AGRICULTURAL SOILS: A SIDEWAYS LOOK - The Soil - page 189

vival as compared with those in unsprayed plots. He showed that this was caused by
decreased predation by ground-living predators which normally climbed the cabbage
plants and consumed the young larvae. The most important predators in this instance
were the ground beetle Harpalus rufipes and the harvest spider Phalangium opilio
both of which were seriously affected by DDT residues in the soil.
Occasionally, sublethal levels of DDT appear to increase the numbers of ground
beetles when these are assessed by catches in pitfall traps (jars sunk in the ground).
But this technique is dependent on the mobility of the beetles and so a spurious in-
crease in numbers may be due to pesticide-induced hyperactivity. Such abnormal be-
haviour is likely to diminish a beetle's chance of survival by attracting the attention
of birds such as skylarks. A diet of affected beetles, in turn, increases the risk of sec-
ondary poisoning, and skylarks have been found to suffer from accumulated doses of
pesticide, like the American robins mentioned earlier.
Chlordane has been used to kill earthworms in sports turf; other organochlorine
insecticides are absorbed and accumulated by worms with no apparent ill effects.
The uptake of pesticides depends on the feeding behaviour of the worms and on
where pesticide residues are concentrated. Lumbricus terrestris can accumulate large
amounts of DDT from fallen leaves in sprayed orchards (It was mainly this species
on which the American robins fed). It acquires less pesticide from residues in the soil
than do shallow-living species which ingest soil. Usually, we know too little about
significant physiological and behavioural differences among soil invertebrates to un-
derstand how they respond to pesticides. The problem with the Arion hortensis group
of slugs was mentioned earlier; pesticides will remain a blunt instrument so long as
differences in ecology are overlooked.
Most organochlorine insecticides have now been superceded by organophos-
phorus compounds, carbamates and others, with names like fonofos, chlorfenvinphos,
bendiocarb and carbofuran. Sometimes the names suggest their chemical affinity but
usually they are simply trade names. The 1988 UK Pesticide Guide gives 19 organ-
ophosphorus and carbamate insecticides that are used to control soil pests, whereas
a little over 20 years ago there was only parathion. Compared with organochlorine
compounds, these substances are much less persistent in soils and in the bodies of an-
imals absorbing them. They still kill non-target organisms and cause imbalances in
the fauna, but the effects are not as long lasting and they are less readily passed on to
predators.
T ABLE 10
Total annual usage of pesticides 1980-83 in agriculture and horticulture. (From
MAFF Pesticide Usage Survey Report No 41) including fruit, vegetables and grass..