High aphid populations challenge Rio Grande Valley pecan growers

One of the biggest challenges New Mexico pecan grower Les Fletcher has faced over the years is reducing the annual variation in production of the alternate bearing trees. Off-year production in 1995, following an unusually large on-year yield of the previous crop, was 65 percent lower than in 1994. Since then, he’s reduced that gap to about a 45 percent difference from one year to the next. “I’ve been trying to better balance production with heavy pruning during the on-year,” says the Las Cruces grower. “But, I may never be able to solve that problem.”

Keep in mind, though, that within the past decade he’s produced several 2-ton crops. That compares to the 1-ton per acre yields when his father planted their first orchards in 1961. Today, Fletcher Farms has 550 acres of bearing pecans, mostly Western Schley, in the Rio Grande Valley of Dona Ana County.

Meanwhile aphids continue to threaten his profits and those of other growers in the area. “We’ve had tremendous aphid pressures all through the valley this year,” Fletcher says.

These pests include yellow and black-margined aphids, both of which cause damage by feeding on nutrients in the trees. The other is the black pecan aphid. “We don’t see it as often and it’s easier to kill than the other two species,” he says. “But, due to its biology, this aphid is much more dangerous because it can defoliate a tree.”

Spraying trees with organic phosphate or synthetic pyrethroids or a combination of the two has worked relatively well in keep aphid numbers in check, he reports. Fletcher has also had success controlling aphids with imidacloprid, since he began using the soil-applied systemic insecticide five years ago. That is until this spring.

“Imidacloprid had been working beautifully,” he says. “But this year, May and June were extraordinarily cool. Apparently, there’s a relationship between heat and the tree’s absorption of the insecticide. In late June, it wasn’t showing up in the leaves. But, after nearly two weeks of 100-degree temperature in the first half of July, we’re finding the chemical in the leaves. Right now, we have no aphids.”

In 2007 and 2008, New Mexico State University researchers Bryan Fontes and Brad Lewis tested the effectiveness of imidacloprid in controlling the black-margined aphid in commercial pecan orchards hear Las Cruces. They found no significant differences between distribution of the insecticide between the top and bottom of the tree and no significant differences around the canopy. However, soil applications of imidacloprid did not result in acceptable control of the pest compared to the untreated control. The scientists attribute these findings to one of three possibilities:

• Concentrations of the insecticide in xylem sap did produce effective concentrations in the tree leaves;

• Imidacloprid levels in the sap may not be a good indicator of its ability to control the black-margined aphid;

• The aphid is showing isolated instances of tolerance to imidacloprid.

The researchers expect that imidacloprid has and will continue to play an important role in managing western pecan black-margined aphid population. Currently, they are trying to correlate concentrations of imidacloprid in xylem sap with leaf tissue concentrations in an attempt to establish baseline information for managing resistance.

TAGS: Tree Nuts
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