We know the importance of scouting new seeding alfalfa which is particularly sensitive to potato leafhopper feeding damage. While corn and soybean prices are high, hay and forage values have increased in a tight supply market. This is not the year to fall behind on potato leafhopper scouting. Both new seeding and established stands should be monitored closely by regular sweep net scouting.
Time to Work Potato Leafhopper Scouting into Hay and Forage Schedule
Eileen Cullen, Field Crops Extension Entomologist
Visiting with Tom Anderson, UW-Extension Shawno County Dairy and Livestock Agent late last week, Tom observed significant acreage of new alfalfa seeding in his county. We know the importance of scouting new seeding alfalfa which is particularly sensitive to potato leafhopper feeding damage. While corn and soybean prices are high, hay and forage values have increased in a tight supply market. Tom stressed the point, and I agree, that this is not the year to fall behind on potato leafhopper scouting. Both new seeding and established stands should be monitored closely by regular sweep net scouting.
Recently I gave an interview to Hay and Forage Grower magazine where we discussed the importance of timely scouting to keep ahead of potato leafhopper populations and protect hay and forage yield and quality by employing economic thresholds and treatment when necessary. At the same time, it’s important not to act prematurely and apply insecticide at numbers far below threshold, nor immediately after baling before regrowth has occurred.
Tom Anderson reported sweeping three Shawno County seeding year alfalfa fields to find a variety of insects (spittle bug spittle masses, plant bugs, pea aphid, etc.), as well as potato leafhopper nymphs. Tom said that none of the insects were at economic threshold, and he did not see beneficial insects in his sweep samples. This may be due to the fact that regrowth was a few inches in height and insects were still re-colonizing the field.
Economic thresholds have not been developed for multiple pest insect species combined at varying ratios in alfalfa (e.g. “1/3 potato leafhopper threshold + 2/3 plant bug threshold”). Population dynamics (reproductive rates, response to temperature, number of generations, key predators) and seasonal timing of different pest species vary from each other and from year to year, as does the length of time that species overlap with each other on a particular alfalfa growth stage (plant height) and days until cutting. The best approach is to be knowledgeable about key alfalfa insect pest species and their individual economic thresholds. This will provide the IPM decision framework to evaluate pest populations of concern when they occur, as well as days until cutting (can the crop be harvested rather than sprayed?) and growing conditions (is the crop under drought stress in addition to insect pest stress?).
Spittlebug damage to alfalfa is uncommon. In fact adults do not damage alfalfa. When nymphs hatch they form a spittle mass which is used to prevent desiccation and for protection from predators. Nymphs stay in this spittle mass until late first crop or early second crop. Although rare, an average of 1 spittle bug nymph/stem is necessary before control would be considered. Spittle bugs are only an occasional pest of alfalfa, and not normally a cause for concern.
Potato Leafhopper Reports
We have been picking up low levels of potato leafhopper adults and nymphs over the last week in our research plots (seeding year) in Columbia County (Arlington) comparing glandular-haired and smooth stem alfalfa (each variety as direct seeded, alfalfa/grass, and oats/alfalfa establishment) in an organic system. WI DATCP pest survey reported June 27th that nymphs are becoming more common in the state survey and a combination of nymphs and adults range from 0.4 to 1.1 per sweep in 14-18 inch second growth alfalfa. Populations are low to moderate relative to the economic threshold of 2.0 per sweep for 12 inch alfalfa, but are increasing. http://pestbulletin.wi.gov/
This is the time to begin scouting alfalfa fields on a weekly basis, if you have not already done so, and continue through the growing season.
Potato leafhoppers are present every summer in Wisconsin. This insect moves north in early summer, arriving with weather fronts from the southeastern United States. Migrant adult potato leafhoppers produce nymphs. Once reproduction has started, just 10 days are required for populations with overlapping generations to double in size.
Potato leafhopper nymphs
Potato leafhopper adult
Potato Leafhopper Scouting
To obtain an accurate population estimate, sample for leafhoppers when alfalfa is dry, and avoid field edges. Cold, wet or windy conditions may temporarily knock adults and nymphs from plants, resulting in an inaccurate sweep count. Use a standard 15-inch diameter insect sweep net. A total of 100 sweeps should be taken throughout the field, in an approximate “M-shape” to obtain 20 consecutive sweeps in each of 5 randomly selected areas.
Economic thresholds are based on average number of potato leafhoppers/sweep. Keep a running total of the number of leafhoppers caught and divide by 100. Nymphs are not generally recovered in the bottom of the sweep net with adults, but found along the collar of the net (along
the wire hoop rim of the net). Nymphs move quickly, typically sideways, when disturbed.
The UW-Madison’s Nutrient and Pest Management Program and IPM Program produced a one-page publication titled Cut, Bale, Scout!. It identifies potato leafhopper nymphs and adults, plant damage (“hopper burn”), describes the scouting method, and economic threshold by plant height. It can be downloaded from
Economic Thresholds
The larger the alfalfa the more potato leafhoppers it takes to cause economic damage. This occurs both because the older plant is more tolerant of damage and because the PLH will be on the plant less time before it is harvested.
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Alfalfa Height (inches)
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Potato leafhoppers per sweep
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Do not spray if you are within 7 days of your normal cutting schedule, in that case cut if possible*
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<3 inches (make sure there is regrowth)
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0.2
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3 to 6 inches
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0.5
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8 to 11 inches
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1.0
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12 inches or taller
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2.0
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