How to Manage Pests

UC Pest Management Guidelines

Alfalfa

Spotted Alfalfa Aphid

Scientific Name: Therioaphis maculata

(Reviewed 11/06, updated 4/08)

In this Guideline:


DESCRIPTION OF THE PEST (View photos to identify aphids)

The spotted alfalfa aphid is a small, pale yellow or grayish aphid with four to six rows of spined black spots on its back. Mature females may either be wingless or have wings with smoky areas along the veins. This aphid prefers warm weather and is generally found during summer months. In the Imperial Valley, high populations may continue into fall and winter.

DAMAGE

Spotted alfalfa aphids inject a toxin into the plant as they feed. Severe aphid infestations stunt plants, reduce yield, and may even kill plants. These aphids also secrete large quantities of honeydew. Plants become very sticky at relatively low aphid densities, and a black fungus that grows on the honeydew excreted by the aphid reduces palatability to livestock and lowers the alfalfa's feed value.

MANAGEMENT

The use of resistant varieties and encouragement of natural enemy populations help to control spotted alfalfa aphids. Border harvesting or strip cutting can be important for preserving natural enemies. In the event that host plant resistance fails or natural enemies do not hold aphid numbers below economic threshold levels, insecticide treatments may be necessary.

Resistant Varieties
Planting alfalfa varieties resistant to spotted alfalfa aphid has been the most effective means of controlling aphids in alfalfa. However, biotypes of spotted alfalfa aphid that are capable of infesting previously resistant varieties are constantly evolving, and even fields planted to resistant varieties should be checked frequently. When selecting varieties, consult your farm advisor for information on varieties suited to your area, or check the current leaflet Winter Survival Fall Dormancy & Pest Resistance Ratings for Alfalfa Varieties (PDF) from the National Alfalfa Alliance Web site. Additionally, a yearly alfalfa variety report can be found at http://alfalfa.ucdavis.edu.

Biological Control (View photos of natural enemies)
Common reddish lady beetles, including the convergent lady beetle, attack and consume this aphid. Green lacewings can also be important in regulating aphids and many other predators including bigeyed bugs (Geocoris spp.), damsel bugs (Nabis spp.), and syrphid flies also play a role. An introduced parasite, Trioxys complanatus, has become established on the spotted alfalfa aphid. Brown aphid mummies attached to leaves and stems of alfalfa plants indicate the presence of this parasite. Caution should be exercised in treating for aphids when the parasite is present.

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 and biological and cultural controls 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 but studies have not been conducted in California to determine their effectiveness.

Monitoring and Treatment Decisions

It is important to sample all fields, even those with resistant varieties, frequently during periods of maximum aphid activity. Start sampling for spotted alfalfa aphid in June and continue until fall. To combine monitoring with cowpea aphid see APHID MONITORING.

In addition to monitoring aphid populations, also take sweep net samples for lady beetles and record all counts on a monitoring form (100KB, PDF).

Time of occurrence No. of aphids per stem
Spring months 40 aphids per stem*
Summer months 20 aphids per stem*
After last cutting in the fall 50 to 70 aphids per stem
Newly seeded alfalfa in lower desert 20 aphids per stem

*Do not treat if the ratio of lady beetles to aphids is equal to or exceeds the following:

No. of lady beetles per sweep No. of aphids per stem
ON STANDING ALFALFA
1 or more adults 5 to 10 aphids
3 or more larvae 40 aphids
 
ON STUBBLE
1 or more larvae 50 aphids
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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