Enlist sod to economically fight broadleaf weeds and insect pests in soft-fruit orchards
By Renee Stern
Keeping orchards weed-free also can simplify pest-control efforts.
A sod ground cover in drive rows chokes out broadleaf weeds that host spider mites and a group collectively known as catfacing insects, which includes tarnished plant bug and lygus, says Peter Shearer, Extension specialist in tree-fruit entomology at Rutgers University's Bridgeton, N.J., Agricultural Research and Extension Center. When those host plants are mowed or begin to die, the pests relocate to fruit trees to feed.
Without the broadleaf hosts, most of those pests stay out of the orchard in the first place. For East Coast peach and other soft-fruit growers, that can mean up to a 75 percent reduction in insecticide use, Shearer says.
"It's almost cheaper to control the weeds than the insects," he says. Selective herbicide sprays help maintain that pest shield.
"The results are pretty quick," says Richard Hilton, an entomologist at Oregon State University's Southern Oregon Research & Extension Center in Central Point. Lygus in particular are unlikely to migrate into an orchard without broadleaf weeds.
Other orchard crops could benefit from this approach, depending on the pests involved, says Mike Hardman, research scientist at the Atlantic Food and Horticulture Research Centre in Kentville, Nova Scotia, Canada. Hardman has focused on controlling tarnished plant bugs and two-spotted spider mites in apple orchards.
Sod trumps beneficial insect habitat
An alternate approach to provide host plants for beneficial insects doesn't work as well against many East Coast orchard pests, Shearer says. Those plants seem to attract target pests along with their predators, creating no net benefit in control.
Diverse habitat to attract beneficial insects "is a double-edged sword," Hilton says. Lygus have a wide host range that spans hundreds of plants, including mustards and legumes.
Ron Thomas, field foreman at Sunny Slope Farms in Bridgeton grows sod between tree rows in the company's 800 acres of peaches and 200 acres of nectarines.
He planted 10-foot-wide strips of Kentucky 31 tall fescue grass initially to cut costs associated with clean cultivation rather than to control either weeds or pests. Sod needs less maintenance, reducing fuel use, and holds down mud and dust that can make orchard work difficult, Thomas says.
"We can get back in the field sooner with sod centers," he says. "After a 2-inch rain, we can run the sprayers right after. And it's more worker-friendly."
Once established, the tough grass grows thickly and allows little foothold for weeds, needing only small amounts of 2,4-D, Stinger (clopyralid) or Prowl (pendimethalin) to knock down a few annual weeds, he says.
After a few years, he noticed that primary pest populations showed dramatic declines, observations that Shearer's research confirmed. Thomas hasn't quantified the effect on his pesticide use, given the continued need to control secondary pests, but he says it has decreased.
Herbicide sprays cut fruit damage
Until recently researchers have had difficulty showing that controlling pests on the orchard floor translates into results in the tree, says David Granatstein, sustainable agriculture specialist at Washington State University's Tree Fruit Research and Extension Center in Wenatchee.
"Now we have the technology to track the movement of pests," he says. "Until now we haven't been able to say who's been where and eating what."
Hardman says weed control in apple tree rows and laneways reduces the number of pests moving up into the tree. Herbicide sprays in both areas nearly cut in half the amount of fruit damage from tarnished plant bugs.
Best results came from mowing and selective spraying in drive rows with Lontrel (clopyralid) and 2,4-D amine, combined with spraying tree rows with Roundup (glyphosate) and 2,4-D amine in May and Ignite (glufosinate-ammonium) in late June or July.
Twospotted spider mites also are attracted to broadleaf weeds and have become a greater concern among local growers now that predators successfully control European red mite, Hardman says. That predator mite, which lives only in trees, isn't as well-matched against the twospotted spider mite.
subhead: Determining grassy strip width
Eliminating weeds in tree rows and laneways held twospotted spider mite populations in the canopy to a more manageable level for predators. "That was what tipped the scale," he says.
Hardman is experimenting with herbicide strips to determine the optimum width for mite control. A narrower strip is more effective at preventing migration into the tree, but too narrow reduces tree growth and fruit yield.
So far his research suggests keeping the strip to about the drip line. But, he says, growers may be willing to trade a slight reduction in yield from a narrower strip for an accompanying drop in pest and disease problems.
He's also looking at which grasses offer the most weed control and pest protection without affecting tree health or fruit production. Shade-tolerant grasses can be grown closer to trees, while low-growing varieties need less mowing and don't shelter rodents or compete for water and nutrients.
Orchard ecosystems are complicated systems, and finding the right cover crop to provide maximum benefit against local pest complexes isn't easy, Granatstein says.
"It takes time to work out the kinks," he says.
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