Carabid beetle communities and invertebrate weed seed predation in conventional and diversified crop rotation systems in Iowa
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Crop rotations affect the nutrient inputs, pesticide inputs, and tillage regime required to maintain desirable yields. However, little is known about how crop rotations affect invertebrate natural enemy communities and consequent levels of pest biological control. In this thesis, we examined how a conventional corn/soybean rotation compared to a low-external-input corn/soybean/triticale-alfalfa/alfalfa rotation affected carabid beetle communities and invertebrate predation of giant foxtail (Setaria faberi) weed seeds. We hypothesized that reduced herbicide and fertilizer in the four-year compared with the two-year corn and soybean would result in a greater abundance and diversity of carabid beetles and other invertebrate weed seed predators. We further hypothesized that the triticale-alfalfa and alfalfa phases of the four-year rotation would benefit carabids and other ground-dwelling, granivorous invertebrates because those crops also require few chemical inputs, provide plant cover for extended periods increased soil disturbance necessary to control weeds in four-year corn and soybean. In concurrence with our original hypothesis, the addition of triticale-alfalfa and alfalfa to a rotation did increase the number of carabid species caught within the four-year rotation. However, these additional species were typically only encountered once or a few times during the growing season. In addition to answering our original hypotheses about crop rotations, this thesis has also determined other interesting information about invertebrate weed seed predation. Crickets, especially Gryllus pennsylvanicus, were determined to be the main invertebrate weed seed predators in this study. Invertebrate seed predation was low or non-significant through July and early August and sharply increased to levels as high as 100% late August until frost. Invertebrate predation of giant foxtail was significantly higher in corn and soybean compared to triticale-alfalfa and alfalfa treatments. Laboratory feeding assays revealed that weed seed preferences differ between invertebrate predators and may be affected by the presence of alternative invertebrate prey.