How to Manage Pests
UC Pest Management Guidelines
Spotted Alfalfa Aphid
Scientific Name: Therioaphis maculata
(Reviewed 1/17, updated 1/17)
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 southern desert, high numbers may continue into fall and winter.
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 feeding value.
Use resistant varieties and encourage natural enemies to help 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 applications may be necessary.
Planting alfalfa varieties resistant to spotted alfalfa aphid has been the most effective means of controlling these 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 a list of alfalfa varieties provided by the National Alfalfa & Forage Alliance. Additionally, a yearly alfalfa variety performance 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 spraying for aphids when the parasite is present.
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 aphids, also take sweep net samples for lady beetles and record all counts on a monitoring form .
UC IPM Pest Management Guidelines: Alfalfa
Insects and Mites
L. D. Godfrey, Entomology, UC Davis
Acknowledgment for contributions to Insects and Mites:D. R. Haviland, UC Cooperative Extension, Kern County
M. Rethwisch, UC Cooperative Extension, Riverside County (Blythe)
C. G. Summers, Entomology, Kearney Agricultural Center