Description of the Pest
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.
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 .
|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 apply a pesticide 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|
|1 or more larvae||50 aphids|
|Common name||Amount per acre||REI‡||PHI‡|
|(Example trade name)||(hours)||(days)|
|Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least harmful to natural enemies, honey bees, and the environment are at the top of the table. When choosing a pesticide, consider information relating to air and water quality, resistance management, and the pesticide's properties and application timing. Always read the label of the product being used.|
|The following pesticides have not been tested under California conditions but have been found to be effective in other areas.|
|(Sivanto 200SL)||7–10.5 fl oz||12||7|
|(Sivanto Prime)||7–14 fl oz||12||7|
|MODE-OF-ACTION GROUP NUMBER1: 4D|
|COMMENTS: Do not apply more than 28.0 fl oz of Sivanto Prime or 200SL (0.365 lb flupyradifurone)/acre per calendar year on alfalfa, regardless of product or formulation.|
|(Dimethoate 2.67EC)||Label rates||48||See comments|
|MODE-OF-ACTION GROUP NUMBER1: 1B|
|COMMENTS: Check label to see if product allows only one application per year or per cutting. Preharvest interval (PHI) is 10 days for harvest or pasturing; for alfalfa seed: do not feed or graze livestock on treated crops, hay threshings, or stubble within 20 days of application. Do not apply when bees are present.|
|(Malathion 8-E)||1–1.25 pt||12||0|
|MODE-OF-ACTION GROUP NUMBER1: 1B|
|COMMENTS: Use only when other products cannot be used. Do not apply when bees are present.|
|‡||Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.|
|*||Permit required from county agricultural commissioner for purchase or use.|
|1||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 to help prevent the development of resistance. 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 group numbers are assigned by IRAC (Insecticide Resistance Action Committee).|