the product in combination with biological control might result in a sufficient control of the pest insects, even if neem provides no strong knock-down effect and has no significant impact on adult aphids (Chapter 2).For further investigations on the potential of azadirachtin to control aphids and to act as an antifeedant or repellent to adult aphids, experiments on the systemic action and feeding activity of M. rosae were conducted (Chapter 3). Our experiments indicate a basipetal and acropetal systemic translocation of active ingredients from NeemAzal-T/S. Consequently, it was concluded that active ingredients are present in the phloem – the preferred feeding site of most aphids. However, no significant effects were found testing the repellence of adult aphids in choice tests with treated and untreated rose plant. Furthermore, we found no significant effects on the feeding activity, assessed as the honeydew production of mixed populations of M. rosae (quantitative approach) and via an electrical penetration graph (EPG) experiment with adult aphids (qualitative approach). The effects of NeemAzal-T/S on M. rosae was considered to be a result of insect-growth regulating effects after feeding on treated plants, not on starvation.As the efficacy of neem products is not as high as those of synthetic pesticides and as our results show no knock-down effect, knowledge about environmental influencing factors and application timing is important to improve the effect of the product. To evaluate the impact on the efficacy of NeemAzal-T/S, greenhouse trials were conducted with M. rosae at different temperatures and at different UV-A intensities. In contrast to our expectations, NeemAzal-T/S can be well used in the tested temperature range and also at higher UV radiations. An application in the evening resulted in significantly better control of M. rosae in outdoor trials. however, this effect seems to be a formulation contact effect and is not dependent on azadirachtin (Chapter 4).The efficacy and deterrent effects of neem formulations on spider mites (T. urticae) were tested in laboratory and greenhouse trials (Chapter 5). With NeemAzal-T/S, 90 % efficacy was reached 33 days after the first application (26 days after the second treatment). The synthetic standard, an acequinocyl product, was only applied once and provides a much stronger knock-down effect with 60 % efficacy seven days after the application. Our experiments indicate a significant formulation effect. A further investigation of the formulation effect shows that both oil and surfactant were significant factors influencing the efficacy also short-term, while azadirachtin seem to be responsible for a long-lasting effect on spider mites. The deterrent effect of the formulation we observed in laboratory trials is only relevant for a short time after application. A soil application on rose plants, to exclude any contact effect of the formulation had no impact on spider mites. For an efficient spider mite control, the contact knock-down and deterrent effect of the neem formulation should be exploited.Neem products are good alternatives to synthetic products for the home and garden use, as their human safety is high and their impact on the environment is low. In addition, for professional horticultural production of high value crops, products based on azadirachtin are valuable and can be part of IPM strategies. They slow down the population growth of aphids and in combination with other control agents such as beneficials, control levels similar to those of synthetic standard product might be reached. Studies with neem products should be conducted in the greenhouse or field to be as close as possible to realistic conditions. Further investigations under practical conditions of such IPM conform combinations or of spraying sequences, in which azadirachtin is replacing synthetic products, are needed to show the full potential.