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As summer heat sets in, it’s time to keep a close eye on alfalfa for insect pressure — particularly from potato leafhoppers and aphids. Both pests thrive in hot, dry weather and can do serious damage if left unchecked, especially in regrowth after first or second cutting.
Potato leafhoppers are tiny, wedgeshaped insects that feed by piercing plant tissue and sucking out sap. The result is a yellow, V-shaped pattern on leaf tips — commonly called hopperburn. This feeding not only stunts growth and reduces leaf production but also lowers overall forage quality. The discoloration is caused by a toxin the insect injects as it feeds. Because of their size and mobility, leafhoppers often go unnoticed until symptoms become visible.
That’s why regular scouting with a sweep net is important. Take 20 sweeps per field sample and count what you find. Economic thresholds vary depending on stand age and growth stage, but in young or stressed stands, even one leafhopper per sweep may warrant control. Older or taller stands may tolerate slightly higher numbers. A helpful 2019 article from UNL’s CropWatch, titled “Potato Leafhoppers in Alfalfa,” includes threshold charts to guide decisions.
Leafhoppers are generally easy to control with insecticides. However, if the stand is already yellow and stunted, it’s best to mow first and remove the affected tissue. This clears out the toxic, damaged growth and allows for better regrowth after treatment. Aphids are another pest to watch. Pea aphids are most common in Nebraska, but spotted alfalfa aphids, cowpea aphids, and blue alfalfa aphids can also cause issues. Seasonality and damage potential will vary with species. These soft-bodied insects feed in clusters on stems, terminals, and the undersides of leaves. As they feed, they remove plant sap and inject saliva, which reduces vigor and interferes with nutrient flow. Heavy infestations can cause leaf curling, wilting, and stunted growth.
Aphids also excrete a sticky substance called honeydew, which can coat leaves and create a host for sooty mold fungi — reducing palatability and making hay hard to dry or bale. In some cases, aphids may also transmit viral diseases, though this is less common in most forage systems.
Scout using sweep nets or by checking plant terminals and leaf undersides. Thresholds for treatment typically range from 40 to 75 aphids per stem depending on plant height and growing conditions. A 2022 CropWatch article titled “Watch for Aphids and Potato Leafhoppers in Nebraska Alfalfa” includes a table with species-specific treatment thresholds. Natural predators like lady beetles and lacewings often help suppress aphids, so consider their presence before spraying. However, hot, dry conditions can reduce predator activity, allowing aphid populations to surge.
Fortunately, aphids respond well to insecticides if caught early. Rotate modes of action when treating repeatedly to avoid resistance, and always check pre-harvest intervals before spraying.
— Ben Beckman is a beef systems Extension Educator in northeast Nebraska. He is based in the Cedar Coimty Extension office in Hartington. You can reach him by phone: (402) 254-6821 or email: [email protected] mailto:[email protected]
