Avoid weedy orchard floors
Weeds harbor fruit-feeding pests.
Flowering weeds and legumes (left) attract bees and are hosts for damaging nematodes. Clean tillage (right) suppresses insect pests, but repeated tillage damages soil structure.
Photos by Bradley Majek
A decade and more ago, it was thought that plant diversity in fruit orchards was a good thing, that clover and broadleaf weeds provide shelter and alternative food sources for beneficial insects and mites that feed on or parasitize insect and mite pests. But now, the thinking is, plant diversity is more beneficial to diseases and pests than it is to the beneficials that prey on them.
Dr. Peter Shearer, an entomologist at Oregon State University’s Mid-Columbia Agricultural Research and Extension Center in Hood River, Oregon, participated in much of the research after he began work at Rutgers University in New Jersey in 1996. He still uses that decade’s worth of data and those conclusions in making recommendations to growers.
“I was once a proponent of plant diversity,” he said. “But it seems pests prefer these alternate hosts more than the beneficials do.
“Our research at Rutgers and on growers’ farms demonstrated the importance of removing broadleaf weeds to minimize damage from several key pests,” he said. “Managed-sod drive rows and weed-free tree rows reduce catfacing insect abundance and damage in peaches.”
“Clean” orchards—whether clean tilled or with grass sod alleys—reduced damage by 60 percent, he said, and similar research in Oregon and Canada showed reduced damage in pears and apples as well.
In peaches, at least eight arthropod pests are associated with orchard ground cover, he said. These include tarnished plant, stinkbugs, green peach aphids, tufted apple budmoth, two-spotted spider mites, false chinch bugs, leafhoppers, and thrips.
Tarnished plant bugs cause the most damage to New Jersey peaches, where they are season-long pests from prebloom to harvest. They, and stinkbugs, cause catfacing from feeding on the fruit.
“We know we can get reduced pest pressure by controlling weeds,” he said.
In his studies, he found that keeping orchards totally free of vegetation—by use of herbicides or tillage—effectively reduced the level of tarnished plant bug to just above zero, even when no insecticides were used to control it.
With no insecticides, orchards kept vegetation-free using herbicides had 3 percent damage from tarnished plant bugs. Grassed alleys containing fescues or Kentucky bluegrass did shelter more tarnished plant bugs, but less than half the number that were found in orchards with white clover or weeds, where damage levels in the study were about 10 percent. Weed-free sod ground cover also delayed the onset of tarnished plant bugs in the orchard by a month, he said, reducing the number of sprays growers needed to apply. Damage by thrips and Japanese beetle was also lower in clean-tilled orchards or those with sod alleys.
Grasses are not good hosts for pests, but they need to be mowed to suppress flowering and the formation of seed heads, he said.
Shearer also reminds growers that peaches have extrafloral nectar glands at the base of leaves, providing beneficial insects with an in-orchard food source even when there are no flowers.