Hull-split sprays have began across the southern valley. These sprays are made to protect the crop from the infestation by Navel Orangeworm (NOW). During this time, it is common to receive questions regarding spray coverage.
A common question received is about the comparison of electrostatic versus air-blast sprayers. A spray trial was conducted at the Nickels Soil Lab near Arbuckle, CA to compare the coverage and NOW control of an axial fan/air-blast and electrostatic spray rig. Sprays were applied between 9:30 – 11:45 PM, when temperatures were below 75oF and relative humidity above 65%, to minimize evaporation and maximize spray coverage. The replicated trial consisted of three treatments:
- Axial fan, air-blast sprayer (John Bean Spraying Co., 500 gallon tank, 36” fan, PTO driven; 143 gpa, 1.75 MPH, 170 psi system pressure)
- Air-shear, air-blast sprayer with electrostatic droplet charging (Progressive Ag, Model 2250, PTO driven; 50 gpa; 3.3 MPH and 30 psi system pressure).
- Air-shear, air-blast sprayer without electrostatic droplet charging (Progressive Ag, Model 2250, PTO driven; 50 gpa; 3.3 MPH and 30 psi system pressure).
Eighth leaf ‘Fritz’ trees were sprayed with Delegate WG insecticide at hull-split timing. Applications included a molybdenum tracer to help determine spray deposition.
Results: The spray coverage and NOW control results from the study (1 Day After Treatment) can be found in the table and summarized into four points.
|Sprayer Treatment||Hull Mo Deposition – Upper canopy (15-20 ft)||% NOW survival
|Hull Mo deposition – Lower Canopy (5-7’)||% NOW survival
|Standard Axial-Fan/Airblast Sprayer||0.06 a||1.64% a||0.09 a||0.19% a|
|Electrostatic Sprayer with electrostatic charge||0.05 a||3.20% b||0.18||1.01% b|
|Electrostatic Sprayer w/o electrostatic||0.07 a||—||0.15 b||–|
Different letters indicate different statistical groupings at p<0.05. -Results pooled with those from air-shear sprayer without electrostatic for this analysis.
- At lower spray volumes and faster ground speed, the electrostatic sprayer – with or without electrostatic charging – provided similar hull deposition compared to conventional, axial fan airblast sprayer in the same region of the canopy.
- Conventional airblast sprayers (Higher volume, slow ground speed) had better pest control when NOW eggs were placed in the suture — a hard-to-reach area of the nut – compared to a typical electrostatic sprayer (low volume, high ground speed spraying with or without electrostatic charging).
- The same amount of active ingredient was used in these studies, indicating that the improved pest control was obtained with a lower pesticide concentration per droplet when using an airblast sprayer (higher spray volume) Joel Siegel has also shown better NOW control at hull split with 150 gpa vs 100 gpa at the same pesticide rate per acre. When using labeled rates, diluting the pesticide by increasing the spray volume per acre DOES NOT harm control when good spray coverage is delivered.
- The use of the tracer element was helpful in determining canopy deposition (where the spray went in the canopy), but not NOW damage. This has been observed in other studies and suggests that fine particles may be important in providing effective NOW control most likely due to increase deposition in the hull crack.
This may be interpreted as an electrostatic sprayer is not an effective spray technology. That is not true. Other studies have found that when driven at slower speeds and higher gallons per acre, electrostatic sprayers can provide as good as control as an airblast sprayer, supporting the saying that “Speed Doesn’t Kill.”
This article is based on the annual report of Project 13-Water3-Giles/Markle titled “Improving Spray Deposition and Reducing Drift in Almond Orchards,” a project funded by the Almond Board of California.