How do soybean cyst nematode populations change over time?
Fig. 6. Seasonal change of SCN egg population densities in plots planted with susceptible soybean and in fallow in a Minnesota field in 2008.
Fig. 7. Relationship of SCN egg population density at harvest with number of years of corn following SCN-susceptible soybean during 1996-2004 in a field in southern Minnesota.
SCN population density is affected by a number of environmental factors as well as host status. The most important environmental factor is probably the temperature. Consequently, seasonal changes in SCN population densities vary in different geographic locations.
In Minnesota, after the soil has thawed and temperature increased in April, second-stage juveniles (J2) start to hatch from eggs. After planting soybean, J2 hatch increases due to chemical stimulants from soybean roots. Egg population density in soil declines gradually due to the hatch of J2 until late June to early July when the females of the first generation become mature and produce eggs, and egg population density starts to increase in SCN-susceptible soybean. From late July or early August to the end of the season, SCN egg population density can increase rapidly (Fig. 6).
Egg population densities in susceptible soybean at harvest can be as low as a few thousand to as high as tens of thousands per 100cc of soil (Fig. 7). Average annual reduction of egg population density in nonhost corn plots is about 50% (Fig. 7). It takes about 5 years to lower the egg population density from 10,000 to approximately 300 eggs/100 cc of soil, a level at which there is limited or no damage to soybean.