A Full Sweep Net

April 24, 2024 10:32 AM
Blog Post

I’ve been scouting several alfalfa fields in East-Central Iowa this spring, and it’s always interesting to see what you get in your sweep net. While I’ve been able to find quite a few alfalfa weevil adults and larvae in fields this spring, that’s not all I’ve found!

Alfalfa weevil: The adults are brown in color with a narrow stripe down their back and have a blunt snout. The larvae are yellow to green in color with a black head and are legless. The larvae feed on the terminal leaves and under a heavy infestation, a field may have a frosted or silver appearance.


Adult alfalfa weevil are brown in color with a narrow stripe down their back and have a blunt snout. Photo courtesy of Rebecca Vittetoe. April 15, 2024. 


Alfalfa weevil larvae of varying sizes. Photo courtesy of Rebecca Vittetoe. April 22, 2024. 

 

Pea aphid: These are common to find in alfalfa, and range in color from light green or yellow to a light or pale pink. The adults are about ¼ inch in length. They prefer to feed on stems and new leaf tissue. If pea aphid pressure is high enough (50-100/stem), they can cause alfalfa leaves to become more yellow and stunt alfalfa growth.


Pea aphids, circled in red, can range in color from pink to green. Photo courtesy of Rebecca Vittetoe. April 23, 2024. 

Lygus bug or tarnished plant bugs These true bugs feed on alfalfa and other legumes. The adults are about ¼ inch and are green to brown in color. A key trait to look for is a distinct white triangular mark on their back. The nymphs are more light green in color and don’t have fully developed wings. The adults and nymphs feed on buds, growing points, and flowers of alfalfa. It’s rare to see these reach economic threshold (40 or more nymphs and adults/10 sweeps).  


A key trait to look for when identifying a lygus bug, circled in red above, is a distinct white triangular mark on their back. Photo courtesy of Rebecca Vittetoe. April 23, 2024. 

Parasitic wasps: These are “good guys” or beneficial insects and help to keep alfalfa weevils in check. The adults will lay eggs in the alfalfa weevil larvae, which ultimately kills it before turning into an adult.


Not all insects are pests, above is a parasitic wasp, which is a beneficial insect for alfalfa weevil. Photo courtesy of Rebecca Vittetoe. April 24, 2024. 

As you get out and scout your alfalfa fields, what insects are you seeing this growing season? Let us know! Feel free to tag us on X (@ISUCropNews, @rkvittetoe, @ashleyn_dean @erinwhodgson). 

Author: 

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...