How to Manage Pests

UC Pest Management Guidelines

Alfalfa

Blue Alfalfa Aphid and Pea Aphid

Blue alfalfa aphid: Acyrthosiphon kondoi
Pea aphid: Acyrthosiphon pisum

(Reviewed 11/06, updated 4/08)

In this Guideline:

DESCRIPTION OF THE PESTS (View photos to identify aphids)
The pea aphid and the blue alfalfa aphid are large green aphids with long legs, antennae, cornicles, and cauda. They are very similar in appearance but can be distinguished from each other by examining the antennae: the antennae of the pea aphid has narrow dark bands at the tip of each segment, whereas those of the blue alfalfa aphid are uniformly brown.

A pink biotype of the pea aphid has recently been found in the central valley of California, including Fresno, Kings, Tulare, Yolo and Sacramento counties. Except for its pink color, it is identical in appearance to the green biotype. The pink biotype causes similar damage to the green pea aphid and management practices are the same, but some studies have suggested it may be partially resistant to parasitization by Aphidius ervi and may also circumvent some of the pea aphid resistance bred into many alfalfa cultivars.

Both the blue alfalfa aphid and the two strains of the pea aphid prefer cool temperatures (optimal temperature for development of blue alfalfa aphid is 60°F) and reach damaging levels in spring, but blue alfalfa aphid is more tolerant than pea aphid of cool temperatures and appears earlier in spring. Pea aphid often reoccurs in fall as well. Both species may be present in alfalfa fields at the same time as the alfalfa weevils. The blue alfalfa aphid prefers the plant terminals while pea aphid is usually more generally distributed. Both species prefer the stems to the leaves.

DAMAGE

These aphids feed on alfalfa and inject a toxin that retards growth, reduces yield, and may even kill plants. Damage can also reduce the alfalfa's feed value. A black fungus, sooty mold, grows on the honeydew excreted by the aphid reduces palatability to livestock. Damage is more severe on short hay than on taller alfalfa for both species. The toxin injected by the blue alfalfa aphid is more potent than that of the pea aphid.

MANAGEMENT

Using resistant varieties of alfalfa and encouraging populations of natural enemies are very important in managing blue alfalfa aphid and pea aphid. It is important to distinguish these two species 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 treatment. Aphids frequently become problems when their natural enemies are disrupted by weevil sprays. Border harvesting or strip cutting can be important for preserving natural enemies.

Resistant Varieties
Planting alfalfa varieties resistant to blue alfalfa aphid and pea aphid has been the most effective means of controlling aphids in alfalfa. Prolonged periods of below-normal temperatures, however, may lower resistance to blue alfalfa aphid injury and result in some crop injury. Studies in the eastern U.S. have shown that the pink biotype of the pea aphid easily overcame resistance in a number of cultivars with the exception of CUF 101. When selecting varieties, consult your farm advisor for information on resistant varieties suited to your area, or check the 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 online)
The most significant aphid predators are several species of lady beetles, including Hippodamia convergens and Coccinella septempunctata that attack and consume both of these aphid species; treatment thresholds for pea aphid are based on the number of lady beetle adults and larvae present. 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. The major parasite of the pea aphid is Aphidius smithi while the parasite A. ervi attacks both species. However, several studies have suggested that the pink biotype of pea aphid shows signs of partial resistance to A. ervi. Large golden-brown aphid mummies on the upper surfaces of leaves indicate parasitization. When parasites are present, be careful when treating for aphids and other insects. Parasites frequently provide adequate control. Aphids may also be controlled by a naturally occurring fungal disease, which is most prevalent during cool, rainy, or foggy weather.

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 some suppression of populations but do not control them.

Monitoring and Treatment Decisions
Start to monitor fields in February for blue alfalfa aphid and pea aphid and continue monitoring through spring. In fall resume monitoring pea and blue alfalfa aphids by combining with cowpea monitoring as described in APHID MONITORING.

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

Common name		Amount/Acre**	R.E.I.+	P.H.I.+
(trade name)			(hours)	(days)
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.
A.	CHLORPYRIFOS*
	(Lorsban) 4EC	0.25–0.5 pt	24	7
	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.
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.
	**  See label for dilution rates. + 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/.

PUBLICATION

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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