Agriculture Reference
In-Depth Information
promoting leaf burial and decomposition (Laing et al. 1986; Kennel 1990), thus reducing their
potential to damage the orchard trees. However, leaf-litter burial also reduces populations of the
natural biocontrol agents (brachonid wasps) of these insects (Laing et al. 1986).
Potential Role of Earthworms in Increasing Plant Disease or Pest Problems
Several species of plant pathogenic fungi have been found in earthworm casts (Hutchinson and
Kamel 1956; Hoffmann and Purdy 1964; Thornton 1970; Melouk and Horner 1976; Toyota and
Kimura 1994), and plant parasitic nematodes may survive passage through the earthworm gut
(Ellenby 1945; Russom et al. 1993). However, there are relatively few data available on the potential
negative effects that earthworm-induced microbial dispersal may have on incidence of plant diseases
(of fungal or bacterial origin) or nematode damage.
Increased dispersal of a plant pathogenic fungus,
Syntrichium endobioticum,
the causal agent
of wart disease of potato, by
L. terrestris
and various other (probably lumbricid) earthworms was
reported by Hampson and Coombs (1989), resulting in increased infection of several potato plants.
Similarly, Melouk and Horner (1976) reported infection of mint seedlings by verticillium wilt
(
Verticillium dahliae
) when the plants were grown with earthworm casts that contained viable
spores of these pathogens.
Dispersal of plant parasitic nematodes by earthworms was reported by Ellenby (1945) and
Russom et al. (1993), but the potential of this for increased damage to plant roots was not evaluated.
Casts of the Nigerian earthworm species
Agrotoreutus nyongii
had larger and more diverse popu-
lations of parasitic nematodes than did the surrounding soil (Russom et al. 1993). Casts of
Apor-
rectodea longa
contained nematode cysts with greater fertility, viability, and germination potential
than those in surrounding soil (Ellenby 1945). Ilieva-Makulec and Makulec (2002) reported an
increase in plant parasitic nematode populations in soil cores inoculated with
Lumbricus rubellus
after 60 and 90 days, but no negative effects on growth of grass roots were observed.
The interactions between earthworms and plant insect pests still remain poorly explored. Kirk
(1981) reported large numbers of the northern maize rootworm (
Diabrotica
: Coleoptera) eggs in
earthworm burrows and suggested that this may contribute to the spottiness of rootworm distribution
and damage often observed in maize fields. More recently, Wurst and Jones (2003) and Scheu et
al. (1999) showed effects of lumbricid earthworms (
Aporrectodea
sp.) on increased numbers of
leaf sap sucking aphids (
Myzus persicae
) and their offspring.
3. E
ARTHWORMS
AND
P
LANT
G
ROWTH
-R
EGULATING
AND
G
ROWTH
-I
NFLUENCING
S
UBSTANCES
The first suggestion that earthworms might produce plant growth regulators (PGRs) was by Gavrilov
(1963). This was supported by the first report of the presence of PGR substances in the tissues of
Aporrectodea caliginosa, L. rubellus,
and
Eisenia fetida
by Nielson (1965), who extracted indole
substances from earthworms and reported increases in the growth of peas because of them. He also
extracted a substance that stimulated plant growth from
A. longa, L. terrestris,
and
Dendrobaena
rubidus,
but his experiments did not exclude the possibility that the PGR substances he obtained
came from microorganisms living in the earthworm guts and tissues.
The presence of PGR substances in the tissues of
A. caliginosa, L. rubellus,
and
E. fetida
was
confirmed by Nielson (1965), who isolated indole substances from whole earthworm tissues. This
was confirmed for
A. rosea
and
A. caliginosa
by Nardi et al. (1988). More recently, El Harti et al.
(2001a,b) isolated indole acetic acid (IAA)-like substances from gross extracts of tissues and feces
of
L. terrestris
. These substances stimulated rhizogenesis and enhanced root growth of
Phaseolus
vulgaris
(common beans) in a manner very similar to that of IAA.
Graff and Makeschin (1980) tested the effects of substances produced by
L. terrestris, A. calig-
inosa,
and
E. fetida
on the dry matter production of ryegrass. They added liquid eluates from pots
