Pesticide spray coverage: searching for the Goldilocks zone
Getting an airblast sprayer ready to use in an apple orchard involves a fair amount of tweaking. Sprayer pressure, nozzle size, and travel speed may need to be adjusted to spray for insect pests and diseases in the upcoming growing season. The goal is to get adequate coverage. But how can you make pesticide spraying more efficient?
Dr. Heping Zhu of the USDA Agricultural Research Service Spray Lab in Ohio works on optimizing sprayer design. Zhu talks about a measurement called “spray coverage,” which is the percentage of the targeted crop surface on which spray droplets land.
A long-standing tradition for apple growers, who typically use airblast sprayers, has been to spray pesticides until runoff – in other words, spray until the pesticides are dripping off the foliage. But spraying to runoff can also mean applying more pesticide than needed.
Despite providing thorough coverage, airblast sprayers waste quite a bit of spray. More than half ends up in the air or on the ground rather than on the trees. This inaccuracy wastes money and time besides contributing to environmental pollution. The ideal spray application puts on just enough pesticide to get adequate pest and disease control - with minimal waste.
Airblast sprayers operate “blindly” in that they don’t adjust their output for differences in tree size, foliage density, or even the absence of trees. The inflexibility of using a constant flow rate leads to over-spraying.
The Intelligent Sprayer, developed by Dr. Zhu’s team, uses new technology that varies the spray rate and direction based on orchard canopy structure and the stage of the growing season. Specifically, it uses laser beams to sense apple tree structure. The sensors mean that the Intelligent Sprayer can “see” the tree, allowing more accuracy during pesticide applications. The Intelligent Sprayer has already shown so much promise that it’s available commercially.
So how can you use this new technology to hit the optimal level (i.e., not too much, not too little) of percent coverage? In other words, how can you get to the pesticide spray coverage ‘Goldilocks’ zone? In limited field trials by Dr. Zhu and his collaborators, pesticide spray coverage as low as 30% was sufficient to handle pests and diseases. That means the spray particles landed on less than a third of the leaf area. This seemingly low percentage of spray coverage was effective because the pesticide droplets deposited on leaves could spread in rainfall and dew. In addition, systemic pesticides could enter and move around inside leaves.
How low can you go when it comes to percent spray coverage on apples? Will 30% coverage do an adequate job on all pests and diseases, year in and year out? Field trials in many orchards, with many varieties, over multiple years will help us zero in on optimal coverage levels for efficiency and crop protection. Dr. Zhu and colleagues collected coverage and deposition data from apple tree canopies, comparing the performance of the Intelligent Sprayer and constant-rate airblast sprayers. They found that the Intelligent Sprayer had better uniformity in spray coverage than the standard airblast at multiple growth stages throughout the season. Intelligent Sprayer technology, with its superior spray deposition uniformity, may soon help growers to zero in on the Goldilocks zone.