Agriculture Reference
In-Depth Information
soil and colonize plant roots extensively, so earthworms may act as important dispersal vectors for
them (Rouelle 1983; Stephens et al. 1993b; Doube et al. 1994b).
Populations and activity of several groups of beneficial soil organisms important in plant litter
decomposition and nutrient mineralization processes in soils (e.g., microfauna, mesofauna, and
macroinvertebrates) may be affected by earthworms (Brown 1995). For instance, protozoa may be
part of earthworm nutrition (Miles 1963; Flack and Hartenstein 1984; Bonkowski and Schaefer
1997), but many protozoan cysts can survive passage through the earthworm gut and can hatch,
become more active, and reproduce rapidly in earthworm casts and earthworm-worked soils (Shaw
and Pawluk 1986; Barois 1987; Winding et al. 1997; Binet et al. 1998).
Earthworm casts may benefit bacteriophagic nematode populations preferentially over those of
other nematode trophic groups (Roessner 1981, 1986; Senapati 1992), but the total numbers of free-
living nematodes in earthworm-worked soils may be reduced (e.g., Alphei et al. 1996; Dominguez et
al. 2003) or increased (Winding et al. 1997), depending on the situation. Populations of other organisms,
such as enchytraeids and various micro- and macroarthropods, may also be increased (e.g., in anecic
earthworm middens) or decreased because of changes in microbial populations and food resources in
earthworm-worked soils (Brown 1995). However, most of the consequences to plant growth of changes
in the populations and activity of micro and macroinvertebrates in earthworm middens, castings, and
earthworm-worked soils are unknown and deserve much more attention.
2. C
HANGES
IN
P
OPULATIONS
AND
I
MPACTS
OF
P
LANT
P
ESTS
, P
ARASITES
,
AND
P
ATHOGENS
As with beneficial microorganisms, earthworm feeding, burrowing, casting, and dispersing activities
can alter the distribution of populations of plant pathogens such as viruses, bacteria, fungi, parasitic
nematodes, or insect pests in soils. Furthermore, by making plants more or less susceptible to these
pests, parasites, and pathogens, earthworms can affect root health (Brown 1995). These relationships
are illustrated in a modified version of the classic Ñplant-disease triangleÒ (
Figure 2.3
)
in which
plant root growth and development are shown as a function of the interactions between a favorable
environment for both roots and pathogens and the presence or activity of ÑvirulentÒ of ÑinfectiveÒ
plant pathogens. The result of these interactions (i.e., plant health status) may therefore be influenced
directly or indirectly by earthworm activities.
Earthworms are known to transport and consume a wide variety of plant pathogenic fungi and
bacteria and plant-parasitic nematodes (Brown 1995). If populations of these organisms are reduced
either directly by transit through the earthworm gut or indirectly via changes in the soil environment,
then the indirect consequences to plant growth may be important, particularly when disease or nematode
pressure is reducing crop yields. The role of earthworms as vectors of plant diseases, parasites, and
pests depends on the type of organism and species ingested, the amount of soil and inoculum ingested,
the extent of beneficial or antagonistic intestinal secretions, the number of organisms digested in the
earthworm gut, the amount of organisms deposited in casts, the infectivity of surviving organisms
deposited in casts, the feeding and casting behavior of the earthworms (dependent on the earthworm
species and ecological category), and the mobility and behavior of the earthworm.
Potential Role of Earthworms in the Reduction of Plant Disease and
Pest Problems
Several reports of beneficial results to plants of earthworm-induced reductions of plant pathogens are
known. For instance, work in the Soil Ecology Laboratory at The Ohio State University has shown
that vermicomposts can suppress plant diseases such as
Pythium
and
Rhizoctonia
(Chaoui et al. 2002,
2003) in the greenhouse and
Verticillium
in the field. When cabbages were grown in the presence of
the earthworm
Pheretima hilgendorfi
, Nakamura et al. (1995) observed lower incidence of club-root
disease (
Plasmodiophora brassicae
) damage in the seedlings. They attributed this decrease to the
