By Daniel Marzu

Crop scouting — I’m not talking about where you jump in the pickup truck, drive past the field, take a quick look and say, “Yep, looks good.” Or if you do get out of the truck, you walk in a few rows, do a quick look around and then leave. I know, it’s summer, and there are better things to do. Right? Well, chances are you’ll end up finding that a pest caused a yield or forage-quality reduction that could have been prevented. 

When done regularly, crop scouting is a valuable tool for identifying potential pests, and it allows you time to decide whether a rescue treatment is needed. While there are many diseases and weeds that may appear during the summer months, this article focuses on scouting pests in alfalfa, corn and soybeans. Examples given describe the process of scouting for insects — this is not intended to be an exhaustive list of potential pests.

Do’s and don’ts
To scout alfalfa for insect pests, you will need a 15-inch-diameter insect sweep net. When scouting, walk in a zigzag pattern, and take 20 consecutive sweeps at five random areas throughout the field. Avoid scouting when plants are wet or during windy conditions, as this may cause insects to be knocked off plants prior to you being there. After each set of sweeps, count the number of insects you are scouting for. The best way is to pinch the net in your hand toward the top and slowly open it. You may have to count fast as the insects jump or fly out.

Once you have done this in five random areas, divide the total number of insects by 100 (the total number of sweeps). This number will be used to determine the economic threshold, or the point at which the cost of the crop damage exceeds the cost of treatment. This number is sometimes related to a certain plant stage or height.

For example, if 0.2 potato leafhopper per sweep is found in 3-inch alfalfa, it warrants a treatment. Once the alfalfa grows to 6 inches, this number increases to 0.5 leafhopper per sweep. With alfalfa 8 to 12 inches tall, there needs to be one leafhopper per sweep, and two leafhoppers per sweep with alfalfa more than 12 inches tall.

When scouting corn and soybeans, you may need to examine the whole plant. Corn rootworm beetles may be found on the tassel, ear, leaf surface and behind the leaf axils. Check five plants in 10 areas of a field for rootworm beetles before 70% of the plants begin to silk. Check the leaf axils by pulling the leaf away from the stalk. Place your hand over the silks, and while peeling back the husk from the ear tip, slowly open your hand and count. Add the number of beetles in the 10 locations and divide by 50. If there is silk clipping and the economic threshold of more than five to six beetles per plant is reached, a treatment may be necessary.

In soybeans, be sure to check the undersides of the leaves throughout the canopy. Pests such as the two-spotted spider mite hide under the leaves. Spider mites occur during hot, dry conditions, causing the leaves to start turning bronze. To scout, use a piece of paper and gently tap the plant. You will see black spots moving on the piece of paper if spider mites are present. If the soybeans are between bloom and pod fill, there is more than 15% discoloration of the leaf area, and the hot, dry conditions continue, a treatment may be necessary.

As previously mentioned, this article provides some tips on how to scout; it does not give specifics on every pest in your fields. Scouting and management strategies for other pests can be found in Extension publication A3646, Pest Management in Wisconsin Field Crops, available for download or purchase at the University of Wisconsin Extension Learning Store. The University of Wisconsin Crop Manager and the Department of Agriculture, Trade and Consumer Protection’s Wisconsin Pest Bulletin are two resources that provide information on pests statewide. Your county Extension agricultural agent/educator will also have information on local pest issues. 

Marzu is the Extension agriculture educator for Lincoln and Langlade counties.