Compared to pesticidal seed treatments, host-plant resistance is a more reliable tool for soybean IPM that can regularly suppress pest populations, especially when dealing with those that are difficult to monitor, such as nematodes in the soil, or those that can rapidly increase in population size, such as aphids (Ajayi-Oyetunde et al. 2016). Suppression of H. glycines with resistant cultivars may indirectly affect A. glycines reproduction through conferred host-plant defenses or alterations to host-plant quality, but further studies are needed on the interactions between these pests (Hong et al. 2010, McCarville et al. 2014a). Pesticidal seed treatments may help reduce A. glycines on susceptible cultivars, but resistant cultivars appear to not benefit from
a seed treatments. Furthermore, pesticidal seed treatments may not consistently preserve yield or help reduce H. glycines populations (Figs. 3, 4, S1). In the North Central region, soybean
growers should consider using cultivars with host-plant resistance to manage A. glycines and H.
glycines, but not use seed treatments with the nematicide abamectin or the insecticide
thiamethoxam for these pests. If a grower does not have access to A. glycines-resistant cultivars, the most cost effective tool to preserve soybean yield would be scouting and application of foliar insecticides if A. glycines populations reach the economic threshold (Myers et al. 2005, Hodgson et al. 2012, Krupke et al. 2017). Soybean production in North America relies heavily on
cultivars with the PI88788 resistance gene. Thus, soybean producers should implement rotation schemes with non-host crops and different sources of resistance to help delay virulence by H.
glycines to PI88788.
Acknowledgments
David Soh assisted with nematode experiments in the greenhouse and helped with data collection. Present and past employees of Iowa State University, including Chris Marett, Greg Gebhart, Stith Wiggs, Mark Mullaney, Greg VanNostrand, Michael McCarville, Adam
Varenhorst, Kiley Mackereth, Sarah Van Tol, Ethan Sprung, Anzelika Marie, Hailey Lambert, Kate Gundry, and Sarah Rueger, assisted with experiments in the field and lab. This work was partially supported by the Iowa Soybean Association and the soybean checkoff.
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Tables
Table 1: Mixed model of analysis for cumulative aphid days in the field experiment.
Fixed effect d.f. F-value P-value
Cultivar 3, 69 19.36 <0.0001
Seed treatment 2, 46 3.54 0.0371
Cultivar × seed treatment 6, 138 4.68 0.0002
Random effect d.f. χ2 P-value
Study site 1 126.0 <0.0001
Study site (block) 1 0.4 0.2635
Study site (block) × cultivar 1 99.3 <0.0001
Study site (block) × seed treatment 1 2.3 0.0647
Table 2: Mixed model of analysis for soybean cyst nematode (Heterodera glycines) population densities at the time of harvest in the field experiment.
Fixed effect d.f. F-value P-value
Cultivar 3, 69 11.96 <0.0001
Seed treatment 2, 46 1.24 0.2996
Cultivar × seed treatment 6, 426 0.72 0.6307
Random effect d.f. χ2 P-value
Study site 1 483.8 <0.0001
Study site (block) 1 240.3 <0.0001
Study site (block) × cultivar 1 3.9 0.0241
Table 3: Mixed model of analysis for soybean stand density in the field experiment.
Fixed effect d.f. F-value P-value
Cultivar 3, 69 8.78 <0.0001
Seed treatment 2, 46 14.86 <0.0001
Cultivar × seed treatment 6, 138 6.00 <0.0001
Random effect d.f. χ2 P-value
Study site 1 217.0 <0.0001
Study site (block) 1 8.4 0.0019
Study site (block) × cultivar 1 7.3 0.0034
Study site (block) × seed treatment 1 7.6 0.0029
Table 4: Mixed model of analysis for soybean yield in the field experiment.
Fixed effect d.f. F-value P-value
Cultivar 3, 69 2.53 0.0646
Seed treatment 2, 46 1.38 0.2614
Cultivar × seed treatment 6, 138 0.75 0.6070
Random effect d.f. χ2 P-value
Study site 1 463.0 <0.0001
Study site (block) 1 443.3 <0.0001
Study site (block) × cultivar 1 32.7 <0.0001
Study site (block) × seed treatment 1 0.6 0.2193
Table 5: Mixed model of analysis for soybean cyst nematode (Heterodera glycines) females per soybean plant in the greenhouse experiment.
Fixed effect d.f. F-value P-value
Cultivar 3, 12 5.54 0.0127
Seed treatment 2, 8 0.29 0.7555
Cultivar × seed treatment 6, 24 1.44 0.2392
Random effect d.f. χ2 P-value
Study site 1 44.4 <0.0001
Study site × cultivar 1 2.2 0.0690
Table 6: Minimum yield increase, or the breakeven point, in kilograms per hectare that is needed to compensate for the cost of a seed treatment. Soybean value ($/kg)a ST ($/ha)b $0.24 $0.28 $0.31 $0.35 $0.37c $0.39 $0.43 $0.47 $0.51 $0.55 $37.07 154.44 132.38 119.56 55.80 100.18d 95.04 86.20 78.86 72.68 67.39 $39.54 164.73 141.20 127.54 69.74 106.85 101.37 91.94 84.12 77.52 71.88 $42.01 175.03 150.03 135.51 75.22 113.53 107.71 97.69 89.38 82.37 76.38 $44.48 185.33 158.85 143.48 97.63 120.21 114.05 103.44 94.63 87.21 80.87 $46.95 195.62 167.68 151.45 111.60 126.89 120.38 109.18 99.89 92.06 85.36 $49.42 205.92 176.50 159.42 125.54 133.57 126.72 114.93 105.15 96.90 89.85
a The column headings in bold represent the value of soybeans per kilogram.
b The row headings in bold are the costs of a seed treatment (ST) per hectare based on the seeding rate of 346,000 seeds per hectare. c $0.37 USD kg-1 represents the recent market value of soybean.
d 100.18 kg ha-1 would be the breakeven amount for the fungicide, insecticide, and nematicide (FIN) seed treatment used in the field experiment,
which was estimated to cost at least $37.07 ha-1, when soybean was valued at $0.37 USD kg-1.
Figures
Figure 1: Average cumulative aphid days in the field experiment. Bars represent combinations of cultivar × seed treatment. Error bars represent standard error of the means. Bar groups are separated by soybean cultivar. SCN = soybean cyst nematode. SBA = soybean aphid. Shading or patterns in the bars represent seed treatment. F = fungicides; I = insecticide; N = nematicide. Capital letters above the bars represent significant means separation (P < 0.05) by soybean cultivar. Lowercase letters above the bars denotes a significant difference between means for seed treatments within a soybean cultivar.
Figure 2: Average number of H. glycines eggs per 100 cc soil at the time of harvest in the field experiment. Bars represent combinations of cultivar × seed treatment. Error bars represent standard error of the means. F = fungicides; I = insecticide; N = nematicide. Bar groups are separated by soybean cultivar. SCN = soybean cyst nematode. SBA = soybean aphid. Shading or patterns in the bars represent seed treatment. Letters above the bars represent significant means separation (P < 0.05) by soybean cultivar.
Figure 3: Average soybean stand density in the field experiment. Bars represent combinations of cultivar × seed treatment. Error bars represent standard error of the means. Bar groups are separated by soybean cultivar. SCN = soybean cyst nematode. SBA = soybean aphid. Shading or patterns in the bars represent seed treatment. F = fungicides; I = insecticide; N = nematicide. Capital letters above the bars represent significant means separation (P < 0.05) by soybean cultivar. Lowercase letters above the bars denotes a significant difference between means for seed treatments within a soybean cultivar.
Figure 4: Average soybean yield in the field experiment. Bars represent combinations of cultivar × seed treatment. Error bars represent standard error of the means. Bar groups are separated by soybean cultivar. SCN = soybean cyst nematode. SBA = soybean aphid. Shading or patterns in the bars represent seed treatment. F = fungicides; I = insecticide; N = nematicide.
Figure 5: Average number of H. glycines females per plant after 30 days in the greenhouse experiments. Bars represent combinations of cultivar × seed treatment. F = fungicides; I = insecticide; N = nematicide. Error bars represent standard error of the means. Bar groups are separated by soybean cultivar. SCN = soybean cyst nematode. SBA = soybean aphid. Shading or patterns in the bars represent seed treatment. Letters above the bars represent significant means separation (P < 0.05) by soybean cultivar.
a Cultivar: S24-K2 is susceptible to both SBA and SCN; S23-P8 is resistant to SCN but susceptible to SBA; S25-F2 is resistant to SBA but susceptible to SCN;
S21-Q3 is resistant to both SBA and SCN.
b Seed treatment: untreated has no chemicals; ApronMaxx has the fungicides fludioxonil and mefenoxam; Avicta Complete has the same fungicides in ApronMaxx
in addition to the insecticide thiamethoxam and nematicide abamectin.
c SCN females per plant: average number of SCN cysts (females) washed from the roots of a single soybean plant with standard error in parentheses.
Supplementary Table 1: The number of soybean cyst nematode females (cysts) on single soybean plants grown in the greenhouse for 30 days. Uppercase letters
following the value in a column represent separation of means for soybean cultivars for one study site and lowercase letters following the value in one column represent separation of means for the combinations of cultivar × seed treatment based on Tukey’s Studentized Range (HSD) Test using a 95% confidence level.
Female index
Year Study site HG-type on PI88788 Cultivara Seed treatmentb SCN females per plantc
2013 Northeast 2.5.7 12% S24-K2 Untreated 140.67 (18.28) Aa ApronMaxx 149.33 (11.78) Aa Avicta Complete 134.33 (13.40) Aa S23-P8 Untreated 68.50 (10.44) Bb ApronMaxx 62.00 (9.18) Bb Avicta Complete 67.00 (9.15) Bb S25-F2 Untreated 156.83 (11.28) Aa ApronMaxx 135.33 (27.03) Aa Avicta Complete 143.40 (12.68) Aa S21-Q3 Untreated 90.33 (24.41) Ba ApronMaxx 88.00 (7.43) Ba Avicta Complete 86.40 (11.99) Ba
2013 Central 2.5.7 43% S24-K2 Untreated 153.33 (46.53) Aba
ApronMaxx 167.33 (24.57) ABa
Avicta Complete 229.83 (39.59) ABa
S23-P8 Untreated 121.40 (17.10) ABa
ApronMaxx 168.33 (19.94) ABa
Avicta Complete 167.17 (18.52) ABa
S25-F2 Untreated 188.67 (48.86) Aa ApronMaxx 198.83 (26.10) Aa Avicta Complete 232.33 (36.86) Aa