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Alfalfa

Aphid Monitoring

(Reviewed 11/06, updated 9/07)

In this Guideline:

How to sample
Procedure and treatment threshold
Treatment thresholds

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 cause the population numbers to decrease.

See monitoring form (100KB, PDF) with detailed treatment threshold information.

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 syrphid flies and lacewing larvae. Natural enemies can quickly reduce the aphid population.
Record results on a monitoring form. A treatment may be warranted if natural enemies fail to keep the aphid population in check.

PROCEDURE AND TREATMENT THRESHOLD

Treatment threshold

Stand establishment and established stands (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

February during stand establishment

Early summer (June for spotted alfalfa aphid)

How often

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.

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 population declines. If it does, 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 treat 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/sweep to every 50 aphids/stem).

PUBLICATION

UC IPM Pest Management Guidelines: Alfalfa
UC ANR Publication 3430
General Information
C. G. Summers, Entomology, Kearney Agricultural Center, Parlier
P. B. Goodell, Entomology, Kearney Agricultural Center, Parlier

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