Alfalfa Insects
Spotted Alfalfa Aphid, Therioaphis maculata.
Closeup of spotted alfalfa aphid.
Winged adults and nymphs.
Description: Spotted alfalfa aphids are pale yellow-green in color with conspicuous rows of black spots on their backs. Each spot is tipped with a short spine. The winged form has smokey-colored areas along the wing veins. Mature aphids are only about 1/16-inch-long and one may need magnification to clearly see the spots.
Distribution: The spotted alfalfa aphid, Therioaphis maculata (Buckton), was introduced into central New Mexico in 1954. It has since spread throughout the alfalfa-growing areas of the United States.
Life History: Like many other aphids most individuals are wingless females that can produce living young without mating. Each female can give birth to 100 or more offspring. The spotted alfalfa aphid goes through four instars in as little as five days. The entire life cycle may be completed in less than seven days. In the fall, males can be produced which mate with oviparous (sexual form) females. The eggs that are laid from this sexual cycle are the overwintering stage.
Damage: Spotted alfalfa aphids prefer to feed on alfalfa, although they will infest some clovers. These aphids inject a toxic salivary secretion into the plants, which causes yellowing at the feeding site and along the veins of the leaf. The location of the spotted alfalfa aphid on the plant is greatly influenced by the relative humidity. In general, they are found on the underside of lower leaves but will move up the plant as these leaves are killed. Uncontrolled populations have reduced the yield and protein content by half on established stands and have destroyed new stands. In addition, the honeydew secreted and the sooty mold that subsequently develops on the leaves reduces hay quality and makes harvesting difficult. Susceptible varieties may be severely damaged by little more than one aphid per seedling, but host-plant resistance can reduce damage. Luckily many modern varieties are resistant to the spotted alfalfa aphid.
Scouting Methods: Randomly select complete stems throughout the field and count the number of spotted alfalfa aphids per stem. Be aware that these aphids may jump or fall from the stem as it is removed so be careful when collecting the sample. Continue this procedure until you have sampled at least 20 to 30 stems. Calculate the average number of aphids per stem and measure each stem and calculate the average stem length. Also record any parasitized (mummified) or diseased (brown and flattened) aphids.
Treatment Threshold: On seedling stands, insecticides should be applied when two to three aphids per seedling are present. Plants 2 to 3 inches tall can tolerate 4 to 5 aphids per plant. Use at least 12 gallons of water per acre and apply when daytime temperatures are expected to be above 65°F. In established alfalfa, higher populations are required to justify treatment. On 10-inch tall alfalfa, treatment generally is not needed until 50 aphids per stem are present. On 20-inch tall alfalfa, twice as many aphids per stem would be needed to justify treatment.
Treatment Options: Using resistant varieties is often the only way to establish a stand in a drought if spotted alfalfa aphids are present. Our data on efficacy of insecticides for spotted alfalfa aphid control in Kansas is limited.
Please refer to the most recent Alfalfa Insect Management Guide for specific control options.
Page last updated 10/29/2013 by J.P. Michaud.