If one takes a pretreatment sample from every plot, this will be considerably more expensive than if samples are only taken from untreated checks, or a composite taken from each block in the experiment. In many instances, for annual cropping situations, although the plot area has a detectable nematode population, many samples from individual plots will be below the detection level. If this is the case, considerable effort is expended with little useful data obtained.
This provides some justification for only sampling from the untreated check plots or taking a composite sample from each block to provide an indication of the type of variability within the trial area.
How frequently to sample a field following preplant applications depends on the nematodes of interest, on the initial nematode population, and on the expected rate of population increase.
If for example, in an annual cropping situation one is dealing with root-knot or cyst nematodes which are initially at just barely detectable levels, then penetrate roots, establish a feeding site, and then mature and lay eggs, a considerable period of time may pass (depending on soil temperature) during which juveniles may not be present in the soil.
If on the other hand, one is dealing with an ectoparasite in a perennial cropping situation, the numbers will likely fluctuate throughout the year and could conceivably be higher several months after treatment than before in both treated and untreated areas.
A consideration of what has been done previously with particular crops or nematodes can provide indications of optimum times to sample in a particular situation.
A sample taken at harvest of an annual crop establishes the final population (Pf) for a trial. At harvest, grading may also be done of nematode damage to crop quality either above or belowground or both.
Root gall ratings are a common practice for root-knot nematode infested crops. This is a somewhat subjective but widely practiced procedure in which individual roots are graded by number based on the degree of galling observed. Different researchers will have different scales which they prefer to use (e.g. 1-10, 0-5, 1-5, etc.). The particular rating used is not critical as long as the researcher records the method used.
Here is an example of tomato roots rated from 1 to 10:
Rating #1
Rating #2
Rating #3
Rating #4
Rating #5
Rating #6
Rating #7
Rating #8
Rating #9
Rating #10
Bioindicators
One method that has been utilized is to place a small quantity of soil infested with a high population of nematodes from another area into a cotton or nylon mesh bag. The bag is then buried in the soil and treatments are applied. At appropriate times following treatments, bags are recovered and analyzed for surviving nematodes. Citrus nematode (Tylenchulus semipenetrans) is one nematode that has been useful as a bioindicator because it has a relatively narrow host range and will typically have several thousand nematodes in 50 ml of soil. It should not be used in fields to be planted to host crops.
This method can be used, for example, to determine the zone of control surrounding a point source injection of a fumigant, as in the example for Telone II in a high organic matter soil. Bags were buried at intervals of 6 inches to 36 inches deep in the soil.
Seeds have been used to determine the effectiveness and depth of penetration of Metam-sodium.
Insects such a mealworms have been used to evaluate the effectiveness of organophosphate and carbamate nematicides.
Earthworms have been placed inside of glass containers with perforated stainless steel lids and buried to evaluate the effectiveness of various fumigant nematicides.