With ideal conditions, 50 or 60 potato leafhoppers can produce from 500 to 1,000 offspring within a five-week period. These insects bite into the alfalfa leaf vein and stop nutrients from flowing to the rest of the leaf causing a yellow triangle, or hopperburn, to develop. Subsequently, nutrient values are diminished and alfalfa volume is lowered.

“Leafhoppers are the same color as the plant, making them hard to identify in time to spray,” said Dale Hewett, Syngenta field agronomist. “Generally we recommend that growers select the highest-yielding variety for their conditions, scout for PLH and spray if necessary. If the grower chooses not to scout and spray, then best way to lower PLH risk is to plant an alfalfa variety with PLH resistant traits.”

When scouting for PLH, growers should use a sweep net to obtain PLH estimates to help determine if treatment is a viable economic option or if harvesting the alfalfa is best at that time. Note that if alfalfa is within seven days of a normal cutting, spraying is not recommended. It is then better to harvest as soon as possible.

To obtain PLH estimates, move a 15-inch sweep net vigorously through alfalfa in 180 degree arcs at several locations in each field. At least 25 sweeps should be taken from about four locations in each field. The number of leafhoppers per sweep is obtained by counting the leafhoppers in the net and dividing by the number of sweeps (or arcs) taken in the field.

Once estimates are gathered, the grower can determine the economic threshold, or the pest density that, if the alfalfa is left unsprayed, will cause yield loss greater than the cost of an insecticide treatment. Thus, the gain in yield from spraying an insecticide is greater than the cost of the insecticide treatment.

According to Iowa State University, there are three things to consider when determining the economic threshold of an alfalfa field. First, determine if the field is leafhopper-tolerant alfalfa or traditional alfalfa.

It takes about 10 times more leafhoppers in an established stand of tolerant alfalfa to cause the same amount of loss in traditional alfalfa. Therefore, these alfalfa types have different economic thresholds.

Second, estimate the height of the alfalfa when scouting for potato leafhoppers. Taller alfalfa is damaged less by the PLH than shorter alfalfa.

Third, estimate the cost per acre of an insecticide treatment. Using these three variables and Tables 1 and 2, growers can calculate the correct economic threshold.

Alfalfa growers are urged to begin sampling fields in late May and June and to continue on at least a weekly schedule through the remainder of the season. It is unwise to wait until the damage appears before making a decision to manage this insect.

“Scouting for leafhoppers is a necessary evil whether you plant alfalfa that is tolerant of them or otherwise,” said Hewett.

“PLH tolerant alfalfa does provide protection for those growers that cannot spray and there is also the convenience factor to be considered. When planting PLH tolerant alfalfa, concerns are reduced if spraying is delayed and there is the longer protection window as well.”

Growers who suspect potato leafhoppers in their field should contact their local agronomist or seed dealer to learn more about how to scout for potato leafhoppers and best practices for protecting alfalfa against these pests.  FG

—Source: Syngenta press release