How to Manage Pests

UC Pest Management Guidelines

Alfalfa

Cowpea Aphid

Scientific Name: Aphis craccivora

(Reviewed 1/17, updated 1/17)

In this Guideline:


DESCRIPTION OF THE PEST  (View photos to identify aphids)

Cowpea aphid is readily distinguishable from other aphids inhabiting alfalfa because it is the only black aphid found infesting the crop. It is a relatively small aphid and the adult is usually shiny black while the nymph is slate gray. The appendages are usually whitish with blackish tips.

In the Sacramento Valley, cowpea aphid numbers are highest from April to September; numbers peak from October to January in the desert; and in the San Joaquin Valley, cowpea aphid can reach treatable levels from March to October. Cowpea aphids are a sporadic pest in the Intermountain Region.

This aphid has an extensive host range, including beans, cotton, and weeds.

Damage

Cowpea aphid injects a powerful toxin into the plant while feeding and, when their numbers are high, this can stunt or even kill plants. While feeding, this aphid produces a considerable amount of honeydew upon which sooty mold can grow. The black sooty mold reduces photosynthesis and may make leaves unpalatable to livestock. The honeydew also makes the alfalfa sticky, which causes problems with harvest.

Management

There are no known varieties of alfalfa that are resistant to cowpea aphid and economic thresholds have not been developed specifically for this pest. Treatments may be necessary if large numbers of cowpea aphids are present. Border harvesting or strip cutting can be important for preserving natural enemies.

Biological Control  (View photos of natural enemies)

Two common aphid parasites,Lysiphlebus sp. and Diaeretiella sp., have been identified from the desert production areas. Although parasitism as high as 95% has been documented, aphid numbers can become so high that enough nonparasitized individuals remain to cause significant injury.

This aphid is also susceptible to the usual complement of aphid predators including lady beetles (convergent lady beetle, multicolored Asian lady beetle, twicestabbed lady beetle), lacewings, bigeyed bugs, damsel bugs, and syrphid flies. Early in the season (February and early-March) many of these predators are generally not active, but in the low desert the sevenspotted lady beetle, Coccinella septempunctata, is abundant and feeding on the aphid.

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

Use biological and cultural controls on organically certified crops. Organically certified insecticides such as azadirachtin (Neemix), neem oil (Trilogy), and pyrethrin (PyGanic) are also registered for use on alfalfa to control aphids. Studies conducted in California, however, have shown that at best they provide some suppression of populations but do not control them.

Monitoring and Treatment Decisions

Cowpea aphid infestations are typically patchy in a field, especially early infestations. Stems on alfalfa plants in infested areas are often completely covered with aphids, whereas plants in other areas of the field may appear aphid-free. Because of the spotty distribution of cowpea aphid infestations, spot treatments may be feasible, especially if the infestation is on the field border.

On dormant alfalfa, pay close attention to plants as they begin breaking dormancy. If shoots fail to grow normally and cowpea aphid is present, consider control measures.

Start to monitor fields in February for cowpea aphid and continue to monitor this aphid through fall; monitoring can be combined with that of blue alfalfa and pea aphid as described in APHID MONITORING. (During summer months, monitoring of cowpea aphid can be combined with that of spotted alfalfa aphids.)

Record counts on a monitoring form (PDF).

No guidelines or economic threshold levels have been established for cowpea aphid in alfalfa. Until economic thresholds are developed for the cowpea aphid, use the following thresholds, which were developed for the blue alfalfa aphid:

Plant height Aphids
Under 10 inches 10 to 12 per stem
Over 10 inches 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.
 
The following pesticides have not been tested under California conditions but have been found to be effective in other areas.
 
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 pint/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: Check label to see if product allows only one application per year or per cutting. 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. Do not apply when bees are present.
 
E. MALATHION
  (Malathion 8-E) 1–1.25 pt 12 0
  MODE-OF-ACTION GROUP NUMBER1: 1B
  COMMENTS: Use only when other products cannot be used. Do not apply when bees are present.
 
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.
 
H. BETA-CYFLUTHRIN*
  (Baythroid XL) 2.8 fl oz 12 7
  MODE-OF-ACTION GROUP NUMBER1: 3A
  COMMENTS: 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.
 
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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