Pesticide Residues in Soil Invertebrates - Pesticides: Evaluation of Environmental Pollution - page 341

Earthworms are generally found in the top 12″ to 18″ of the soil because this is where
food is most abundant. The worm ingests soil and organic matter, which is swallowed
and ground in the gizzard. The ejected material, or casting, is used to line the burrow or
deposited at the entrance. Earthworm activity depends directly on the soil moisture and
temperature. They become active when the soil thaws in the spring and move deeper in
late summer as the soil dries.
The use of pesticides that may reduce the numbers or activity of earthworms can result
in adverse effects on soil fertility, which in turn may influence crop yields. Second, reduc-
tions in the populations of earthworms may lead to a decrease in food supply for their
predators. Third, the presence of pesticide residues in earthworms may lead to the poison-
ing of predators after consumption. Bioaccumulation may occur at secondary levels, with
earthworm predators such as birds, or at tertiary levels, for example, when a fox preys
upon a bird that has consumed earthworms containing pesticide residues.
DDT residues in soil and earthworms from 50 sites in Delhi were monitored. DDT
was detected in all but two samples each of soil and earthworms. Among DDT residues,
p,p′-DDE was most common and was found in 48 samples each of soil and earthworms;
p,p′-DDT was detected in only 43 soil samples and 46 earthworm samples. p,p′-TDE and
o,p′-DDT were also present in smaller concentrations in 29 and 15 soil samples and in 43
and 25 earthworm samples, respectively (Pillai 1986). A maximum total DDT concentra-
tion of 2.6 ppm was detected in the soil from Durga Nagar in the vicinity of a DDT factory.
The highest concentration of 37.7 ppm total DDT in earthworms was also obtained from
the same site. The maximum concentration factor found in the earthworms was 551. The
total DDT concentration in the earthworms and soil showed significant correlation (Yadav
et al. 1981).
The most important consequences of pesticide contamination of earthworms were seen
in the 1960s and 1970s on species at higher trophic levels in food chains, when adverse
effects on predators of earthworms were extensively reported. In addition to the example
of local declines in the population of American robins noted earlier, poisoning of birds
after eating DDT-contaminated earthworms has been reported from several countries.
In the UK, several studies carried out in orchards in the early 1970s reported deaths of
blackbirds ( Turdus merula ), song and mistle thrushes ( Turdus philomelos and T. viscivorus ),
gray partridge ( Perdix perdix ), red-legged partridge ( Alectoris rufa ), pheasant ( Phasianus col-
chicus ), and tawny owl ( Strix aluco ). Blackbirds and song thrushes found dead in a British
apple orchard contained 81-128 mg/kg DDE in breast muscle, and the authors suggested
that, based on the residues detected in other live birds, most thrushes in the orchard were
carrying near lethal amounts of DDT (Bailey et al. 1974). However, in this study, the con-
centrations in the earthworms were much lower than the threshold value that Beyer and
Gish (1980) suggested: a concentration of 32 mg/kg of total DDT in earthworms could be
a hazard to reproduction in some bird species, implying the lower risk to the earthworms
in most of the leisure areas in Beijing in this present condition. Although DDTs are of
little risk to the worms, it was probably virulent for the higher trophic organisms in the
urban environment because of the bioaccumulation of DDTs through the food chain such
as soil-earthworm-bird.
Work carried out in a Norfolk orchard during the 1960s showed that thrushes could accu-
mulate DDT in their fat deposits from eating worms and other invertebrates. The death of
birds was associated with the release of DDT into the bloodstream, following the mobiliza-
tion of fat reserves during periods of stress (Edwards 1973; McEwan and Stephenson 1979).
DDT passed to them by the consumption of contaminated earthworms may also affect ver-
tebrates other than birds. Frogs of the species Rana temporaria may be affected by DDT arising