Should aphids be capitalized in a sentence?

No, "aphids" should not be capitalized in a sentence unless it is the first word of the sentence or part of a proper noun.

Examples:

* Correct: Aphids are small insects that feed on plants.

* Correct: The Aphid Research Institute is dedicated to understanding these pests.

* Incorrect: Aphids are a common problem in gardens.

Let me know if you have any other grammar questions!