Biological control of pests involves the use of natural enemies of pests to control their population. An example of biological control of pests is using ladybird beetles to feed on aphids, which are pests that damage crops. Ladybirds are natural predators of aphids, so introducing them into a field can help to reduce the population of aphids without the use of harmful chemicals. Therefore, the correct answer is "using lady bird beetles to feed on cytoplasm".