Biological Warfare - Soil: Reflections on the Basis of our Existence - page 106

species in the soil that can keep pests at bay in our gardens and vegetable patches.
Farmers and horticulturalists have long sought to harness these biological pow-
ers rather than resort to chemical warfare, but experience has shown that it is not
always so easy.
The tomato plants in my greenhouse grew well to start with. But after a few
years they became thinner and thinner in spite of the household compost and
chicken manure I lavished on them. On pulling up the plants I found small egg-
containing cysts on the roots. The culprits were cyst nematodes—tiny microscopic
worms that congregate in soil where tomatoes are grown year after year.
Crop rotation is potentially one solution to this problem. Another—if, like
me, you want to grow tomatoes every year—is simply to change the soil at regu-
lar intervals. There is also a third solution, one that might save me the hassle of
removing all the soil in my greenhouse. Some soils contain organisms that attack
nematodes and scientists have long looked for ways to use these as biological con-
trol agents. Nematophagous fungi are carnivorous fungi that form sticky networks
Producing natural enemies . Organic pest control involves fighting pests using their natural
enemies instead of toxic chemicals. Ladybirds eat large numbers of aphids and under favour-
able conditions can keep the aphid population in check. But biological control is also a difficult
balancing act. Australia's introduction of cane toads to combat beetles that attacked sugarcane
roots is just one example of pest control gone wrong. In confined spaces, as in a greenhouse, the
results are usually better. Whitefly on tomatoes can be successfully controlled with parasitoid
wasps and predatory mites are used to tackle spider mites