Hull Split Timing and Sprayer Practices for Best Pest Control Results

“If your first hull spray went out after July 5 last year, you weren’t happy with your reject sheets.”

An experienced PCA in the Arbuckle area said that to me last fall. The exact date in 2019 isn’t important as we look to hull split, 2020; every year is different. The key point is: waiting too long to start hull split spraying will cost you. So will a poor spray job. There are two take home messages from this article:

  1. If you wait too long to put on the first hull split spray, it doesn’t matter if it’s done by air, ground or robot, what’s in the tank, how slow you drive or what spray volume you use – you are too late to get the best control possible and it will cost you in both lost crop and quality incentives.
  2. Once you get the timing right, you have to get excellent coverage when you spray or you will not get the best control possible and the best net return.

Early hull split, when the hull begins to open at the suture, is a can’t-miss timing in almond pest management. The splitting hull does two things; releases nut volatiles so the navel orangeworm (NOW) female can find the nut (and lay eggs), and gives wound pathogens like Rhizopus stolonifera and Aspergillus niger an opening to infect the hull. Spraying at the right time (Stage 2c, see photo below) when hull split first begins (in the upper, southwest side of the tree) is critical to effective NOW and Rhizopus hull rot management throughout the orchard.* To get the timing right, get up in the tree in a pruning tower or bring the tree top nuts down to ground level by cutting tree top shoots with extension pruners so you can really see what’s going on. Splitting usually starts on the SW side of the tree tops.

Stage 2c of early hull split; target timing for first NOW spray. The nut can be squeezed from the ends and the entire suture will pop open. (photo credit: UC IPM program;

Stage 2c of early hull split; target timing for first NOW spray. The nut can be squeezed from the ends and the entire suture will pop open. (photo credit: UC IPM program;

Because getting the timing right for the first hull split spray is critical and it takes time to get across orchards spraying by ground, consider 1) going by air to get across the orchard(s) before Stage 2c ends in the tree tops and/or 2) starting spraying by ground a couple of days before nuts get to Stage 2c so you can finish quickly (in a week or less). A second application may be needed within 7-14 days based on trap counts and orchard damage history, with the interval depending on the material. Finally, since outside nuts on the trees on the outside trees in a block split first, consider an “edge spray” by ground, driving around the whole orchard, spraying in from the outside, when the sound nuts start to split on those edge trees. The edges can be ready several days before the rest of the orchard.

For the first hull split spray, air application is effective and fast. Research by Dr. Joel Siegel, USDA-ARS Researcher at Parlier, CA, has documented good NOW control with careful aerial applications (fixed wing or helicopter) with spray volumes as low as 15 gallons per acre (GPA). He reports spray coverage by air is 20% to 25% less than a good ground spray job (150 to 200 GPA, 2 MPH), but control in the tree tops — where the nuts split first — is very good. Dr. Siegel’s research showed 90% NOW control in tree tops by air, though careful ground application (150-200 GPA at 2 MPH) was still superior, with 100% control. (Don’t expect 100% NOW control with hull split sprays. Dr. Siegel’s results were from field sprayed targets exposed to NOW in the lab. The point is that aerial application can do a very good job, compared to ground sprays, in controlling NOW in tree tops.)

If you are going to spray by ground, it pays to do a careful job. To deliver good results, ground rigs must be carefully calibrated and set up for excellent coverage in each orchard you spray. It is a lot like painting a large house. For good weather protection of house siding, even paint coverage (no gaps), is needed. In almonds at hull split, the whole canopy, leaves and all, must be evenly sprayed to protect the nuts —to leave no gaps you have to “paint the whole house”. There are 4-8 acres of leaf surface area in an acre of mature, vigorous almond trees. Multiple studies in almonds from Colusa to Fresno have shown that 150-200 gallons per acre (GPA) spray volume from a ground application delivers better NOW control than 100 GPA.

In big, dense trees, drive slowly. Slow tractor speed — 2 MPH – gives the sprayer fan time to move the spray material throughout the canopy. Check your ground speed using flagging tied to a PVC pole or window-washing extendable pole. If the flagging doesn’t move as the sprayer drives by with the fan on, slow down until you find a speed where the flagging just flutters out 45-90o off vertical.

Watch when you spray. Spraying in dry, warm air (relative humidity below 40% and temperatures above 80oF) can reduce spray coverage due to droplet evaporation. Losses of 50% spray deposition have been measured in treetops using a ground rig at 11:30 AM vs 6:30 AM in June in the Sacramento Valley. Dusk to mid-morning is a good target window. Some aerial applicators are set up for night spraying.

Finally, Dr. Joel Siegel recommends alternating insecticide chemistries between NOW generations to slow pesticide resistance development in the pest. For example, if you use Altacor® or Minecto® Pro at spring (“mummy”) spray timing against the first NOW generation, change chemistry group and use Intrepid® or Intrepid Edge® in late June into July against the second generation.

Hull split is a critical timing for pest control in almonds. NOW and hull rot pressure have increased the last few years. Proper spray timing and delivery will help make reject sheet reading less painful and almond growing more profitable.

*Work by Drs. Mohammad Yaghmour, UCCE Farm Advisor in Kern County, and Themis Michalidies, UC Plant Pathologist based at UC Kearney in Parlier, shows that Aspergillus niger infested more nuts and spurs (through the hull sutures) about a week after Stage 2c of hull split, when the suture is about a third of an inch wide (Stage 3), than at or before Stage 2c.


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