and muscles in many organisms, the specific ion composition of water
bathing a nematode can influence water balance and motility independ-
ently of osmotic pressure. Extreme effects from unbalanced salts were
seen in the experiments of Robinsonet al. (1984b), where monocationic
solutions of Mg2+or Na+, with Cl−orNO 3 −present as the anion, stopped
all movement by the normally highly spontaneously motile J4 of the
foliar parasiteD. phyllobiusin water at 100 mequiv l−^1. Under the same
conditions, sucrose, mannitol and a synthetic soil solution at equivalent
osmolality had no obvious effect on motility or water content. No volume
loss was observed at toxic concentrations of Mg2+or Na+and nematodes
fully recovered from Na+but not Mg2+ exposure when returned to a
synthetic soil solution. Thus, Wright (1998) justifiably criticized the use
of single salt solutions for studying nematode water and ion regulation.
The same warning should be extended to the study of plant-parasitic
nematode behaviour in soil and plant tissues.Temperature
Like most organisms, plant-parasitic nematodes appear to be thermally
adapted to their habitats. Croll (1975) noted that motility optima for
juvenile stages of a stem parasite, a root parasite and two animal parasites
that ascend foliage were around 20–25°C, whereas optima for active
penetrators of warm-blooded hosts were near 40°C. Robinson (1989)
found that, in southern Texas, infectives of two foliar parasites,D. dipsaci
andD. phyllobius, had optima for motility 10°C cooler than those of root
parasites,R. reniformisandT. semipenetrans, from the same locale. This
observation is consistent with the ecological need for the foliar parasites
to be maximally active on foliar surfaces during the cool rainy periods in
the summer when the foliar moisture films required for infection of foliar
buds are present. A similar pattern is apparent among entomopathogenic
nematodes, which, like plant-parasitic nematodes, reproduce within
poikilothermic hosts. Optimal temperature ranges for host infection,
establishment within the host and reproduction for 12 species and strains
of entomopathogenic nematodes from different latitudes around the
world were broadly similar to the climatic conditions of their origin
(Grewal et al., 1994). Further information on thermal adaptation is
available in Trudgill’s (1995a,b) analyses of base temperatures and
thermal constants for embryogenesis and development in relation to niche
temperature and reproductive strategy for more than 20 plant-parasitic,
animal-parasitic and free-living nematodes.
Many nematodes are attracted to heat or repelled by it, and at least
nine species exhibit a preferred temperature towards which they migrate.
The latter include the root parasites Globodera rostochiensis (Rode,
1969a,b), M. incognita (Diez and Dusenbery, 1989a), R. reniformis
(Robinson, 1989; Robinson and Heald, 1989, 1993) andT. semipenetrans
(Robinson, 1989), the foliar parasitesD. dipsaci(Wallace, 1961; Croll,94 A.F. Robinson