Alfalfa Seed

Pea Aphid, Blue Alfalfa Aphid, Spotted Alfalfa Aphid

Sue Blodgett

Spotted alfalfa aphid, left; pea aphids, right

Aphids are important alfalfa seed pests. All aphid species have piercing-sucking mouthparts with which they suck plant sap and cell contents from vegetative plant parts and do not directly damage seed-producing structures. It can be important to distinguish among the species as their damage potential differs. Pea aphid and spotted alfalfa aphid are the most prevalent species in the High Plains. Spotted alfalfa aphid has been found damaging dryland alfalfa.

Identification (life cycle and seasonal history)

Pea and blue aphids are quite similar in appearance but pea aphid is yellow-green in color while the blue alfalfa aphid is bluish-green, although the color difference is not reliable in northern regions. The pea aphid can be distinguished by the dark antennal band around each antennal segment in contrast to the uniform dark antennae of the blue aphid. It can be important to distinguish among the species as their damage potential differs. Blue alfalfa aphids have not yet been detected in Montana. [NE, CO, WY???] Both pea and blue aphids resemble the first two nymphal stages of the lygus bug. However, the lygus bugs can be distinguished from aphids by their brighter color and more rapid movements than the rather slow moving aphids. Aphids reproduce parthogenetically, each female giving birth to 50-100 live young at the rate of 6-7 per day. Multiple aphid generations can occur each year depending on weather conditions and aphid populations can increase quite rapidly. Local, high density populations induce the formation of winged forms, which are able to migrate within and between fields. Pea aphids and blue alfalfa aphids prefer cool, dry conditions and are problems in the first cutting and during spring seedling establishment. Pea aphid tends to congregate along the stems, sterminal shoots and leaves while the blue alfalfa aphid prefers the growing point and buds. Spotted alfalfa aphids prefer hot, dry conditions and generally are a problem on rainfed crops, later cuttings and late summer seedlings. These aphids often feed on the underside of leaves and will move up and down in the canopy, depending on humidity.Each species has a different potential for damaging alfalfa, so it is essential to determine which aphids are present in a field. This information combined with the crop height and stage of development can help determine the need for control (Table 2).

Plant Damage and Response

Large aphid populations may stunt plants, cause foliage to wilt and/or yellow or may prevent or delay flowering. Any delay in flowering can compromise seed maturation and also bee reproduction. If damage is severe and prolonged, the leaves may drop from severely infested plants. Aphids produce a sugary, sticky material called "honey dew," which is visible on plants in moderate to severe infestations.

Economic Thresholds

Table X-21. Economic thresholds for pea, blue, and spotted aphids.

Aphid species	No. aphids per stem
Aphid species	Seedling	0-10 inch stems	20 inch stems
Pea		50	100
Blue		20	50
Spotted	1-3	10-20	40

Biological Control

Aphids are attacked by a diverse community of natural enemies, including specialist parasitoid wasps and number of generalist predators including Nabis and Orius bugs, Coccinellid and Carabid beetles, and web-building spiders. Aphid "mummies" also are common in pea-aphid colonies. These swollen, tan-to-copper aphid skins are the result of parasitism, suggesting that natural control is at work. However, predators cannot exist without insect prey to feed on. Low numbers of aphids (less than 25 aphids per sweep) can help attract and maintain predators within the field. Research by Dr. Bill Synder at WSU has indicated that the impact of parasitism and general predation is additive, with the generalist predators having an immediate negative effect on aphid populations but allowing aphid populations to increase in time. He found that parasitoids have a small initial impact but causing aphid populations to decline. When both parasitic wasps and generalist predators were present, pea aphid dynamics reflected the impact of both generalists and specialists – pea aphid population growth was depressed early, and pea aphid densities strongly declined later. Overall, the density independent impact of generalists and the density dependent impact of specialists were complimentary, so that biocontrol was most effective when both classes of natural enemy were present. The impacts of generalists and specialists were additive despite a high rate of predation among aphid and natural enemy populations, because predators did not alter percent parasitism by the parasitic wasp, A. ervi.

Resistant Varieties

The Alfalfa Council publishes lists of alfalfa varieties including their resistances to pea, blue and spotted alfalfa aphid as well as a number of other pests. Varieties with high levels of resistance to pea aphids or spotted alfalfa aphids, or both, are available.

More information is available in the alfalfa (forage) chapter

Chemical Control

Product List for Pea, Blue Alfalfa and Spotted Alfalfa Aphid: During bloom or Prebloom

Insecticide	Lbs Active Ingredient per Acre (Fl oz. or oz. product)	Preharvest Interval, remarks
bifenthrin^R,1,2	3.9 – 6.4	12 hr REI. Extremely Hazardous to Bees! Do not apply to alfalfa in bloom. Do not apply more than 0.3 ai/A/season or make more than 3 applications/season. Treated seed must be labeled ‘Not for Human or Animal Consumption’.
dimethoate¹	See labels	10 days (harvest, grazing). 48 hr REI. Extremely Hazardous to Bees! Do not apply to alfalfa in bloom. 1 application/ cutting.
Fulfill^1,2	2.75	14 days. 12 hr REI. Do not apply more than 2.75oz/application. Do not apply more than 5.5 oz/A/season. When blooming only apply in early morning or late evening when bees are not foraging. Treated seed must be labeled ‘Not for Human or Animal Consumption’.
lambda cyhalothrin^R,1,2	2.56-3.84	7 days for hay. 24 hr REI. Extremely Hazardous to Bees! Do not apply to alfalfa in bloom. Do not apply more than 0.24 pt /A per cutting. Do not apply more than 0.96 pt/A per season. Advisable to move bees during application and allow 3 (low rate) or 5 (high rate) days before re-introduction of bees
MSR^R,1	1.5 – 2 pts	21 days. Chaff may be used for feed or forage but not cut green for these purposes. Do not apply more than 2 times /season.
Malathion³	See labels	0 days. 12 hr REI. Not effective below 65^oF. Apply to blooming alfalfa only in evening or early morning when bees are not working.
Mustang MAX^R,1	2.24– 4.0	3 days. 12 hr REI. Extremely Hazardous to Bees! Do not apply to alfalfa in bloom. No more than 4 oz per cutting, 12 oz per season
permethrin	See labels	1 day if less than 0.1 lb ai/A. 14 days at rates greater than 0.1 lb ai. 12 hr REI. Do not apply more than 0.2 lb ai per cutting. Extremely Hazardous to Bees! Do not apply to alfalfa in bloom.
Pirimor 50-DF	2-6 oz	1 day. 12 hr REI. Do apply more than 2 times/season or exceed 6 oz/A /season. All treated seed tagged “not for human or animal consumption”.
Proaxis^R,1	2.56-3.84	7 days for hay. 24 hr REI. Extremely Hazardous to Bees! Do not apply to alfalfa in bloom. Do not apply more than 0.24 pt /A per cutting. Do not apply more than 0.96 pt/A per season. Advisable to move bees during application and allow 3 (low rate) or 5 (high rate) days before re-introduction of bees
Supracide 2E^R	2-4 pts	7 days. 48 hr REI. Extremely Hazardous to Bees! Do not apply to alfalfa in bloom. No part of crop or residue may be used for human or animal feed. All treated seed tagged “not for human or animal consumption”.
^RRestricted Use Pesticide ¹ Labeled for chemigation ²24c labels may be available, check with State Dept of Agriculture. ³Several formulations. Additional chemicals for which Special Labels (Section 24c or 18’s) are available include: Capture 2EC, Supracide 2E.
The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality.

Categories: Alfalfa Seed, Insects, Pea aphid, Blue alfalfa aphid, Spotted alfalfa aphid, Cowpea aphid

Date: 11/17/2006