Abundance of Pest and Beneficial Macro-Invertebrates in Crop and Non-Crop Habitats Under Contrasting Insecticide Management Regimes in an Irrigated Cotton Landscape

Abstract

<p>Beneficial and pest invertebrates occur in irrigated cotton crops and in the surrounding landscape. Thus, farming and landscape management are inextricably linked in the challenge to encourage beneficial invertebrate survival and pest suppression. Cotton production has advanced recently in the cultivation of genetically modified cotton, areawide crop management and a reduction in reliance on insecticides. All these strategies contribute to enhancing the number of beneficial invertebrates and reducing the prevalence of pests in cotton crops. Beneficial and pest invertebrates also appear in other common winter crops such as wheat. However, only a small body of research has been conducted on the landscape-wide abundance of beneficial and pest invertebrates, and even less attention has been given to the non-crop habitat near to cotton crops that may play an essential role in beneficial and pest invertebrate abundance in cotton. </p> <p>This study was undertaken in a cotton-growing area in the Namoi Valley near Boggabri, in northern New South Wales, and an a landscape matrix including riparian river red gum (<i>Eucalyptus camaldulensis</i>) woodland, poplar box (<i>E. populnea</i>) woodland, and native grassland dominated by weeds such as <i>Ammi majus, Aster subulatus, Avena fatua, Bromus catharticus, Cynodon dactylon, Urochloa panicoides</i> and <i>Rapistrum rugosum</i>. Gaining an understanding of beneficial and pest invertebrate abundance and their response to different habitat types requires careful consideration of sampling strategy because of the variety of taxa and different habitats to be studied. With regard to sampling strategy, we compared two sampling methods (beat box and D-vacuum) for sampling key pest and beneficial invertebrates in different habitats. The aim of the first experiment was to evaluate the efficacy of the D-vac and beat box in sampling different invertebrate taxa in different vegetation types. The beat box sampling technique with four beats and six subsamples (the level of sampling effort required at each site in each vegetation type) were sufficient to capture the principal beneficial invertebrate taxa (ants, spiders, blue and red beetles, lady beetles and lacewings) and principal pest invertebrates (leafhoppers, apple dimpling bugs, red-shouldered leaf beetles, peanut scarab beetles, brown flea beetles, cotton seed bugs, flower beetles, mirids, Rutherglen bugs and aphids).</p> <p>The aim of the second experiment was to examine invertebrate abundance and irrigated cotton yields associated with changing farm management over time. Five years of beatsheet sampling was conducted on three farms near Boggabri NSW, from the beginning to the end of each summer cotton growing season 2009 till 2014. Beneficial invertebrates were significantly more abundant in the 2013/14 season than in the 2009/10 season, while pest invertebrates were significantly more abundant in 2009/10 than 2013/14. In addition, cotton yields were significantly greater in 2013/14 than 2009/10. These changes were associated with avoidance of insecticide use in cotton over the 5 years, which was valuable for increasing beneficial numbers and suppressing pests in cotton and led to greater yield. </p> <p>The aim of the next experiment was to determine the role of woody native vegetation and weedy native grassland in influencing the abundance of pests and beneficial invertebrates in irrigated cotton crops and the impact of farm management (conventional vs low-insecticide management) on these interactions. Beneficial invertebrates; ants (Hymenopteran), spiders (Araneae), blue and red beetles <i>Tectocoris diophthalmus</i> and lady beetles (Coccinellidae) and pest invertebrates; leafhoppers (Cicadellidae), apple dimpling bugs (<i>Campylomma liebknechti</i>), red-shouldered leaf beetles (<i>Monolepta australis</i>), peanut scarab beetles (Scarabaeidae), brown flea beetles (Chrysomelidae), cotton seed bugs (<i>Oxycarenus hyalinipennis</i>), flower beetles (Coleoptera), mirids (Miridae) and Rutherglen bugs (Lygaeidae) were sampled with beat box on two different types of farm (low-insecticide management vs conventional management) in different habitats (river red gum and poplar box woodland, weedy native grassland, irrigated cotton and refuge crops) in cotton crops stratified by distance to and amount of close by woody native vegetation. Cotton crops with a larger amount of woody native vegetation nearby on low-insecticide managed farms harboured more beneficial invertebrates than on conventionally managed farms. Cotton crops with a larger amount of woody native vegetation nearby on low-insecticide managed farms harboured less pest invertebrates than conventionally managed farms. Woodland harboured more beneficial invertebrates and less pest invertebrates than weedy native grassland, and woody native vegetation on low-insecticide farms harboured more beneficial than pest invertebrates. Cotton crops close to large amounts of native vegetation harboured more beneficials than pests on both types of farm. These results were due to minimal use of insecticides on three farms, which allowed significantly greater pest suppression by beneficial invertebrates. </p> <p>The aim of the last experiment was to understand the role of woody native vegetation, weedy grassland and spring wheat crops in influencing the abundance of pest and beneficial invertebrates throughout the year in cotton landscapes in northern NSW characterised by two types of farm management: conventional vs low-insecticide management. Beneficial invertebrates (ants, spiders, blue and red beetles and lady beetles) and pest invertebrates (leafhoppers, apple dimpling bugs, peanut scarab beetles, brown flea beetles, flower beetles, mirids, Rutherglen bugs and aphids) were sampled with the beat box on two different types of farm (low-insecticide management vs conventional management) in different habitats (river red gum and poplar box woodland, weedy native grassland, and rotation wheat crops) over the summer and winter growing seasons. Rotation crops (winter growing season) were stratified by distance to and amount of nearby woody native vegetation. Blue and red beetles were absent from grassland, while aphids, Rutherglen bugs, mirids and red-shouldered beetles were abundant but only occurred in grassland. Generally, beneficial invertebrates in total were more abundant on low-insecticide managed farms than on conventionally managed farms, although not always significantly so. In contrast, pest invertebrates in total were less abundant on farms managed with a low-insecticide regime than on farms managed conventionally, although the difference were not always significant. Rotation crops with a higher amount of woody native vegetation nearby on low-insecticide farms harboured less aphids than conventionally managed farms, and this was similar to the pattern in ant abundance. The differences in pest and beneficial invertebrates among farms were related to the amount of woody native vegetation, grassland and reduced reliance on insecticides. </p> <p>The results in this thesis will encourage researchers and cotton growers to use the beat box sampling technique to sample invertebrate taxa in different habitats (river red gum and poplar box woodland, weedy grassland, irrigated cotton, refuge crops and rotation wheat crops). However, landscape management through conservation and revegetation of woodland habitats e. g. river red gum and poplar box woodland close to cotton fields is also important for supporting beneficial invertebrate abundance in nearby cotton fields. To protect crops, growers can plant local eucalypt species into weedy grassland to restore woodland habitats, as weedy grassland harbours more pests than beneficial invertebrates. Weeds in grassland are a primary food resource for pest invertebrates, some feeding exclusively in weedy grasslands such as Rutherglen bugs and others preferring weeds when crops are absent, such as mirids. We assumed that pest abundance was high in weedy grassland due to the reduced abundance of beneficial invertebrates and lower predation of pests than in woody native vegetation, as well as due to the feeding preferences of pest invertebrates for weeds in grassland. To manage farms, a reduction in the reliance on insecticides to advantage beneficial invertebrate abundance should lead to environmental benefits as well as increased profit in irrigated cotton production.</p>