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.
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). |