Use 'gadfly' in rural or farming settings where livestock might experience nature's annoyances. During the summer, horses often need protection from gadflies.
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Literal Pest
Refer to the insect itself, especially when discussing its direct impact on animals. She applied repellent to shield her cattle from gadflies.
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Figurative Use
Sometimes used to describe anything irritating, not just the fly. Understand the metaphor. That buzzing phone is like a gadfly during my meditation.