Presenter: Banu Kesavaraju PhD, Global Technical Manager, Valent BioSciences
From a high level, low-rate applications have easy-to-understand safety, economic, and environmental impact benefits. However, the benefits of low-rate applications can only be fully realized with the right product in the right environment, and when equipment has been adequately characterized to ensure application consistency.
According to Kesavaraju, the main benefit for districts performing low-rate aerial applications of granular larvicides for floodwater mosquito control is lower operational costs. Single brood efficiency, application flexibility, and high potency can also be achieved. In addition to potentially lower product costs, a lower use rate means reduced ferry time for aerial applicators: fewer trips back and forth to refill the aircraft. This delivers savings on fuel and means less wear and tear on the equipment. For districts that contract their applications, additional savings are realized through less time in the air. With the introduction of drones, low-rate applications have the additional benefit of maximizing area of application relative to the battery life of the vehicle.
While the benefits can be significant, Kesavaraju cautions that effective low-rate applications require thorough characterization of equipment. He recommends that technicians depend on a variable called coefficient of variation (COV) when conducting their characterization activities. He mentioned that Valent BioSciences recommends a COV of less than 0.3 (or 30%).
Kesavaraju explained that a traditional way of setting up a characterization is to place buckets or tote containers at regular intervals in a field setting, then fly the aircraft over the containers. As the product is applied, granules will drop into each of the containers. By collecting and weighing the amount of material captured in each bucket or tote, technicians can compare samples to evaluate variability of the application in a live setting. COV provides an excellent way to measure variability of application across the target site and is particularly important when characterizing for low-rate applications. According to Kesavaraju, technicians should never rely solely on “mean” or “average” granule weights across containers as they can be quite misleading.
He stressed that the effectiveness of low rate applications is dependent on evenness rather than on average distribution. Since these kinds of applications are already positioned at the low end of the recommended range, product delivered at less than that rate will likely be ineffective. In the same vein, short bursts of overapplication (streaking) may deliver an inordinate amount of material into just a few spots across the swath. This can dramatically change the result, as Kesavaraju demonstrated in one example.
For a characterization run where the targeted calibration was 4 lbs/acre and the minimum preferred rate was 2 lbs/acre, the spreader had an average application rate of 2.6 lbs/acre with containers spaced 5 feet apart. Although the average rate of application was comfortably within the 2-4 lbs/acre range, data showed that two of the containers received approximately 10 lbs/acre each because of improperly calibrated equipment. Furthermore, only one of the containers actually received an amount of product within that desired range. This variation in distribution was captured by the COV value which was greater than 1. (see Uneven Distribution)
Tote or Bucket
Kesavaraju also explained that while helping districts in achieving even distribution (low COV) of product, his team learned that the kind of collection container being used can dramatically impact results. He said many districts default to buckets for materials collection, then demonstrated how tote containers typically deliver more reliable data especially lower COV.
Kesavaraju showed an example of a characterization run using a line of tote boxes on one side and a line of buckets on the other. (see Tote vs Bucket). Evaluating the resulting data, he showed how the totes produce a lower COV value of 0.3 (optimal) compared to a COV of 0.7 (too high) from the buckets. Despite the fact that all of the conditions were the same, the same aircraft, product, wind speed, temperature, and distance interval, he explained that mouth of the tote opening is simply larger than the area of the bucket opening. As a result, tote measurements provide COV values that better represent the consistency of application across the entire surface being treated.
Kesavaraju then went on to explore specific challenges associated with fixed-wing and rotary aircraft. For fixed-wing applications, he noted that streaking can often be overcome by installing a custom roller in the system to help deliver a more consistent rate of product to the boom. This helps applicators improve both the consistency of their swath width and more even distribution across the swath.
According to Kesavaraju, COV challenges in rotary aircraft are often attributable to inconsistencies related to hopper openings and booms. He advised technicians to thoroughly test their hopper openings and booms for even distribution, which will result in more consistent swath widths and distribution.
More new products with lower label rates will come into the market in response to modern needs like drone based mosquito control. Districts can reduce their costs by taking advantage of these products by having a pragmatic approach to characterization and lower COV.
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