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Scout fields for potato leafhoppers now

David Cappaert, Adult leafhopper
ALFALFA YIELD ROBBERS: The potato leafhopper is the worst pest of alfalfa. Pyrethroids are good for controlling severe populations, but experts suggest first scouting for the critters.
Scouting your fields can save you money on unnecessary insecticides.

Seeing potato leafhopper in your alfalfa? You’re likely not alone.

The pests have arrived for the year, so now is a good time to start scouting for them in your fields and, if possible, take a count.

John Tooker, field crops entomologist with Penn State, says that the pest has been reported in Lancaster County, Pa., and it is safe to say that surrounding counties, and Maryland and Delaware, are likely seeing it, too.

“Potato leafhopper is pretty consistent across years, the threat is about the same,” he says. “Some dry years are worse because alfalfa is dealing with lack of moisture and then they get hit by potato leafhopper and their damage is worse.”

Potato leafhopper is the worst pest of alfalfa and, if severe, can reduce yield and quality, especially lower protein content.

Tooker says that pyrethroids offer the best control when things get out of control, but scouting is also key.  

“If growers scout and do not find that their populations exceed the economic thresholds that Penn State has published, then there is no reason to spray,” he says. “In our ongoing work over 10 years, by scouting and timing out insecticide applications well, we typically only need one insecticide application to control [potato leafhopper] all season.”

Now that it’s warm, it takes about three eggs for potato leafhopper eggs to develop into adults.

They have straw-like mouthparts that extract plant juices, causing yellow triangles to form at the leaflet tips or even entire yellow patches in fields.  

Here are some tips from Penn State for scouting potato leafhoppers:

When to scout

For new spring seedings, start sampling when the plants are 3 inches high and resample every week until the field is sprayed or 10 days before harvest.

For new summer seedings, sample when plants are 2 inches high and resample every week until mid-September.

For second and third cuttings, sample when plant regrowth is 2 to 3 inches and resample every week until the field is sprayed, or 10 days prior to harvest.

How to scout

In square or rectangular fields, follow a "U" pattern. In narrow strips, an "I" pattern works best. Sample five sites in each when alfalfa is dry and avoid sampling in cold or windy weather.

Follow these steps when scouting:

1. Sweep properly. Make 20 pendulum sweeps (100 sweeps total) with the net at each site, sweeping 3 to 4 inches below the tops of the plants. Don't stop swinging until you complete 20 sweeps. Collect samples from each site following a zigzag pattern and take one or two steps between each sweep.

2. Keep swinging. When you complete 20 sweeps, continue swinging the net several times to force the insects into the small end of the bag. Grab the bag about 10 inches from the small end of the bag.

3. Count the catch. Only count the pale green (nymphs are yellowish green) leafhoppers and disregard any brown ones.

Bryan Jensen, University of Wisconsin, Bugwood.orgClose up of of severe leafhopper injury to alfalfa

LOOK FOR DAMAGE: Potato leafhoppers have straw-like mouthparts that extract plant juices. This causes discoloration of the plants, including yellow triangles at the leaflet tips or even entire yellow patches in fields.

Be alert because adult leafhoppers are very active and can easily escape without being noticed. Unfold the net slowly and let the insects escape a few at a time, counting them as they appear. Be careful to check the interior walls of the net for nymphs. They cannot fly and will be walking or clinging to the cloth.

Count the total number of leafhoppers. Repeat the same procedure at the next four sites, which completes your 100 sweeps of the field.

4. Determine the population. Calculate the average number of leafhoppers per sweep. For example, if you collected a total of 60 leafhoppers, find the average per sweep by dividing 60 by 100, which equals 0.6 leafhopper.

When the leafhopper population is high (40 or more in 20 sweeps) at the first site, spending time sampling the other four sites is of little value.

Make a sweep net

A sweep net is necessary to monitor potato leafhopper populations in a field. Insect nets can be purchased for around $15 but are not widely available. Materials to make a net can be found on most farms.

Use a solid piece of wood about 3/4 inch in diameter and 2.5 to 3 feet long as the handle. A broom handle cut to length is suitable.

The hoop can be a piece of heavy-gauge wire or thin steel rod. You need a piece 53 inches long. Securely fasten the ends of the hoop to the wooden handle by tightly wrapping them together with light wire, such as baling wire or by slipping a metal sleeve onto the handle and over the hoop ends.

To make the net, you need two pieces of cloth measuring 24 by 36 inches. Heavy muslin or tightly woven nylon cloth will do.

Prepare for next year

Resistant varieties of alfalfa are available. These varieties are covered with fine hairs (glandular trichomes) that decrease leafhopper feeding.  

Another option is to mix other forages in with alfalfa. Alfalfa/orchardgrass stands (or other combinations) are better at tolerating leafhopper damage than pure stands of alfalfa.

Spiders and other natural enemies kill potato leafhoppers, so using integrated pest management and spraying insecticides only when economic populations develop will help maintain these allies in pest control.

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