Penn State entomologist Dr. Larry Hull has spent much of his 35-plus-year career perfecting and advocating a technique called alternate row middle (ARM) spraying for insect control. The technique started in New York, came to Pennsylvania in the 1960s, and quickly caught on and became widely used by growers across the eastern United States.

There are many benefits when compared to every row middle (ERM) spraying, Hull said, including:

  • Reduced overall pesticide load. Growers usually apply a third to two-thirds the full per-acre rate of insecticide. They only spray each tree from one side.
  • Reduced application costs in time, labor, fuel, and machinery devoted to spraying
  • Lower risk of phytotoxicity from using the lower rates
  • Higher probability of survival of natural enemies and enhanced opportunity for biological control. Early work with IPM showed it was better for predators if highly toxic materials like the organophosphate and carbamate insecticides were applied at lower doses but more often.

Moreover, if growers shortened the interval between sprays to five to seven days and then sprayed from the other side of each tree row, they gained the additional benefit of having new material on the trees in the event of wash off or degradation of the pesticide residue. At the low rates recommended in 1970–1990s era of the organophosphate materials like Guthion (azinphos-methyl), the half-life was only about five to seven days. This was a highly effective way to apply these materials, which killed both by direct contact and as stomach poisons.


In the last few years, however, growers have become more nervous about the ARM technique. Lots of changes have taken place in tree size and spacing, in sprayer size and output, and in the type and toxicity of the insecticides being used. Growers are rapidly phasing out organophosphates and carbamates—because of new rules from the U. S. Environmental Protection Agency and because of growing insect resistance to them.

“We should revisit the requirements of ARM spraying, in light of products that likely have less contact activity, are not as broad-spectrum as the older products, and usually must be ingested to obtain their best activity,” Hull said.

Over the last two years, Hull has conducted a series of experiments with the new reduced risk insecticides Belt (flubendiamide), Altacor (rynaxypyr), Delegate (spinetoram), and Voliam Flexi (chlorantraniliprole plus thiamethoxam).

“From our research trial conducted at the Fruit Research and Education Center farm in Arendtsville, we found that the ARM method of applying Altacor and Delegate was very effective against both internal worms (codling moth and oriental fruit moth) as well as leafrollers,” he said. That was in 2008, and since then, testing with other new insecticides has shown them equally effective when using ARM.

“There are several reasons why some growers have seen failures with ARM spraying over the years,” Hull said. A major one is lack of adequate coverage inside the trees and especially on the opposite side of the tree. Sometimes growers have used too little water to get good coverage, reduced insecticide rates too low, or stretched the spray interval too long for the residual activity of the ­insecticide and the rate selected.

When ARM spraying first started, trees were larger, and sprayers were also larger. Early recommendations were that trees needed to be well pruned, airblast sprayers needed an output of 90,000 cubic feet per minute, the spray interval needed to be five to seven days, and pesticides were to be used at two-thirds the recommended rate.

Over the years, sprayers have gotten smaller. Fortunately, so have the trees. But growers need to make sure they put on enough water and use enough air to get at least some coverage on the side of the tree opposite the sprayer, Hull said.


To be successful using ARM, here are Hull’s observations and recommendations:

  • Tree structure and pruning will affect success.
  • Match sprayer size with tree size.
  • You must get some coverage on the off side of the tree.
  • Use the correct water volume for the various tree structures, canopy volumes, and time of season.
  • ARM works best early in the season, but it can work later in the season if the water volume is 100 gallons per acre or more and enough air is used to ­penetrate the canopy.
  •  It is better to be early in your timing than late with an ARM application, especially when controlling internal fruit feeders.
  • Be aware of the pest and predator situation in the orchard. The new materials are generally safer on beneficials.
  • Be flexible—adjust the spray interval as conditions dictate.

ARM works great when using mating disruption as an umbrella approach to control of codling moth and oriental fruit moth, Hull said. It doesn’t work as well when pest pressure is intense.