Scarab beetle larvae feed on plant roots, so they are known as pests in gardening.
Written 黄金虫 in kanji (literally "gold insect"), but the katakana form is standard in zoology and everyday use. Famous in Japan from the children's song コガネムシは金持ちだ ("The scarab beetle is rich").
Similar words
カブトムシ: rhinoceros beetle — larger, with a distinctive horn
クワガタムシ: stag beetle — with large mandibles instead of a horn
カナブン: drone beetle — flatter relative often confused with コガネムシ