Checking the sprayer

David Manktelow, one of New Zealand’s premier spray application gurus, spoke at the strawberry conference this past winter. A comment that stuck with me is about the inefficiency of spraying to achieve pest control. Plants have really complicated architecture and it’s no easy feat to get pest control products to reach their target, especially when they’re hiding under the leaves, where most pests live.

We see this difficulty time and time again when we check spray coverage with water sensitive papers. Folded in half and paper-clipped to plant leaves, they show where and how many spray droplets have landed on both sides of the plant leaf.

I’ve drawn yellow rectangles to show the placement of the water sensitive papers in the bush–we used three, one on each side and one in the middle.

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This spring we have used the water sensitive papers to check coverage for a typical sprayer, and for a drone sprayer.

Here’s a typical sprayer used in blackcurrants, in this case young bushes. Thank you to Quintin from Fruitfed and Rodger from McFarlane’s for sharing their test results!

Papers start out bright yellow, and when where water touches they instantly turn blue. Each paper has been unfolded to show what was the bottom and the top side during the spray pass. The sprayer does two rows in one pass–the top three are row 1 and the bottom three are row 2. Water rate was 500L/ha.

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Studying the picture above, the spray coverage is quite variable–and there’s a lot of yellow area still. Sometimes the top was better covered and other times the bottom was, but the coverage is not great. The spray was oil + wetter, aimed at early season spider mites, products that would need excellent coverage to be effective. The spray didn’t get rid of the mites, which is not surprising when we see how much yellow is left after the spray pass.

The addition of an organosilicon spreader (Du-Wett) will help the droplets that do land to spread, but will not magically fix the yellow areas where there just aren’t enough droplets.

Thank you to Quintin at Fruitfed for the photos and observations pre and post spray.

We did the same thing to assess spray coverage from a drone.

The drone applies chemicals in a mere 50L/ha, and I was skeptical that the coverage would be sufficient.

Again, the papers were folded and clipped over leaves in the canopy, and unfolded after the spray to examine the top and bottom sides. The olive-green haze is spray coverage by 320 micron droplets–like a fog.

Surprisingly, the under-leaf coverage looked to be quite good with the drone, and better indeed than the top of the leaf. Where there are big blue drops/splotches, it appears that the wind from the drone blew around water droplets that were on the leaf margins within the canopy–the sprayer made the very fine haze that looks olive green, and doesn’t show individual droplets.

Weather permitting, the drone operator will be on hand at the Nov 28th field day (hosted by Mike Read in Hinds) to demonstrate his machine. If you haven’t RSVP’d to Jacki yet, please do, as she’s organizing afternoon tea and it’s bound to be good.

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