Evidence of a root mealybug infestation.
Photo courtesy of Juan Manuel Alvarez, University of Idaho, Bugwood.org

by Courtney Llewellyn

In the Battle of Mobile Bay during the American Civil War, Admiral David Farragut uttered the phrase “Damn the torpedoes, full speed ahead!” That same all-or-nothing mentality inspired Stanton Gill, Extension specialist in IPM and entomology at the University of Maryland, to title his virtual presentation at Cultivate’20 “Man the Torpedoes and Hit the Herbaceous Perennial Pests Broadside!”

The United Nations declared 2020 the International Year of Plant Health, and that focus is as important this year as it’s always been. Plant health – and threats of plant diseases and pests – are ever-changing, and Gill noted that.

“The first surprise we’re seeing is scale insects on herbaceous plants, as they’re usually seen on woody plants,” he said. Scale insects tend to thrive in warm, dry environments. They are so named because they’re small, oval and flat, with protective tan to brown shell-like coverings (scales), although each species’ appearance can vary dramatically.

“Their whole thing is to blend in on plant material. They don’t look like typical insects,” Stanton said. He explained they overwinter down in the center of plants; once outside, they produce two generations per year. Scales can generally be controlled with systemic insecticides (such as dinotefuran or Flupyradifurone), as long as they are timed for crawler use. Female scales can produce up to 130 eggs, with crawler emergence about eight days after egg deposition.

“Check down in the base when you bring in new plants,” Stanton said. “If you’re worried, make up a tank solution of SuffOil-X, dunk your foliage – and train your workers to look for scales.”

Particular pests that are becoming more common are the fern scale and the common stalk borer (which is adapting to thrive in ornamental grasses), so growers need to make sure they keep weeds down to spot scales more easily.

Additionally, Stanton said growers are seeing more root mealybug in production nurseries. “They tend to go unnoticed, but they are a big, big deal,” he cautioned. These pests will cause wilting, stunting and yellowing by causing waxy filaments to build up on roots. They affect asters, field hemp and sedums. Stanton said he’s been testing insecticides and nematodes against it in the past five years and found that Mainspring, Acelepyrn, Altus, Beauveria bassiana fungus drenches and weekly applications of Venerate and Grandevo worked well.

“We also know heat kills insects – usually around 120º Fahrenheit,” he said. In trials, he and his team have been practicing dipping infected plants into hot water baths with promising results.

There are always new pressures on plant health – and so the work continues, full speed ahead. Having a good nursery herbicide rotation is one form of ammunition growers can use, and that’s where Chris Marble, assistant professor in the Environmental Horticulture Department of the University of Florida, picked up.

Put bluntly, the benefits of a good rotation are “better weed control (and therefore a better bang for your buck), better crop tolerance and a reduction of resistance development,” according to Marble.

A rotation is a planned but flexible schedule of herbicide treatments based on weed pressure, the season and weed species, crop tolerance, annual limits (rates and the number of legal applications per year) and your specific nursery – no one rotation works for everyone.

In selecting which herbicides to rotate, Marble said growers need to identify the worst weed each season, then secondary weeds. You need to find active ingredients that are effective for the worst weed but also have control over the second, third and fourth most problematic weeds. “Remember, overuse of modes of action can lead to failures,” he said.

A good herbicide rotation:

  • has demonstrated safety on crops (either through testing or by following the label)
  • can control or be effective on target weeds
  • requires no or limited sequential applications
  • can rotate through different modes of action but remain effective
  • and can stay within annual limitations.

Do your research and know what your options are – and see if there’s overlap for different plants. You also need to know which weeds to target and when. Proper identification and recordkeeping are key.

Knowing your nursery pests, whether they’re insects or weeds, is one of the best ways to control what can be controlled, and as G.I. Joe said, “knowing is half the battle.”