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Soybean cyst nematode (SCN), Heterodera glycines, is a threat to soybean production in North Dakota. Studies on the biology of SCN were conducted to improve my understanding and management of this plant parasitic nematode. The objectives of the research were to; (1) determine if SCN reproduces on crops commercially grown or being tested for production in North Dakota, (2) evaluate the effects of SCN on growth of dry bean, (3) determine if there could be a shift in the SCN population toward greater ability to reproduce on dry bean, and (4) characterize the spatial distribution of SCN in research size field experiments. Canola, clover, lentil, and sunflower were nonhosts while borage, camelina, chickpea, crambe, cuphea, field pea, nyjer, and safflower were poor hosts for SCN with female indices (FI) less than 8. Lupines were susceptible hosts with FI’s of 42 to 57. FI’s of dry bean cultivars varied from 5 to 117. Kidney beans averaged the highest FI at 110 followed by navy, pinto and black at FI’s 41, 39, and 16, respectively. Pod number (PN), pod weight (PW), seed number (SN), and seed weight (SW) of GTS-900 (pinto bean) were significantly less at 5,000 and 10,000 eggs/100 cm3 soil compared with the control by 44 to 56% averaged over the two years. Significant reduction in growth of Montcalm (kidney bean) and Mayflower (navy bean) was observed at 2,500 and 5,000 eggs/100 cm3 soils in 2009, but not in 2008. There was no evidence that SCN was increasing reproduction during two 11 month periods of continual reproduction on roots of dry bean cultivars Premiere and Cirrus (navy), Buster and Othello (pinto), and Eclipse and Jaguar (black). The spatial distribution of SCN in field plots was aggregated in nine of ten field sites with large differences in egg numbers between plots. Lloyd’s index of patchiness ranged from 1.09 to 3.34. Spatial distribution of SCN can be an important factor affecting the results of field experiments.

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Soybean leaf consumption for the most common semilooper species, Trichoplusia orichalcea (F.) was established in the laboratory using a leaf area meter. The total consumption per larva was 120,85 cm$ sp2$. The yield reduction of our soybean cultivars (Glycine max (L.) Merrill) being Duiker, Gazelle, Roan and SCS1 was measured in relation to different levels of defoliation. The defoliation was induced manually singly or sequentially over three different growth stages. / At soybean maturity, counts of pods per plant, number of seeds per plant, weight per 100 seeds and weight of seeds per plant were obtained. Yields were converted to a percentage of yield and linear regressions fitted to the relationship between percentage yield and defoliation. The percentage yield was influenced by the cultivars, the levels of defoliation, and the timing of defoliation. / The most critical growth stage for defoliation was the seed development stage, but the differences between them were small and not significant. SCS1 was the most resistant cultivar to defoliation, while Duiker was the most susceptible to single defoliation. / The economic thresholds were determined for the soybean cultivars demonstrating significant linear relations between percentage yield and single defoliation. Based on three insecticides and two application methods, the economic thresholds were determined at the flowering stage for Duiker, at the pod development stage for Duiker and Roan and at the seed development stage for Duiker, Gazelle, Roan, and SCS1 cultivars. The numbers of semilooper caterpillars per metre of row required to reach the economic thresholds, are generally higher than the natural levels of infestation.

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