Leafhoppers are sucking insects and also excrete a toxin while feeding on the sap from alfalfa plants. The toxin is what causes the yellowing and stunts plant growth. Very young alfalfa plants and plants in early stages of regrowth are the most susceptible to leafhopper damage. Infestation is most likely to occur prior to the second and third cuttings of alfalfa in most areas, and in the first cut if it has been delayed. Warm, dry spring weather is much more conducive to leafhopper infestation than is cool, wet spring weather.
Leafhoppers typically migrate from the edge of the field inward.
Simply put - very significant and extremely costly. Literally millions of dollars are stolen from alfalfa growers each year by leafhoppers in terms of lost yield and quality. The loss is not limited to just annual yield reduction. For example, when leafhoppers are controlled, other factors which affect income and profitability are positively impacted. Recovery is improved - plants are more vigourous and regrow faster after each cutting.