1967; Robinson, 1989) andD. phyllobius(Robinson, 1989), the ento-
mopathogenicSteinernema carpocapsae(Burman and Pye, 1980), the
bacteriophagousC. elegans(Hedgecock and Russell, 1975) and the seal
and fish parasiteTervanova decipiens(Ronald, 1960). For plant-parasitic
nematodes within the soil, temperature is probably the most consistent
vertical cue in a world in which most life-limiting factors (root archi-
tecture, oxygen, carbon dioxide, moisture) are to be found by moving
vertically. Several studies have suggested that plant-parasitic nematodes
utilize diurnally fluctuating vertical gradients that extend through the
root zone to locate optimum depths for root finding and survival. Models
proposed by Dusenbery (1988a,c, 1989) and tested by Robinson (1994)
have shown that, when nematodes are exposed to the vertically propa-
gated heat waves that occur naturally in soil, unexpected movement
towards the surface, towards great depths or towards specific depths can
result from interactions between the rate of nematode movement and the
rate of thermal adaptation. The possible role of metabolic heat from roots
as a host-recognition cue is discussed below.Directed Movement
Movement towards carbon dioxide
Carbon dioxide may be the most common and potent nematode attractant
in nature. It is released abundantly by living and decaying plant and
animal tissues, providing an obvious cue to the possible presence of food.
It has also been suggested that carbon dioxide serves as a collimating
stimulus in soil, providing a directional reference for other responses
(Pline and Dusenbery, 1987). Because of their probable importance,
responses to CO 2 are discussed in detail as a prelude to a general
discussion of the literature on host finding by plant-parasitic nematodes.
Plant-parasitic nematode attraction to known sources of CO 2 in vitro
was first observed (or refuted) about 40 years ago (Bird, 1959, 1960;
Klingler, 1959; Rohde, 1960; Johnson and Viglierchio, 1961). CO 2 attracts
nematodes from a wide range of trophic groups, including bacterial
feeders (Balan and Gerber, 1972; Dusenbery, 1985; Viglierchio, 1990),
insect parasites (Gaugleret al., 1980), root parasites (Bird, 1960; Johnson
and Viglierchio, 1961; Pline and Dusenbery, 1987; Robinson and Heald,
1991; Robinson, 1995), tree-trunk parasites (Miyazakiet al., 1978a,b),
foliar parasites (Klingler, 1959, 1961, 1970, 1972), free-living marine
nematodes (Riemann and Schragge, 1988) and vertebrate parasites
(Granzer and Haas, 1991).
Interpreting the effects of CO 2 can be difficult. Most nematodes
are partly or completely anaesthetized in water equilibrated against air
containing more than 5% CO 2 (v/v) and, in at least some published
studies, nematodes have been subjected to flow rates of pure CO 2 that
probably produced anaesthetic concentrations in water agar (Robinson,Host Finding by Plant-parasitic Nematodes 95