Agriculture Reference
In-Depth Information
toxic (Curry 1976; Andersen 1980). However, any adverse effects of moderate slurry applications
are transitory, and the long-term net population response is positive (Curry 1976; Cotton and Curry
1980a,b; Unwin and Lewis 1986).
P
ESTICIDES
AND
P
OLLUTANTS
Earthworms can be exposed to pesticides and other hazardous chemicals while moving through
and ingesting contaminated soil or by ingesting contaminated litter. Varied responses to pesticides
have been reported, ranging from little or none to very severe depressed numbers depending on
the species concerned; the chemicals used; the rates, methods, and frequency of application; and
other factors often not well understood (Edwards and Thompson 1973; Brown 1977; Lee 1985).
Edwards and Bohlen (1992) reviewed the toxicity of nearly 200 chemicals to earthworms,
including inorganic chemicals, 4 biological agents, 19 aromatic agents and organochlorine insec-
ticides, 41 organophosphate insecticides, 13 carbamate insecticides, 6 pyrethroid insecticides, 26
fungicides, 53 herbicides, and 16 organic chemicals, and assigned a chemical toxicity index to
each. Among the older pesticides, lead arsenate and mercuric chloride were highly toxic, as were
soil fumigants such as DD, chloropicrin, methyl bromide, and carbon tetrachloride. Chlordane,
heptachlor, and toxaphene are also very toxic and have been used as vermicides, but few of the
other organochlorines affect earthworm populations to any significant degree at normal rates of
application. Of the organophosphates, phorate and ethoprop are most toxic at normal rates of
application; others, including fonofos, parathion, and thionazin, can be moderately toxic. Carbamate
insecticides, notably aldicarb, carbaryl, carbofuran, and methiocarb, are very toxic to earthworms
(Stenersen et al. 1973; Martin 1976; Stenersen 1979; Edwards 1980, 1983; Clements et al. 1986),
as are benomyl and some related fungicides (Stringer and Wright 1976; Lofs-Holmin 1981).
Herbicides do not appear to be directly toxic to earthworms but can have indirect effects by altering
plant cover and food supply and the microclimate at the soil surface.
It is unlikely that occasional applications of even the most toxic compounds have very serious
consequences to earthworm populations, but their repeated use over a long period can. Long-term
use of copper fungicides for disease control drastically reduced earthworm numbers in an English
orchard (Raw 1962), and frequent treatments with large doses of insecticides (mainly phorate) over
a period of 20 years eliminated earthworms from grassland plots (Clements et al. 1991).
Other potentially hazardous chemicals include heavy metals in metal smelter emissions, in
landspread sewage sludge, and in landspread pig slurry containing copper and zinc. Metals in
organic wastes are not considered toxic to earthworms (Hartenstein et al. 1980; Malecki et al.
1982), but copper toxicity has been suggested as the likely reasons for low earthworm numbers in
land heavily contaminated with pig slurry (van Rhee 1977; Curry and Cotton 1980; Ma 1988).
S
OIL
W
ATER
M
ANAGEMENT
As mentioned in this chapter, earthworm populations and activity are often restricted by unfavorable
soil moisture conditions. Irrigation of dry soils has resulted in significant extensions of the range
of lumbricid species (Barley and Kleinig 1964; Reinecke and Visser 1980) and can allow at least
some species to remain active during the hot dry summer weather in South Australia (Baker 1998).
However, the response to irrigation may depend on the species. Some soil-dwelling earthworm
species such as
A. caliginosa
can suffer high summer mortality in irrigated pastures because of
surface compaction and poaching by grazing animals, and dung and litter-feeding species such as
L. rubellus
are favored by such conditions (de Bruyn and Kingston 1997). High salinity resulting
from excessive irrigation can also limit earthworm populations in some situations (Khalaf El-
Duweini and Ghabbour 1965). Conversely, drainage and reclamation of wetlands such as polders
and peat soils create conditions suitable for earthworm establishment (van Rhee 1969; Curry and
