New Mexico State University entomologists continue their investigation into why soil-applied treatments of imidacloprid sometimes fails to satisfactorily control blackmargined pecan aphid (BMA) in flood- or sprinkler-irrigated orchards in Arizona and New Mexico.
The insecticide can be sprayed on the leaves of pecan trees or applied to the soil either by sprinkler or by chiseling it into the ground.
Unlike foliar insecticides, which may require two or three applications a season to control BMA, just one application of imidacloprid to the soil in the spring can provide season-long control, says Brad Lewis, NMSU research entomologist.
The insecticide has been widely over the past six years or so to treat pecan orchards for these aphids.
“No doubt about it, imidacloprid works,” Lewis says. “But, there are isolated areas where we’re beginning to see poor performance with the product.”
There doesn’t seem to be a single environmental reason, including soil type, to explain this, he says.
It’s not that the trees aren’t taking up the insecticide. NMSU studies confirm that they are. Maybe, then, effectiveness of imidacloprid is being thwarted by the aphids themselves because they’ve become resistant to it — that’s one possibility the entomologists are currently studying, Lewis says.
Another is the idea that imidacloprid may be binding to organic matter in the top two inches or so of the soil, or to clay particles, reducing the amount of the material that reaches the roots of the trees. In fact, organic matter has been building up in the orchard floors as more and more growers mulching their prunings, he notes.
Despite instances where soil treatments have produced poor results, Lewis points out, foliar applications of imidacloprid continue to work. “They remain effective due to the higher concentrations of imidacloprid that contact BMA, compared to soil applications,” he says.