How to Manage Pests

UC Pest Management Guidelines

Alfalfa

Cowpea Aphid

Scientific Name: Aphis craccivora

(Reviewed 11/06, updated 4/08)

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.

Cowpea aphid has been a long time resident of alfalfa in California as well as other states. In the Central Valley, populations are highest from February to April; numbers peak from October to January in the desert; and in the San Joaquin Valley, populations can reach treatable levels in August and September. Cowpea aphids are a sporadic pest in the Intermountain Region and require treatment in some years – mostly in spring, but damage can occur at other times during the growing season.

This aphid has an extensive host range. In addition to alfalfa, it infests many other legumes and cotton, as well as shepherd's-purse, lambsquarters, lettuce, pepperweed, Polygonum sp., and Rumex sp.

DAMAGE

Cowpea aphid injects a powerful toxin into the plant while feeding and, when populations are large, this can stunt or kill plants. While feeding, this aphid produces a considerable amount of honeydew upon which sooty mold grows. 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 populations 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 Diaraetiella sp., have been identified from both the high and low desert. Although parasitism as high as 95% has been documented, aphid population levels 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, 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

The use of biological and cultural controls are acceptable on organically certified crops. 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 some suppression of populations but do not control them.

Monitoring and Treatment Decisions

Aphid infestations in a field are typically patchy, especially an early infestation. 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 at which time 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.

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
10 to 20 inches 40 to 50 per stem
Over 20 inches 40 to 50 per stem
Common name Amount/Acre** R.E.I.+ P.H.I.+
(trade name)   (hours) (days)

  Calculate impact of pesticide on air quality
 
The following materials are listed in order of usefulness, taking into account efficacy and impact on natural enemies and honey bees. When choosing a pesticide, also consider environmental impact. Not all registered pesticides are listed. Always read label of product being used.
 
Note: The following materials have not been tested under California conditions but have been found to be effective in other areas.
 
A. CHLORPYRIFOS*
  (Lorsban) 4EC Label rates 24 see comments
  MODE OF ACTION: An organophosphate (Group 1B)1 insecticide.
  COMMENTS: Do not make more than 4 applications/year or apply more than once per crop cutting. 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 pt/acre, and 21 days for rates above 1 pt/acre.
 
B. DIMETHOATE 2.67EC Label rates 48 10
  MODE OF ACTION: An organophosphate (Group 1B)1 insecticide.
  COMMENTS: Check label to see if product allows only one application per year or per cutting. Do not apply when bees are present.
 
 
Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) is the number of days from treatment until the field can be grazed or cut. In some cases the R.E.I. exceeds the P.H.I.; the longer of these two intervals is the minimum time that must elapse before harvest may take place.
* Permit required from county agricultural commissioner for purchase or use.
1 Modes of action are important in preventing the development of resistance to pesticides. 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. 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 is assigned by IRAC (Insecticide Resistance Action Committee). For additional information, see their Web site at http://www.irac-online.org/.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Alfalfa
UC ANR Publication 3430

Insects and Mites

  • C. G. Summers, Entomology, Kearney Agricultural Center, Parlier
  • L. D. Godfrey, Entomology, UC Davis
  • M. Rethwisch, UC Cooperative Extension, Riverside County (Blythe)
  • D. R. Haviland, UC Cooperative Extension, Kern County
  • P. B. Goodell, Entomology, Kearney Agricultural Center, Parlier
  • R. F. Long, UC Cooperative Extension, Yolo County

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