Good news for bad news

Photos by Jim Dwyer, Unless Otherwise Noted.
This potato leafhopper nymph, and the adult form, can cause damage to over 100 crops in the north by piercing and sucking at the leaves. Inset, This Wisconsin potato crop looks peaceful, but clouds on the horizon could bring the potato leafhopper, a plague from the South that can devastate crops across the northern tier of states.

When you see the damage that the potato leafhopper (PLH) does and start reading about how widespread it is, the word plague comes to mind. University entomology Web sites across the nation, and Canada, have written up information sheets on it, and the list of crops it damages is astounding. This critter is bad news virtually everywhere in the northern and eastern U.S.

There’s good news, too. Entomologists are finding creative ways to combat it in many crops. There are common insecticides available, and researchers are looking at different methods of managing it in the crops grown in their regions.

The potato leafhopper is a small, 1/8-inch, lime-green insect in its adult stage, and the nymphs are smaller, wingless and a paler green. Eggs are white and elongated, and are usually inserted into a petiole or stem. Both adults and nymphs insert piercing/sucking mouthparts into leaves, damaging them and clogging vascular tissue. Yellow, curling leaves are the first sign of infestation, and eventually the entire plant can be stunted and fruit diminished.

On top of that, PLH can descend in enormous numbers. They originate in the pine forests of the South, and in the warm northward air currents of spring they catch the winds and scatter across huge areas well into July. Infestations can descend, often in the rains, in one fell swoop. The females can lay two or three eggs a day for over a month. Two or more generations can develop even in the upper Midwest. The bullet-shaped insects have a distinctive sideways, crab-like walk when disturbed.

It is the range of susceptible crops that is impressive. PLH will feed on-as the name indicates—potatoes, but also on anything from strawberries to apples, from alfalfa to horticultural plants. This makes them a widespread blight across the northern tier of states, and it also gives them a large number of host plants from which they can assault new vegetable and fruit crops. Over 100 host plants have been identified, and many of them are crucial crops in their areas.

Starting in the Northeast, this year threatened to be a bad year for PLH in the Maine potato crop. Jim Dwyer, University of Maine crop specialist and IPM coordinator for potatoes, says that the 52,000-acre crop in Aroostook County was seeing steady pressure by mid-August. His IPM scouts, looking at 125 farms, were seeing leafhopper outbreaks at multiple locations.

“It’s an intermittent problem,” Dwyer says of the pest. “Some years we see it, some years we don’t.” This year they did.

Estimating that the PLH has popped up here in three or four out of the last 20 years, Dwyer says it doesn’t affect any local crops other than potatoes, but it can explode and cause significant damage there. This year, storms that came up the Mississippi River Valley spread the pest through New York, Vermont and New Hampshire before depositing them in Maine.

Dwyer advises avid scouting and control with synthetic pyrethroids as soon as an economic threshold is reached. For PLH, that is a count of 10 nymphs per 100 leaves, because more damage is caused by the nymphs than the adults.

Foliar sprays of pyrethroids are the treatment of choice for two reasons, he notes. First, systemic neonicitonoids will have already been applied to control early infestations of Colorado potato beetle (the worst potato pest) and aphids, and they will be losing efficacy as PLH populations reach treatable levels. Second, pyrethroids are effective and fiscally favorable.

One thing that makes the PLH so dangerous in potatoes is that they generally increase just as the Russetts are reaching the bulking stage. That’s when they are most vulnerable, because with destruction of vascular tissue, the plants increase respiration and waste energy that should go into the tubers. Dwyer says that two applications are generally enough, but he bases sprays on scouting right through summer. The insect can be a pest until first frost.

“It’s a really fascinating insect,” Dwyer says. He has seen them literally rain down from the skies in the midst of a thundershower.

Photo by Russell Groves.
One of the most important aspects of a good IPM program for PLH is scouting, as in this potato field in Wisconsin.

At Michigan State University, small fruit entomologist Rufus Isaacs points out that two of the state’s major crops, wine grapes and berries, have been severely infested with PLH in the last two years. He doesn’t see the migrations worsening, and they aren’t present every year, but in the years that the pests arrive in Michigan, they can be devastating. He cites the pinot gris and chardonnay vines as being highly susceptible.

“We tend to see more impact there,” Isaacs says of PLH in wine grapes, with a lot of yellowing leaves affecting yield and quality. “There’s also a cascade of responses in the plant that can lead to stunted growth.”

Management in grapes is difficult, because often an infestation will arrive just as fresh shoots are forming. That’s prime food, and vines can be stunted at a delicate time. Isaacs recommends early and regular scouting, and says that there are remedies.

Broad-spectrum insecticides are effective, Isaacs points out, and he has seen growers spray as many as three times to control successive generations. Older neonicotinoid materials such as Provado and Assail have been efficacious, as has the newer Venom, as foliar sprays. A new option he has tested is the use of some of these same chemicals as systemic applications to the vine roots. These can be applied through chemigation that will deliver insecticide to the roots and ultimately to the vine.

Some of the benefits of soil applications are that the chemical can be effective for much longer; it isn’t washed off leaves; and there is less exposure to field workers. That is particularly true if it is applied in a fertigation system through drip irrigation, which would also conserve water. Admire and Venom are registered for this usage and have been effective in his recent tests. It’s also a good control method for Japanese beetles if applied at the right time.

Another interesting development in Michigan is with raspberries. Isaacs and his colleague Eric Hanson have been working with new raspberry plants under high tunnels of white plastic, and apart from the other benefits, Isaacs has found that this physical barrier is a wonderful PLH deterrent. “It’s caused a dramatic reduction in potato leafhopper pressure,” Isaacs says.

In Wisconsin, Russell Groves emphasizes one thing when he advises growers on how to deal with PLH. That’s good scouting. “One of the important parts of our treatment is early and frequent scouting,” says Groves, who is the extension vegetable entomologist at the University of Wisconsin.

He says the insect is “one we expect every year,” but with good scouting and accurate timing of treatments, its effects can be minimized. PLH can drop in here any time from late April, along with the astor leafhopper, which is also a problem because it vectors disease. This year the PLH began exceeding economic thresholds by early June, and it has been so injurious to forage crops in Wisconsin, Iowa and Minnesota that a few growers have had to spray.

The vegetable crops most affected in Wisconsin have been snap beans and potatoes, Groves says. He says that PLH has many crops that it feeds on, but not that many where it will stay and reproduce. Unfortunately, potatoes and beans, even dry beans, are among them. When alfalfa is baled, the insects move to veggies.

For potatoes, Groves recommends an at-plant systemic neonicotinoids, such as Platinum, or a seed treatment, such as Gaucho, that will fend off several pests.

“We really benefit from the fact that they control hoppers,” he says, because those materials give good control for the first half of the season and may be applied for more endemic pests. If PLH comes in later, it will also be affected.

Snap beans are also registered in the state for seed treatments such as Gaucho and Cruiser, though these wouldn’t normally be used just for PLH. A treatment for seed corn maggot, though, would also protect against hoppers. When systemic treatments wear out, that’s when the scouting is most important.

Groves points out that it is important to recognize that first economic infestation so treatment with foliar insecticides can begin. Neonicotinoids can be utilized, but usually the less costly synthetic pyrethroids such as Pounce or Ambush would be used for economic reasons.

This is where the scouting becomes so important. Groves says the established economic threshold here is one adult PLH per net sweep. That triggers a treatment, but the scout should also look for nymphs. If they are present, then another treatment should be done in seven days. That’s because the nymphs live lower in the canopy and are more difficult to kill, so the seven-day wait allows them to become adults.

By using this methodology, a grower might get away with a single, well-timed application if there’s not much reproductive activity going on. If there is, two sprays might do the job.

Groves says that it would be nice if more cultural controls could be used, but in his area there aren’t many options for changing cropping patterns, for example. There usually isn’t a choice of moving away from an infested alfalfa crop, but he is intrigued by some new chemicals that are undergoing testing against hoppers. Chemical resistance isn’t a problem with the transitory PLH, but new nicotinoid products could help control them and, at the same time, avoid resistance in the devastating Colorado potato beetle.

The consensus among northern IPM specialists is that the potato leafhopper may not be  the worst pest year in and year out, but in the years when it arrives from the skies, it can be devastating. By using established scouting and treatment regimes, growers of affected fruit and vegetable crops can avoid this plague from the South.

Don Dale is a freelance writer and a frequent contributor. He resides in Altadena, Calif.