Geoscience Reference
In-Depth Information
Before 1964, aldrin and dieldrin were used in dusts, drenches and sprays and
sometimes mixed with fertilizers against a wide range of soil pests including leather-
jackets, wireworms, cutworms, millipedes, symphylids, and the larvae of cabbage
root fly, bean seed fly, carrot and mangold fly. They were also used in seed dressings
against wheat bulb fly. The government-sponsored
Review of Persistent Organo-
chlorine Pesticides
in 1964 recommended withdrawal or restriction of some of these
uses “as soon as practicable”, but the use of DDT was not restricted until the Further
Review in 1969. Then, widespread restrictions were imposed on DDT since it could
be replaced by alternative insecticides, especially in gardens where “the home garden-
er would not be put to a serious disadvantage”.
It is inevitable that such potent and persistent pesticides should produce side ef-
fects on soil organisms, but the ecological consequences are often complicated. The
chemicals themselves react differently in different soils, soil organisms vary greatly
in sensitivity, and a response by some may trigger reactions in others. For example,
ground-living predatory mites, beetles and harvest spiders are often more affected by
persistent pesticides than their prey. Sometimes this is because of greater intrinsic
sensitivity but sometimes a result of their greater activity which renders them more
likely to encounter pesticide residues. In either case, the result is often an upsurge in
numbers of the prey species.
This is normally only noticed when the prey species is or becomes an economic
pest, like the cabbage root fly
Delia radicans
or cabbage white butterfly
Pieris rapae.
T.H.Coaker found that the ground beetles
Bembidion lampros
and
Trechus quadris-
triatus
and rove beetles of the genus
Aleochara
normally consume a significant pro-
portion of the eggs and young larvae of the cabbage root fly before the latter inflict
much damage on crops. However, these beetles were affected by residues of aldrin/
dieldrin at concentrations of less than one part in ten million in the soil, resulting from
sprays or dusts applied in previous years. Unless there was a high concentration of in-
secticide immediately around the plants, the cabbage root fly larvae were unaffected,
and, with less predation, their better-than-normal survival resulted in yield reductions
of up to 70% in cauliflowers, cabbage and brussels sprouts. The repercussions did not
stop there; increased survival of the fly in the presence of low pesticide residues was
thought to have been a major factor in the rapid increase in resistant root-fly popula-
tions which occurred at that time.
A similar story was unravelled in the case of the cabbage white. In this case,
DDT gave very good control of caterpillars on the plant at the time of spraying. Con-
trol was short lived, however, since new leaves were produced and fresh eggs were
laid on these. J.P.Dempster studied the survival of caterpillars in the second gener-
ation for three years, and found that there was a marked improvement in their sur-
