**1. Introduction**

Indoor residual spraying (IRS) is a cornerstone of malaria vector control. It is typically conducted manually by spray operators using compression sprayers, a method that has seen little change since its introduction in the 1940s. Equipment specifications for IRS were first published in 1964 and described a hand-operated cylindrical tank with a hose, lance, and a flat-fan nozzle [1]. Specifications remained relatively unchanged until the addition of a control flow valve (CFV). Regulating the flow of insecticide through the nozzle with a CFV meant that, despite decreasing pressure in the tank while spraying, the emitted spray volume stayed constant. The introduction of CFVs resulted in a recommended application rate of 30 mL/m2, rather than the original 40 mL/m2, reducing the volume (and thus

**Citation:** Snetselaar, J.; Lees, R.S.; Foster, G.M.; Walker, K.J.; Manunda, B.J.; Malone, D.J.; Mosha, F.W.; Rowland, M.W.; Kirby, M.J. Enhancing the Quality of Spray Application in IRS: Evaluation of the Micron Track Sprayer. *Insects* **2022**, *13*, 523. https://doi.org/10.3390/ insects13060523

Academic Editor: Corey Brelsfoard

Received: 22 April 2022 Accepted: 2 June 2022 Published: 6 June 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

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weight) of water needed to spray a surface area. A second change was the shift from stainless steel nozzles to a more durable ceramic nozzle, reducing the risk of inconsistent spray due to wear on the nozzle [2].

Although these innovations in spray equipment have resulted in more accurate IRS applications, the spray technique itself has remained similar to that in early spray campaigns [3]. In control campaigns, manual spraying is prone to variation in the dose applied due to variation in competence and skills, and lapses in concentration between operators. Even in experimental hut trials, human error by individual spray operators can result in large differences in insecticide application rates between walls within a house and between positions on a single wall. Both overdosing and underdosing of IRS products have been reported during the conduct of experimental hut [4–7], highlighting the challenges in spraying IRS products accurately and consistently. Overdosing of IRS products can lead to higher than anticipated costs and potential safety concerns, while underdosing can result in a shorter residual half-life and development of mosquito resistance due to exposure to sublethal doses [8,9]. Consistency in the speed of application and the distance of the sprayer nozzle from the surface are critical to applying the correct dose of insecticide to walls, ceilings, and other sprayable surfaces.

High-quality training and supervision of spray operators, plus good maintenance and calibration of spray equipment, can contribute to the accurate application of IRS products in experimental hut studies (and in IRS spray campaigns). Accurate quality control of spray application, however, remains challenging and relies on complicated, timely, and expensive technology such as HPLC analysis of sprayed filter papers, or methods that are relatively insensitive to variations in dose such as cone assays. Variation in insecticide delivery has impacted the effectiveness of spray campaigns or outcome of regulatory trials. High accuracy in the measuring of spraying is particularly important in experimental hut trials evaluating different dosages of IRS products [10–15], as these trials need to inform development decisions on the most appropriate application rate for novel products prior to regulatory evaluations and subsequent market launch. To improve the consistency of the application of IRS products, the automated mechanical track sprayer was developed by Micron Sprayers Ltd., with support and funding via the Innovative Vector Control Consortium (IVCC). The track sprayer was designed specifically for experimental use, aiming to improve the quality of insecticide application in semi-field experimental hut studies.

This comparative study was conducted to evaluate whether the application of IRS products by mechanical track sprayer gives less variation in spray application rate than conventional manual spraying. The comparison was made during two experimental phases: the first phase, conducted in the laboratory at the Micron Centre in Herefordshire, England, used fluorescein diluted in water; and the second phase, under semi-field conditions at the Kilimanjaro Christian Medical University College (KCMUCo, Moshi, northern Tanzania) used IRS products. The proof-of-concept laboratory phase allowed for a high-throughput and low-cost comparison of both spray methods, whereas the semi-field phase provided the opportunity to test with insecticidal products under more realistic experimental conditions. For both phases, the manual spraying was carried out by an experienced spray operator and the mechanical spraying was conducted using the automated track sprayer.

#### **2. Materials and Methods**
