Agriculture: Alfalfa Pest Management Guidelines

Aphid Monitoring

Using the procedures below, take weekly stem samples for blue alfalfa aphids, pea aphids, cowpea aphids, and natural enemies during stand establishment and in spring- and fall-established stands. Sample for cowpea aphids, spotted alfalfa aphids, and natural enemies in summer-established stands.

Natural enemies can quickly reduce aphid infestations. As aphid threshold populations are approached, check every 2 to 3 days to determine if natural enemies and disease are reducing numbers. For more detailed treatment threshold information, see the monitoring form. (PDF)

How To Sample

(View photos of aphids)

  • Randomly choose 5 stems from each of 4 areas per field, noting if the average plant height is less than 10 inches, 10 to 20 inches, or more than 20 inches.
  • Bend each stem sample over a white pan and tap; dislodged aphids will fall in. The stem can also be shaken into a sweep net if a pan is not available.
  • Take sweep net samples, see SAMPLING WITH A SWEEP NET), for lady beetle adults and larvae, fungal-killed aphids, parasitized aphid mummies, and the presence of other predators such as syrphidflies and lacewing larvae. Natural enemies can reduce aphid numbers.
  • Record results on a monitoring form. A pesticide application may be warranted if natural enemies fail to keep the aphid numbers in check.

Procedure and Treatment Threshold

Treatment threshold Stand establishment and Established stands (winter, spring, and fall) Established stands (summer)
Aphid species Blue alfalfa, Pea aphids, Cowpea aphid, and natural enemies Cowpea aphid, spotted alfalfa aphid, and natural enemies
When to start Late fall Early summer (June for spotted alfalfa aphid)
How often Weekly Weekly
Special instructions If both blue alfalfa and pea aphid species are present, use the blue alfalfa aphid treatment thresholds.
No thresholds are established for aphids on seedling alfalfa; watch for plant damage.
If only spotted alfalfa aphids are present and reach treatment thresholds, be sure to sweep and compare spotted alfalfa aphids and lady beetles according to the following "Treatment Thresholds" table.
If natural enemies are present Check the field every 2 to 3 days to see if the aphid numbers decrease. If they do, insecticides may not be necessary.

Treatment Thresholds (#aphids/stem)

Pest Plants less than 10" Plants 10-20" Plants more than 20" Summer Spring After last fall cutting
Pea aphid 40-50 70-80 100+
Blue alfalfa aphid 10-12 40-50 40-50
Cowpea aphid 10-12 40-50 40-50
Spotted alfalfa aphid 40* 20* 50-70
*Do not apply a pesticide if there are 4 or more adult lady beetles or 3 or more lady beetle larvae per sweep for every 40 aphids counted per stem (on stubble this ratio is 1 larva per sweep to every 50 aphids per stem).
Text Updated: 01/17