Following harvest, nematode numbers typically decrease until another susceptible crop is planted or weeds become available that will support reproduction. Under weed free fallow conditions, root-knot nematode populations may drop as much as 85-90 percent in a years time, while sugarbeet cyst nematode will only decrease by 20 to 60 percent depending on location. This principle is illustrated in this example of population decrease of sugarbeet cyst nematode following harvest of a susceptible crop until the planting of another susceptible crop (cabbage) during which time populations increase dramatically and then decrease once again during successive nonhost crops.
Nematode populations are usually easier to detect at harvest when they are highest than several months following harvest. For example, after several months in a fallow or nonhost crop, populations of root-knot nematode may not be detectable through sampling but may still be high enough to cause significant damage when a host is planted.
Because populations fluctuate, if one wants to compare populations from year to year, it is important to always sample at the same time each year. Also, if treatments are applied to try to reduce nematode numbers, its important to leave some areas untreated for comparative sampling. If this is not done, and samples are taken several months after a treatment, populations may be either lower or higher than before treatment just because of normal populations fluctuations.