Evolution has endowed caterpillars that feed on corn with a unique ability, they can induce the plant to turn off its defence against insect predators. This helps caterpillars to eat more and grow faster. The agent that causes this effect is the caterpillar’s faeces or frass. The find could throw new light on compounds associated with plant response to pathogens like fungi or bacteria.

Explanation:

a) Farmers can use caterpillars to feed on weeds in their crop fields/ plantations.
This option is not supported by the passage. The passage discusses caterpillars feeding on corn, not weeds. Moreover, it doesn’t suggest using caterpillars as a form of weed control.

b) This finding can help in the development of clinically useful antimicrobial compounds.
While the passage mentions that the find could throw light on compounds associated with plant response to pathogens, it doesn’t directly link this to the development of antimicrobial compounds for clinical use. This connection is a bit of a stretch.

c) This finding can help in the development of organic, ecologically sustainable pesticides.
This option is the most logical and practical inference from the passage. The discovery that caterpillar frass can turn off a plant’s defense mechanism could lead to the development of new, nature-inspired pesticides. Understanding how pests manipulate plant defenses could help in creating more targeted and potentially organic pest control methods.

d) Caterpillars can be genetically modified to be predators of the other plant pests.
This option is not supported by the passage. The text doesn’t mention genetic modification or suggest using caterpillars as predators of other pests.

Therefore, the most logical, rational, and practical message conveyed by the passage is option c: This finding can help in the development of organic, ecologically sustainable pesticides.