Established nematode populations are very persistent in the soil. It is known that they need sufficient soil moisture for movement, feeding and reproduction (fig. 5), and that there are adverse soil moisture conditions which they cannot survive. The influence of soil moisture on survival of nematodes and nematode populations is the topic of this study. The investigations are made under laboratory conditions and are concentrated on ectoparasitic root-infesting nematodes, particularly Tylenchorhynchus dubius
and Rotylenchus robustus,
in fallow soil and in vitro. The animals chosen represent the large group of unspecialized plant-parasitic nematodes, which pass their entire life in the soil and which do not possess any apparent mechanisms against drought. All investigations on populations have been carried out with natural soils and their indigenous nematode populations (2.1.1. and 2.2.1.).
The gross relationships between the three major soil phases and nematodes are summarized in figure 1; the nematodes themselves, though numerous, may be neglected as a factor in soil formation or as a soil component. Figure 2 lists the various soil moisture potentials which may affect nematode survival. The relationships between pF value and soil moisture content (figs. 3 and 6) and between pF' value and relative humidity of the air (fig. 4) are discussed as tools in this study (see also chapter 2). Distinction between soil moisture quality and quantity, and regarding the latter between three essentially different situations, viz. water-saturated soil, unsaturated moist soil and dry soil, appeared to be useful. The limit between unsaturated and dry soil cannot be indicated accurately with respect to nematode biology, because essential data about the physical and chemical properties of nematodes are lacking. The limit may vary for different species, stages of development or even individual nematodes (1.3.3.).
The extensive literature survey (chapter 1) shows that many incidental data are available, but the information is yet too erratic to draw a coherent picture of the relations between soil moisture and nematode survival. This is particularly so because quantity and quality of soil moisture are often not considered separately. One conclusion may be that nematodes generally are proof against moisture conditions which both chemically and physically differ widely from their normal average environment. Reduced activity is the first visible symptom of a reaction to environmental stress, which may affect various species differently (1.2.).
In water-saturated soil, nematode populations were reduced to about 20 % after 20 weeks (fig. 9) and there were indications that considerable reduction had occurred already after 12 weeks. This confirms the general opinion that saturated soil is unfavourable for most nematodes due to chemical properties of the soil solution as a result of microbiological activity (1.3.1).
The results of experiments in unsaturated soil indicate that survival of ecto-parasitic nematodes in fallow soil at pF values between 0.5 and 4 is not directly affected by soil moisture quantity (figs 10, 11A and 12A). This can also mean that these nematodes are not or hardly active in moist fallow soil (3.3). Under such conditions also the quality of the soil solution, as influenced by normal manuring, has no measurable influence on survival of populations (figs. 11 B,C and 12B,C). Great qualitative modifications by excessive dressings with chemicals may cause damage (table 15), as indicated in the literature (1.3.2).
Drought is generally considered harmful to nematodes and especially to ectoparasites, although in literature several cases of drought-tolerance in such nematodes are mentioned (table 1). When investigating the response of nematodes to desiccation, the graduality of the processes of dehydration and rehydration appear to be most important. Dehydration must proceed gradually, particularly for in vitro studies, because nematodes are killed by unnatural, rapid loss of water. For that purpose three slightly different desiccation techniques have been developed, making use of the relationship between the concentration of aqueous glycerine solutions and the relative humidity of the air in a closed room (2.3).
Rehydration of desiccated nematodes and regaining activity thereupon requires time. This necessitates the application of adapted methods for the extraction of nematodes from dry soil, because otherwise only a fraction of the living nematodes would be caught (figs. 13 and 14).
Experiments in soil as well as in vitro have demonstrated that ectoparasitic root- infesting nematodes generally are drought-tolerant to a certain extent (figs. 16, 17A, 19, 20). As already mentioned, desiccation survival depends on the rate of water loss (table 11), but also the degree and duration of desiccation are important (figs. 20 and 21). Diurnal changes of the relative humidity of the soil air aid nematodes to survive (table 12). Both experiments in vivo and in vitro showed great differences in drought-tolerance between nematode species (figs. 16, 17A, 19-22) and also between various stages of development (figs. 17B and 22). The general assumption that nematode eggs are important for restoration of populations upon severe drought, as a result of their exceptional drought-resistance, is contradicted by the experimental results for ectoparasites 4.3.2. and 5.7).
Regarding the influence of chemical properties of soil moisture in dry(ing) soil, more research, distinguishing primary and secondary effects, appears to be necessary. The experimental results indicate fundamental differences, viz. increased drought-tolerance of one species and intoxication of another species under the same conditions (fig. 19, table 14), and also that loss of water in a dry atmosphere affects nematodes other than loss of water to a surrounding solution (5.10).
Systematic differentiation of the reactions of nematodes to drought or any other environmental stress, analogous to systems used for plants, seems to be useful (5.10).
Eradication of ectoparasitic root-infesting nematodes in fallow soil merely by maintaining non-extreme soil moisture conditions is hardly possible (figs. 9, 10, 11). Under extreme moisture conditions nematodes can be eradicated, but the process requires a long time (table 8, figs. 9, 16, 17, 19). However, partial nematode control with about 80% mortality, which is comparable with the effect achieved by chemical control, is considered sufficient for safe crop growth. In rapidly and strongly desiccated soil (pF 5.5) 80 % mortality can be achieved in a rather short period of time. For a number of susceptible species, such reduction occurred in 4 weeks or less and for more tolerant species the period varied from 10 to more than 24 weeks (table 8, figs. 16, 17, 19). Long lasting periods of severe drought occur in and and semi-arid areas (4.4). Efficient control of nematodes by dry fallow, possibly in combination with soil tilling at the right time, seems to be possible in these areas.