Agro-ecological management of the wheat bug, Nysius huttoni (Hemiptera: Lygaeidae) and other pests in brassicas : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy

Modern agriculture offers a range of benefits including sufficient food production for a constantly increasing human population. Improved living standards, enhanced social stability and avoiding food insecurity are other advantages of agricultural intensification. Unfortunately, such agricultural intensification relies heavily on anthropogenic agricultural inputs such as high-yielding varieties, fertilizers and chemical pesticides. Some aspects of these practices are associated with human health problems, reduced biodiversity, degradation of soil fertility, air and water pollution, eutrophication of rivers and lakes, pollinator decline as well as impacts on atmospheric constituents and global warming. In New Zealand, wheat bug, Nysius huttoni, is considered an economic pest of forage brassicas and many other cultivated crops such as cereals and vegetables. This bug damages forage brassicas; greater economic losses have been recorded at the germination/seedling stage (90% plant loss in extreme situations). Insecticides as seed coatings and sprays are frequently used to manage this and other New Zeland forage brassica pests. Although seed coatings represent selective placement of the toxin, it is still true that large quantities are applied. A high proportion of these compounds enters the soil and leads to pesticide resistance, and they impact beneficial arthropods and soil microorganisms creating an adverse effect on ecosystem services (ES). This study developed a habitat management protocol using trap plant species in a ‘sustainable intensification’ approach, which is an alternative, more benign approach to pest management. Specifically, the study developed trap-crop technologies to draw N. huttoni away from kale seedlings. The use of less susceptible kale cultivars and integrating these into the trap cropping technology are important pest management strategies in integrated pest management (IPM) and potentially reduce over-reliance on orthodox pesticides on brassicas. Flowering trap plants can improve conservation biological control (CBC) and improve multiple ES in and off-farm in brassicas. A range of laboratory, field-cage and open-field experiments were carried out at Lincoln University (43° 38' S; 172° 27' E), New Zealand, during 2016 and 2017, and at Chitwan (270 37’ N; 840 22’ E), Nepal, during 2018 to: 1) evaluate host plant selection by N. huttoniof a range of potential trap plant species; 2) evaluate the susceptibility of kale cultivars to N. huttoni; 3) assess the growth stage of alyssum (Lobularia maritima) preferred by N. huttoni; 4) evaluate potential trap plant species for the N. huttoniin forage brassicas; and 5) improve CBC by using alyssum floral strips in a radish field. A series of laboratory choice, no-choice and paired-choice tests were conducted to evaluate the preference of N. huttonifor seedlings of eight potential trap plant species: L. maritima (alyssum), Triticum aestivum (wheat), Phacelia tanacetifolia (phacelia), Fagopyrum esculentum (buckwheat), Coriandrum sativum (coriander), Trifolium repens (white clover) and Medicago sativa (alfalfa). These species were compared with Brassica oleracea (kale) as a potentially susceptible control. Alyssum and wheat were the most favoured potential trap plants for N. huttoni, with a significantly higher survival rate, earlier feeding damage and quicker settling time. Laboratory bioassays were performed to evaluate N. huttoni preference for a range of kale cultivars: Kestrel, Gruner, Sovereign, Regal, Corka and Colear. Kestrel and Coleor are the most popular kale cultivars used as forage brassicas in New Zealand but they are the most susceptible to N. huttoni. Corka and Regal were the least susceptible cultivars; the others showed medium susceptibility cultivars to the N. huttoni. However, farmers mostly consider other agronomic factors such as yield and disease resistance during cultivar selection. The less susceptible kale cultivars can be integrated into an IPM strategy with trap cropping, biological and microbial approaches, for future low-pesticide management of the bug. Laboratory bioassays of two growth stages of alyssum were performed to evaluate N. huttoni preference for the growth stages of alyssum. Flowering alyssum was significantly more suitable for N. huttoni than seedlings. Assessment of bug preference for the various growth stages of alyssum plants suggests appropriate planting times for the trap and main crop. Efficient trapping of N. huttoni in brassica fields can be achieved if flowering alyssum strips are maintained at the brassica seedling stage in fields. Field cages and open-field experiments were established at the Biological Husbandry Unit (BHU), Lincoln University, to evaluate the performance of L. maritima and T. aestivum as a potential trap plants of N. huttoni compared with kale. In field cages, the most suitable trap plants, L. maritima and T. aestivum, were compared with the least suitable plants, C. sativum and T. repens, and all were compared with kale. In open field experiments, alyssum, wheat, ‘alyssum plus wheat’ and kale were used; other species were discarded based on their poor performance in the field-cage experiments. In field cages, alyssum was the most suitable trap plant of the bugs followed by wheat; this was also true in the open-field experiments. However, the ‘alyssum plus wheat’ trap strips have a greater potential to trap N. huttoni than wheat alone, but less potential than alyssum alone. In open fields, flowering, fruiting and senescent alyssum stages, and ripening and senescent wheat stages were significantly more suitable for trapping the bug than the vegetative stage. This information is important; it is necessary to maintain the flowering or fruiting stages of potential trap plants at the brassica seedling stage to reduce pest pressure in brassica fields. Nysius huttoni populations declined with distance from the edge trap strips. That significantly higher numbers of N. huttoni were intercepted at the edge trap strips suggests focussing N. huttoni management practices, such as ‘soft ‘chemicals, at the edges rather than other parts of fields, which would reduce pesticide cost. Less damage was recorded on kale seedlings next to wheat trap strips followed by alyssum, ‘alyssum plus wheat’ and kale strips. Flowering alyssum strips also provide habitat for many beneficial arthropods such as spiders, seven-spotted ladybirds (Coccinella septempunctata), and lacewings (Micromus tasmaniae), that could potentially kill N. huttoni and other brassica pests in forage brassicas. A study in Nepal to test alyssum (L. maritima), as a potential trap plant for the N. huttoni in CBC of pests in radish fields. Alyssum in radish fields significantly increased beneficial arthropods such as hoverflies (Diptera: Syrphidae), ladybirds (C. septempunctata), and spiders and reduced the pest pressure of aphids (Myzus persicae) and other pests. These findings are useful in developing a pest management protocol for N. huttoni using a ‘push-pull’ strategy in which less susceptible kale cultivars can be used as a ‘push’ component and alyssum plants as a ‘pull’ component. The less susceptible kale cultivars can also be used as a ‘push’ component and highly susceptible kale cultivars as a ‘pull’ component in a ‘push-pull’ strategy of pest management. Maintaining potential trap plant species at the flowering stage or growing highly susceptible kale cultivars at the edge of the main field can keep the wheat bugs away from the main crop and keep them from entering the main field. Flowering alyssum can also improve CBC and multiple ES in brassica fields and improve the quality of landscape.