Aphids Observed in Alfalfa Fields

June 5, 2020
ICM News

Alfalfa weevils aren’t the only insect pest being found in alfalfa fields this spring. Reports of aphids, particularly cowpea aphids and pea aphids, have been made around the state. This article will discuss identifying the common aphid species found in Iowa as well as scouting and management recommendations for aphids in alfalfa.

Aphid species

In general, aphids are soft bodied and pear-shaped insects. They have a piercing-sucking stylet (mouthpart) and feed on sap within the plant phloem. The two cornicles, which are like tailpipes, on the tip of the abdomen is an important diagnostic feature to distinguish aphid species. Table 1 lists the four common aphid species found in Iowa. Additional information on these different species can be found online.

Table 1. Common alfalfa aphid species found in Iowa.

Aphid species
 (common name)

Size and color description

Population peak

Blue alfalfa aphid

3/16”; blue with black cornicles

March – June

Cowpea aphid

1/8”; shiny black with black cornicles

April

Pea aphid

¼”; pale green or pink with dark cornicles

April – November

Spotted alfalfa aphid

1/8”; pale yellow with dark spots on abdomen and short cornicles

May – October

Aphid scouting, thresholds, and management

The good news is harvesting alfalfa is an excellent method to control aphids as very few aphids will survive. Many fields have already been harvested or will soon be harvested. Additionally, natural enemies, like ladybeetles or damsel bugs, can help keep aphid populations in check. As the alfalfa starts to regrow below are guidelines on how to scout and monitor fields for aphid pressure. 

To scout for aphids, count the number of aphids on at least 30 alfalfa plant stems. Average the number of aphids per stem. Since populations can be spotty across fields, it is recommended to sample multiple areas across the field when doing the stem counts. The economic thresholds for aphids are summarized in Table 2.

Table 2. Economic thresholds for pea aphids, blue aphids, cowpea aphids, and spotted alfalfa aphids. (Source: IPM-58).

Plant Height

Pea aphids/stem

Blue aphids and Cowpea aphids/stem

Spotted alfalfa aphids/stem

< 10”

30-50

10-50

10-20

10-20”

50-75

30-50

20-40

>20”

100

>50

>40

Spraying before the economic threshold is met can lead to an aphid resurgence. Only spray once the threshold is reached and if the field is not ready to be harvested. If an insecticide is warranted, always read and follow the label instructions. Pay attention to pre-harvest interval when spraying, and make sure to use sufficient volume and pressure when spraying to ensure contact with aphids on the lower parts of the plants.


A sweep net can be used to check for the presence of aphids in alfalfa fields. (Photo taken July 3, 2003 by Brian Lang).

Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on June 5, 2020. The information contained within may not be the most current and accurate depending on when it is accessed.

Authors: 

Rebecca Vittetoe Field Agronomist in EC Iowa

Rebecca Vittetoe is an extension field agronomist in east central Iowa. Educational programs are available for farmers, agribusiness, pesticide applicators, and certified crop advisors.

Areas of expertise include agronomy, field crop production and management of corn, soybeans, and...

Brian Lang Field Agronomist in NE Iowa

Brian Lang conducts Iowa State University Extension and Outreach programs in crop production and protection in northeast Iowa. Frequent clients include farmers, ag chemical and fertilizer dealers, seed dealers, crop consultants, and farm managers.  Provide timely in-season crop management inform...

Erin Hodgson Professor

Dr. Erin Hodgson started working in the Department of Entomology at Iowa State University in 2009. She is an associate professor with extension and research responsibilities in corn and soybeans. She has a general background in integrated pest management (IPM) for field crops. Dr. Hodgson's curre...