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 |
Pesticides and Natural Enemy Releases
Not all registered pesticides are listed. The following are ranked by their IPM value, with the most effective and least harmful to natural enemies, honey bees, and the environment listed at the top of the table. When choosing a pesticide, consider information related to water and air quality, resistance management, and the pesticide's properties and application timing. Use PestManage to compare summarized management options for different pests in the same crop. Always carefully read the label of the product being used and take all necessary precautions when handling pesticides.
| Rank | Active ingredient | Example trade name | Group | Group Order | MoA 1 | Amount per acre | REI (hours) ‡ | PHI (days) ‡ | Comments | Selectivity 2 | Bees 3 | Predatory mites 4 | Predators 5 | Parasitoids 5 | Residue duration 6 | Leaching(fish) 7 | Adsorbed runoff(fish) 8 | Solution runoff(fish) 9 | Leaching(human) 10 | Solution runoff(human) 11 | Last updated 12 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
|
|
Baythroid XL | Application Timing Varies (See UC IPM Pest Management Guidelines and Label) | 8 |
3A |
2.8 fl oz | 12 | 7 | Can be disruptive to natural enemies. Hi... | broad | high | high | high | moderate | intermediate | intermediate | extra high | very low | very low | 04/2024 | ||
|
|
Dimethoate 2.67 EC | Application Timing Varies (See UC IPM Pest Management Guidelines and Label) | 8 |
1B |
0.75–1.5 pt | 48 | See comments | Check label to see if product allows onl... | broad | high | high | high | long | very low | low | low | intermediate | high | 04/2024 | ||
|
|
Beleaf 50 SG | Application Timing Varies (See UC IPM Pest Management Guidelines and Label) | 8 |
29 |
2.8 oz | 12 | 14 | Use allowed under a 24c registration (SL... | narrow | low | low | low | short | very low | very low | low | low | intermediate | 04/2024 | ||
|
|
Sivanto 200 SL | Application Timing Varies (See UC IPM Pest Management Guidelines and Label) | 8 |
4D |
7–10.5 fl oz | 12 | 7 | Do not apply more than 28.0 fl oz of Siv... | narrow | — | — | — | — | very low | low | low | very low | low | 04/2024 | ||
|
|
Sivanto prime | Application Timing Varies (See UC IPM Pest Management Guidelines and Label) | 8 |
4D |
7–14 fl oz | 12 | 7 | Do not apply more than 28.0 fl oz of Siv... | narrow | — | — | — | — | very low | low | low | very low | low | 04/2024 | ||
|
|
Warrior II | Application Timing Varies (See UC IPM Pest Management Guidelines and Label) | 8 |
3A |
1.28–1.92 fl oz | 24 | See comments | Preharvest interval (PHI) is 1 day for f... | broad | high | high | high | moderate | intermediate | low | extra high | low | high | 04/2024 | ||
|
|
Malathion 8-E | Application Timing Varies (See UC IPM Pest Management Guidelines and Label) | 8 |
1B |
1–1.25 pt | 12 | 0 | Use only when other products cannot be u... | broad | high | high | high | moderate | high | intermediate | extra high | very low | very low | 04/2024 | ||
|
|
Mustang | Application Timing Varies (See UC IPM Pest Management Guidelines and Label) | 8 |
3A |
2.4–4.3 fl oz | 12 | See comments | Preharvest interval (PHI) is 3 days for... | broad | high | moderate | moderate | moderate | intermediate | low | extra high | low | high | 04/2024 |
Legend
- ‡ a b Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without personal protective equipment. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases, the REI exceeds the PHI. The longer of the two intervals is the minimum time that must elapse before harvest.
- 1 ↩ Group numbers for insecticides and miticides are assigned by the Insecticide Resistance Action Committee (IRAC). Insecticides with unknown modes of action are assigned mode-of-action group numbers (MoAs) that begin with UN. Rotate pesticides 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; insecticides with a 1B group number should be alternated with insecticides that have a group number other than 1B.
- 2 ↩ Range of insect and mite groups affected by a pesticide. Broad means the pesticide affects most groups of insects and mites; narrow means the pesticide affects only a few specific groups.
- 3 ↩ Risk of harm to honey bees. For more information, see Bee Precaution Pesticide Ratings.
- 4 ↩ Risk of harm to predatory mites. Toxicities are generally to western predatory mite, Galendromus occidentalis. Where differences have been measured in toxicity of the pesticide-resistant strain versus the native strain, these are listed as pesticide-resistant strain or native strain.
- 5 a b Risk of harm to parasitoids and general predators. Toxicities are averages of reported effects and should be used only as a general guide. Actual toxicity of a specific insecticide depends on factors including the application rate, environmental conditions, and the life stage and species of a parasitoid or predator.
- 6 ↩ Length of time residue affects natural enemies. Short means hours to days; moderate means days to 2 weeks; and long means many weeks or months.
- 7 ↩ Risk of harm to fish from leaching, based on USDA Natural Resources Conservation Service Windows Pesticide Screening Tool (WIN-PST).
- 8 ↩ Risk of harm to fish from adsorbed runoff, based on USDA Natural Resources Conservation Service Windows Pesticide Screening Tool (WIN-PST).
- 9 ↩ Risk of harm to fish from solution runoff, based on USDA Natural Resources Conservation Service Windows Pesticide Screening Tool (WIN-PST).
- 10 ↩ Risk of harm to humans from leaching, based on USDA Natural Resources Conservation Service Windows Pesticide Screening Tool (WIN-PST).
- 11 ↩ Risk of harm to humans from solution runoff, based on USDA Natural Resources Conservation Service Windows Pesticide Screening Tool (WIN-PST).
- 12 ↩ Date information was last updated in the UC IPM Pest Management Guidelines.
UC IPM Pest Management Guidelines: Alfalfa
UC ANR Publication 3430
L.D. Godfrey, Entomology, UC Davis
E.T. Natwick (emeritus), UC Cooperative Extension Imperial County
P.B. Goodell (emeritus), UC IPM and Kearney Agricultural Research and Extension Center, Parlier
R.F. Long (emeritus), UC Cooperative Extension Yolo County
V.M. Barlow, UC Cooperative Extension Riverside County
I.M. Grettenberger, Entomology, UC Davis
M.M. Leinfelder-Miles, UC Cooperative Extension San Joaquin, Contra Costa, Sacramento, Solano, and Yolo counties
M.D. Rethwisch, UCCE Riverside Co. (Blythe)
Acknowledgement for Contributions to Invertebrates
D.R. Haviland, UC IPM and UC Cooperative Extension Kern County
C.G. Summers (emeritus), Entomology, UC Davis and Kearney Agricultural Research and Extension Center, Parlier
