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How to Manage Pests

Identification: Key to Aphids Commonly Found in San Joaquin Valley Alfalfa and Cotton

Note: This key includes only the aphid species that are most commonly found in alfalfa and cotton in California's San Joaquin Valley. If you are unable to identify an aphid using this key, see your local Cooperative Extension farm advisor for assistance.

Aphids may be difficult to identify as they attack California alfalfa and cotton.

The alfalfa aphids, pea aphid (Acyrthosiphon pisum (Harris)) and blue alfalfa aphid (A. kondoi Shinji), are similar in appearance. Recently, the cowpea aphid, Aphis craccivora Koch, has become a pest of alfalfa and has been found colonizing cotton in California’s San Joaquin Valley. The cowpea aphid on cotton is easily confused with cotton aphid, A. gossypii Glover, at certain times of the year.

Since economic thresholds have been developed for individual species, proper aphid management requires accurate identification.

Use this key to identify an aphid. While aphids appear to be very simple insects, they are really very complex, both in their biology and their morphology. As with other insects, external morphological features are used to identify individuals to the species level.

Diagram of a winged aphid, showing antenna, tubercle, head, thorax, abdomen, cornicle, and cauda. This drawing of a typical aphid shows a number of morphological features important in identification.
  1. Look at an aphid through a 10x hand lens to see the important characters that distinguish the aphids included in this key.
  2. On each page of the key, click on the box that best matches the character of the aphid you’re trying to identify.

Begin the Key

Additional reading

Statewide IPM Program, Agriculture and Natural Resources, University of California
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