Which aphid species is also known as the cotton aphid?

The correct answer is Aphis gossypii, commonly referred to as the cotton aphid. This species is named for its significant impact on cotton crops, where it can cause damage by feeding on plant sap and transmitting various plant viruses. Aphis gossypii has a wide host range, affecting many other plants beyond cotton, but its association with cotton gives it the common name "cotton aphid."

The other options do not refer to the cotton aphid. Macrosiphum euphorbiae is known as the potato aphid, which primarily affects potato plants and other members of the Solanaceae family. Paratrichodorus spp. and Longidorus africanus are not aphids at all; they are nematodes, which are totally different organisms that target plant roots rather than the foliage or sap of plants. Understanding these distinctions is critical for effective pest management and crop protection strategies.