- A multi-year field study was conducted in Oregon and Washington to evaluate the influence of nitrogen fertilization rate and timing on cone quality, nitrate accumulation in cones, severity of powdery mildew (caused by Podosphaera macularis), arthropod pests, and the stability of conservation biological control of the twospotted spider mite (Tetranychus urticae Koch). Nitrogen rates ranged from 44.8 to 269 kg/ha, and nitrogen fertilization timing ranged from pre-bloom, during bloom, and after bloom. The impact of nitrogen rate on cone yield, levels of bittering acids, total oil content, color, and nitrate level were year dependent. However, when data were aggregated over years and analyzed using a mixed effect model, α-acids, β-acids, and total oil volume decreased linearly with increasing nitrogen rate, whereas cone color, expressed as the degree of greenness of cones, and nitrate content of cones increased linearly with nitrogen rate.
Alpha- and beta- acids decreased and nitrate concentration increased when nitrogen was applied after bloom. With regards to arthropod pests, hop looper, Hypena humuli Harris, can cause substantial defoliation and crop damage by feeding on hop leaves and cones. A four-year field study conducted in western Oregon evaluated the abundance of hop looper larvae and associated defoliation of leaves. There was annual variation in abundance of hop looper and defoliation, with a tendency for increasing nitrogen rate to increase both abundance of hop looper and defoliation. A mixed model analysis with data combined from 2014 to 2017 found that abundance of hop looper was linearly related to nitrogen fertilizer rate, with a 2.5 increase in hop looper-days per kg of nitrogen fertilizer applied.
Similarly, based on data from 2015 to 2017, defoliation associated with hop looper increased 0.031 percent with each kg of nitrogen fertilizer applied. The twospotted spider mite can be controlled by resident predatory arthropods in certain situations. This research quantified the stability and resiliency of established biological control of the twospotted spider mite in hop over a five-year period associated with nitrogen fertilization rate and use of a broad-spectrum insecticide. Biological control generally was stable and resilient over a six-fold range of nitrogen fertilization rates, and in only one of five years did elevated nitrogen rates significantly affect populations of spider mites. In contrast, one application of the insecticide bifenthrin was sufficient to disrupt biological control and induce a severe outbreak of twospotted spider mites. The complex of natural enemies suppressed the outbreak during the same year in which bifenthrin was applied, but only after populations of spider mites exceeded levels associated with economic damage. However, in the following year the system returned to an equilibrium state where spider mites were suppressed below economically damaging levels. Established biological control in hop appears stable and robust to factors, such as nitrogen fertilization, that increase reproductive rates of spider mites but sensitive to factors, such non-selective insecticides, that are lethal to natural enemies. Therefore, reducing nitrogen rates, applying nitrogen before and up-to bloom, as well as avoiding non-selective insecticides can help to reduce pest pressure and improve cone quality.