Grasshoppers have been a sporadic pest of seedling cotton for 10 or more years. Growers have reported high numbers of grasshoppers in fields across the state this spring. As of May 1, 2020 the grasshopper population has largely matured in to adults, which are more difficult to manage than immatures. Environmental conditions influence overwintering populations. Rainfall is likely more important than temperatures. Dry winters are favorable for the grasshopper population because they overwinter as eggs in the soil. Grasshopper problems are sporadic and almost always associated with reduced-tillage fields.
Damage by Grasshoppers
The primary grasshopper that damages cotton is the differential species that also overwinters as eggs in the soil. Eggs hatch from late March throughout April, May, and June as soil temperatures rise and spring rains occur. The first nymph to leave the egg pod makes a tunnel from the pod to the soil surface through which the succeeding nymphs emerge. Nymphs feed and grow for 35 to 50 days before becoming adults when they can then fly. The nymphs or immatures can only jump. Their development proceeds most rapidly when the weather is warm but not too wet. Mature grasshoppers mate and continue feeding on plants. About 2 weeks later, females begin to deposit clusters of eggs in the soil. Soil particles are glued together around the eggs to form a protective pod. Each pod may have 25 to 150 eggs. Most grasshopper species only complete one generation per year.
In fields with historical grasshopper problems, growers may want to take a more preventative approach by adding a grasshopper insecticide to their burndown herbicide. Because not all grasshoppers emerge from the egg stage at the same time, a long residual insect growth regulator (IGR) insecticide could also be used. Dimilin has proven to be a good management tool for grasshoppers. It has extended residual effects that provide good control of immature grasshoppers but will not control adults.
There are no established thresholds for grasshoppers in cotton and will likely never be because their feeding habits are so unpredictable. Some fields and some years may have grasshopper damage while other fields and years have the same level of grasshoppers but no damage. Preventative insecticide applications for grasshoppers are a judgment call. When grasshoppers are observed and cotton is in the susceptible stage, treatments can be based on the risk level that an individual grower is willing to take.
Problems Greater in Lighter Soils
Grasshopper problems are greater in lighter soils or soils with higher sand content. Damage often occurs in the same fields or farms from year to year. Grasshopper damage as stated is unpredictable but can potentially threaten a stand. Grasshoppers may feed on foliage, but most economic damage occurs when grasshoppers feed on the main stem of emerging (in the crook or cracking stage) seedlings. In some cases, grasshoppers may completely sever the stem, but more often they will chew partially through the stem weakening the plant, which will fall over at the feeding site.
Most all-cotton insecticides will control immature grasshoppers when applied at a low labeled rate. Later into the spring, adult grasshoppers are very difficult to control with any insecticide, even at a high labeled rate. Acephate (Orthene) at 0.6 pounds to 0.75 pounds active per acre has proven to be the most effective grower treatment for adult grasshoppers.