How to Manage Pests

UC Pest Management Guidelines

Alfalfa

Blue Alfalfa Aphid

Scientific Name: Acyrthosiphon kondoi

(Reviewed 1/17, updated 1/17)

In this Guideline:


DESCRIPTION OF THE PESTS  (View photos to identify aphids)

The blue alfalfa aphid is a large blue-green aphid with long legs, antennae, cornicles, and cauda. It is similar in appearance to the pea aphid but can be distinguished by examining the antennae. The antennae of the pea aphid have narrow dark bands on each segment, whereas those of the blue alfalfa aphid gradually darken to brown as you near the tip of the antennae.

Both the blue alfalfa aphid and the pea aphid prefer cooler temperatures (optimal temperature for development of both blue alfalfa and pea aphid is around 60°F); however, the blue alfalfa aphid is more tolerant than the pea aphid of cool temperatures and appears earlier in growing season. The biology and population levels of the blue alfalfa aphid has changed in recent years. Damaging numbers are most common in the mid- to late winter in the desert production areas, in the late winter to early spring in the Central Valley, and the spring to early summer in the intermountain areas. However, blue alfalfa aphids may also be found in the fall in many areas.

The blue alfalfa aphid and the pea aphid may occur in mixed populations. Historically, both species were present in alfalfa fields at the same time as the alfalfa weevils. However, now both blue alfalfa and pea aphids can be present in the fall, winter, and spring. The blue alfalfa aphid colonizes the plant terminals while pea aphid is usually more generally distributed. Both species prefer to feed on the stems rather than the leaves.

Damage

While feeding on alfalfa, the blue alfalfa injects a toxin that retards growth, reduces yield, and may even kill plants. The toxin injected by the blue alfalfa aphid is more potent than that of the pea aphid (pea aphid toxin itself is not particularly damaging to alfalfa plants). Toxin that remains in the stems and crown after harvest of the upper plant material may continue to retard stem growth and elongation and may carry over to the next cutting, or even the subsequent two cuttings.

Damage can also reduce the alfalfa's feeding value. A black fungus, sooty mold, grows on the honeydew excreted by the aphid and reduces palatability to livestock. Damage is more severe on short growth alfalfa than on taller alfalfa for both species.

Management

Using resistant varieties of alfalfa and encouraging populations of natural enemies are very important in managing blue alfalfa aphid. It is critical to distinguish between the blue alfalfa and pea aphids because blue alfalfa aphid causes more damage than pea aphid, and the two species have different treatment thresholds. Natural enemies, especially lady beetles, are monitored along with the aphids to determine the need for a pesticide application. Aphids may become a more severe problem when weevil sprays reduce the numbers of natural enemies. Border harvesting or strip cutting can be a useful tool for preserving natural enemies.

Resistant Varieties

Planting alfalfa varieties resistant to blue alfalfa aphid has been the most effective means of controlling aphids in alfalfa. Prolonged periods of below-normal temperatures, however, may lower plant resistance to blue alfalfa aphid when it is most needed and some crop injury may occur. At around 70°F, the host plant resistance should be fully expressed.

When selecting varieties, consult your cooperative extension advisor for information on resistant varieties suited to your area, or check a list of alfalfa varieties provided by the National Alfalfa & Forage Alliance. Additionally, a yearly alfalfa variety performance report can be found at http://alfalfa.ucdavis.edu.

Biological Control  (View photos of natural enemies)

The most significant aphid predators are several species of lady beetles, including Hippodamia convergens and Coccinella septempunctata that attack and consume both the blue alfalfa and pea aphid species. Green lacewings can also be important in regulating aphids and many other predators including bigeyed bugs (Geocoris spp.), damsel bugs (Nabis spp.), and syrphid fly larvae also play a role.

Aphidius smithi and A. ervi are parasites of alfalfa aphids. Large golden-brown aphid mummies on the upper surfaces of leaves indicate parasitization. When a high level of parasitization is present, carefully consider the need to apply insecticides for aphids. Parasites frequently provide adequate control.

A naturally occurring fungal disease, which is most prevalent during cool, rainy, or foggy weather, may also control aphids.

Cultural Control

Use border-strip cutting during harvest to help maintain populations of parasites and predators within the field. For more details, see BORDER-STRIP HARVESTING.

Organically Acceptable Methods

The use of resistant varieties, biological control, and cultural control are acceptable to use on an organically certified crop. Organically certified insecticides such as azadirachtin (Neemix), neem oil (Trilogy), and pyrethrin (PyGanic) are registered for use on alfalfa to control aphids. Studies conducted in California, however, have shown that at best they provide slight suppression of aphids but do not control them.

Monitoring and Treatment Decisions

Start to monitor fields as soon as the first aphids are observed. Use monitoring as described in APHID MONITORING.

If natural enemies fail to keep the aphids in check, an insecticide application may be necessary. Economic treatment thresholds for both blue alfalfa and pea aphids are as follows (if both species are present, use the blue alfalfa aphid treatment levels):

Plant height Pea aphids Blue alfalfa aphids
Under 10 inches 40 to 50 per stem 10 to 12 per stem
10 to 20 inches 70 to 80 per stem 40 to 50 per stem
Over 20 inches 100 + per stem 40 to 50 per stem
Common name Amount per acre** REI‡ PHI‡
(Example trade name)   (hours) (days)
 
Calculate impact of pesticide on air quality
Bee precaution pesticide ratings
The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least harmful to natural enemies, honey bees, and the environment are at the top of the table. When choosing a pesticide, consider information relating to air and water quality, resistance management, and the pesticide's properties and application timing. Not all registered pesticides are listed. Always read the label of the product being used.
 
A. FLUPYRADIFURONE
  (Sivanto 200SL) 7–10.5 fl oz 12 7
  MODE-OF-ACTION GROUP NUMBER1: 4D
 
B. FLONICAMID
  (Beleaf 50SG) 2.8 oz 12 62
  MODE-OF-ACTION GROUP NUMBER1: 29
  COMMENTS: Use allowed under a 24c registration (SLN CA-140006).
 
C. CHLORPYRIFOS*
  (Lorsban Advanced) 1–2 pt 24 See comments
  MODE-OF-ACTION GROUP NUMBER1: 1B
  COMMENTS: Do not apply when bees are present. Avoid drift and tailwater runoff into surface waters. Preharvest interval is 7 days for cutting and grazing when 0.5 pt/acre used, 14 days for 1 pint/acre, and 21 days for rates above 1 pint/acre. Certain formulations emit high amounts of volatile organic compounds (VOCs); use low-VOC formulations. Regulations affect use for the San Joaquin Valley from May 1 to October 31, 2017. Review the Department of Pesticide Regulation's updated fact sheet.
 
D. DIMETHOATE
  (Dimethoate 2.67EC) Label rates 48 See comments
  MODE-OF-ACTION GROUP NUMBER1: 1B
  COMMENTS: Preharvest interval (PHI) is 10 days for harvest or pasturing; for alfalfa seed: do not feed or graze livestock on treated crops, hay threshings, or stubble within 20 days of application. Check label to see if product allows only one application per year or per cutting. Do not apply when bees are present.
 
E. METHOMYL*
  (Lannate SP) 0.5–1 lb 48 7
  MODE-OF-ACTION GROUP NUMBER1: 1A
  COMMENTS: Highly toxic to bees: do not spray directly or allow drift onto blooming crops or weeds where bees are foraging. Can also kill natural enemies.
 
F. LAMBDA-CYHALOTHRIN*
  (Warrior II with Zeon) 1.28–1.92 fl oz 24 See comments
  MODE-OF-ACTION GROUP NUMBER1: 3A
  COMMENTS: Preharvest interval (PHI) is 1 day for forage and 7 days for hay. Can be disruptive to natural enemies. Highly toxic to bees; do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging.
 
G. ZETA-CYPERMETHRIN*
  (Mustang) 2.4–4.3 fl oz 12 See comments
  MODE-OF-ACTION GROUP NUMBER1: 3A
  COMMENTS: Preharvest interval (PHI) is 3 days for cutting or grazing and 7 days for harvesting seed. Can be disruptive to natural enemies. Highly toxic to bees; do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging.
 
** See label for dilution rates.
Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.
* Permit required from county agricultural commissioner for purchase or use.
1 Rotate chemicals with a different mode-of-action group number, and do not use products with the same mode-of-action group number more than twice per season to help prevent the development of resistance. For example, the organophosphates have a group number of 1B; chemicals with a 1B group number should be alternated with chemicals that have a group number other than 1B. Mode-of-action group numbers are assigned by IRAC (Insecticide Resistance Action Committee).

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Alfalfa
UC ANR Publication 3430

Insects and Mites

L. D. Godfrey, Entomology, UC Davis
E. T. Natwick, UC Cooperative Extension, Imperial County
P. B. Goodell, Entomology, Kearney Agricultural Center, Parlier
R. F. Long, UC Cooperative Extension, Yolo County
V. M. Barlow, UC Cooperative Extension, Riverside County and UC IPM Program

Acknowledgment for contributions to Insects and Mites:
D. R. Haviland, UC Cooperative Extension, Kern County
M. Rethwisch, UC Cooperative Extension, Riverside County (Blythe)
C. G. Summers, Entomology, Kearney Agricultural Center

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