Study: Promising Fenugreek Gum Polymer: Adjuvants for Drone-Based Aerial Chemical Applications to Mitigate Off-Target Drift
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https://www.mdpi.com/2504-446X/8/11/667
Excerpts:
Abstract
“Off-target drift from aerial pesticide applications in croplands can be a major source of pesticide exposure to pollinators. Pesticide adjuvants (PAs) are added to pesticides but can be as toxic as pesticides’ active ingredients. Ongoing experiments have identified sodium alginate (SA) as a drift-reducing PA less toxic to honeybees. Hence, SA and fenugreek polymer (FP) have been tested as drift-reducing PAs for aerial applications using the Remotely Piloted Aerial Application System (RPAAS). Two spray experiments were carried out in the field: (i) water only (W) and (ii) water and adjuvant (WA). Droplet spectrum and on-target coverage were collected using a VisiSize P15 image analyzer and kromekote cards, respectively. The drift reduction potentials (DRPs) of the adjuvants were analyzed based on droplet size (diameters of 10%, 50%, and 90% volume) and the proportion of driftable volume with droplets < 200 µm. Compared to the W only, the W-A treatment produced larger droplets, suggesting the presence of DRP. There were 14.5%, 8.3% to 14.4%, and 2.3% to 7.7% driftable fines in the W, WA (SA), and WA (FP) treatments, respectively. The FP treatment improved the on-target coverage (3.0% to 3.1%) compared to water (2.7%). Our results indicate that SA and FP have the potential to mitigate off-target drift and protect pollinator health.
Keywords: sodium alginate; fenugreek; UAS; polymer-based pesticide adjuvant; rhodamine dye; kromekote; spray coverage; AccuStain; drift mitigation
1. Introduction
Profitable commercial crop production often involves the use of pesticides to maintain pest densities below economic thresholds. Most pesticide applications in the lower Mississippi Delta (LMD) are carried out by piloted agricultural aircraft. The drone-based aerial application of pesticides to large agricultural areas, such as LMD, has not been widely practiced due to the newness of the technology. However, to address existing concerns, such as off-target drift [1,2,3], aerial applicator fatalities with conventional manned agricultural aircraft [4,5], and high costs of repair and maintenance, drone-based aerial pesticide applications can be a valuable alternative.
Like piloted aerial applications, the distance between the nozzle and the target remains high in the RPAAS, which favors conditions for off-target drift. In addition, the high concentration of pesticide spray because of low volume applications (because of small tank size and limited battery capacity in most cases) affects the spreading and volatilization of the pesticide. Although most of the pesticide labels have recommendations on doses required to improve spray characteristics, they are tailored for ground-based and piloted large-scale aerial applications only. Therefore, research is needed to develop pesticide adjuvants specific to the RPAAS platform to mitigate drift and improve pesticide efficacy.
Adjuvants have been used in conventional aerial pesticide applications for many years, and in drone-based aerial applications, in recent years [6,7,8]. Several adjuvants added to pesticides to improve the efficacy, adhesion, and incorporation into plant tissues and reduce off-target drift may be equally or more toxic than the active ingredient [9,10,11]. Commercially available adjuvants such as polyacrylamides, polyvinyl chlorides, sodium salts of fatty acids, alkyl phenyl ethoxylates, and other petroleum-based distillates are also not environmentally friendly [12]. The most-used adjuvants in drone-based aerial applications include vegetable oils, organosilicon, and high-molecular polymers, which mostly act to decrease the surface tension and therefore increase the wetting, spreading, and retention of spray droplets on leaves [6,8]. These adjuvants, especially the petroleum-derived ones, are environmentally harmful and could be toxic to pollinators. This calls for the development of pollinator and environmentally friendly adjuvants with better efficacies.
The dependence of crop production on agrochemicals creates a need to improve our understanding of how these practices affect our ecosystem, notably the detrimental effects on pollinating insect populations. The impacts of pesticides on pollinator insects such as honeybees were well documented [13,14,15,16,17,18,19,20]. To protect insect pollinators from exposure to pest control products, there is an urgent need to develop environmentally safe adjuvants. Those adjuvants should have the following characteristics: (i) compatible with commonly used herbicides, fungicides, and insecticides; (ii) not adversely affecting pesticide efficacy against target crop pests; and (iii) not having adverse effects on beneficial insects, such as bees and other insect pollinators. Such adjuvants will have an enormous positive impact on the pesticide market and an increased acceptance among crop producers.
In this study, we tested two plant-derived polymers for their potential as drift-reducing pesticide adjuvants to reduce pesticide exposure and related toxicity to insect pollinators. Most of the plant-based polymers are found widely in nature, including in algae (as alginate), plants (as cellulose, pectin, cyclodextrin, and starch), microorganisms (as dextran), and animals (as chitosan) [21,22]. These polymers can be linear (for example, chitosan) or cyclic (such as cyclodextrin) and differ in their charge, which can be neutral, positive, or negative [23]. The advantages of plant-based polymers include low cost, environmental friendliness, and abundant availability, which can facilitate their large-scale production. Data from our prior and ongoing projects provided the rationale for conducting the study reported here. Our ongoing research with sodium alginate (SA) as an adjuvant to commonly used insecticides in the LMD region resulted in less toxicity to honeybees. Our prior study in ground-based field herbicide application and in laboratory settings revealed the drift reduction potential (DRP) of SA [24]. Prior study results demonstrated that when added to commonly used insecticides as an adjuvant, SA did not interfere with targeted insect kill mechanisms [25]. Preliminary no-choice honeybee visitation field experiments were carried out using sunflower (Helianthus annuus) plants with flowers. We sprayed water (control), fenugreek polymer (FP) suspension (FP is a nontoxic food-grade polysaccharide derived from the seeds of fenugreek (Trigonella foenum-graecum L.), a legume (Fabaceae) grown in North Africa and Asia), insecticides imidacloprid, and bifenthrin as different treatments. Bee visitation results for fenugreek solution were similar to water and significantly higher than insecticide-alone treatments (unpublished).
The major goal of this study was to evaluate the drift reduction potential of sodium alginate (SA) and fenugreek polymer (FP) for aerial pesticide applications using the Remotely Piloted Aerial Application System (RPAAS). The specific objectives include the analysis of (i) the droplet spectrum of spray volume; (ii) the proportion of driftable droplets (driftable fines) in the spray volume; and (iii) the on-target spray coverage. With better on-target coverage, we can expect less off-target drift. Therefore, on-target coverage was used in combination with droplet spectrum to determine the drift reduction potential. The data required to estimate DRP were obtained by spraying water (control) and water with adjuvant (treatments). To our knowledge, this is the first study in which SA and FP were tested as drift-reducing adjuvants when aerially applied with the RPAAS platform.
“The major goal of this study was to evaluate the drift reduction potential of sodium alginate (SA) and fenugreek polymer (FP) for aerial pesticide applications using the Remotely Piloted Aerial Application System (RPAAS).”
“We sprayed water (control), fenugreek polymer (FP) suspension (FP is a nontoxic food-grade polysaccharide derived from the seeds of fenugreek (Trigonella foenum-graecum L.), a legume (Fabaceae) grown in North Africa and Asia), insecticides imidacloprid, and bifenthrin as different treatments. Bee visitation results for fenugreek solution were similar to water and significantly higher than insecticide-alone treatments (unpublished).”
The fenugreek polymer (FP) is a nontoxic food-grade polysaccharide derived from the seeds of fenugreek (Trigonella foenum-graecum L.), a legume (Fabaceae) grown in North Africa and Asia. FP is partially soluble in water and completely soluble in one molar of sodium hydroxide (NaOH) [37]. The pH of FP aqueous suspension was found to be almost neutral. FP is an amorphous non-ionic polymer made up of D-galactose and D-mannose with a galactose-to-mannose ratio of 1:1.2 [38]. FP has shown promising results as a plant-derived flocculant in water treatment studies [39]. FP was prepared by following the method of Srinivasan et al. [37]. Briefly, fenugreek seeds (purchased from a local grocery store) were soaked overnight in approximately 3 to 5 times the volume of deionized (DI) water. The soaked seeds were finely blended, the dissolved mucilage was filtered with a muslin cloth, and the remnants were discarded. Then, the mucilage was precipitated with 99% isopropyl alcohol at a ratio of one part extracted mucilage solution to three parts isopropyl alcohol. The precipitated FP was separated using vacuum filtration, washed with acetone two to three times to remove impurities, and then dried in a hot-air oven at 70 °C. The dried FP was blended into powder and stored at 4 °C for future use.””
“4. Discussion
The preliminary results from some of the ongoing experiments indicate that when SA was added to a pesticide mixture as an adjuvant, it (i) reduced the toxicity of the pesticide mixture to honeybees (Table 8), (ii) did not interfere with the targeted pest kill mechanisms [25], and (iii) reduced drift in ground-based pesticide applications (Table 9) [24]. In addition, our results showed that SA is compatible with commonly used herbicides and insecticides [24,25]. Therefore, the use of SA and FP as potential pesticide adjuvants and their use in drone-based aerial applications were investigated in this study.”
“5. Conclusions
Ongoing laboratory and field experiments suggest that some plant-based polymers could be developed as drift-reducing pesticide adjuvants (PAs) to reduce pesticide exposure to insect pollinators inhabiting/foraging areas adjacent to croplands. In this study, sodium alginate (SA) and fenugreek polymer (FP) were tested as drift-reducing PAs when aerially applied with the RPAAS platform. Two spray experiments were carried out in the field: (i) water only as control (W) and (ii) water and adjuvant (WA). The drift reduction potential (DRP) of the adjuvants was determined based on the droplet size, on-target coverage, and proportion of droplets less than 200 µm (driftable fines).
Compared to the control, the WA treatments produced larger droplets (9% to 18% larger for FP and 7% larger for SA), suggesting the occurrence of DRP. Notably, 14.5 percent of the total spray volume comprised driftable size droplets for W, whereas only 8.3 to 14.4% and 2.3 to 7.8% of the spray volume were driftable for the SA and FP treatments, respectively. Compared with the W only (2.7%), the FP treatment improved on-target coverage (3.0% to 3.1% average coverage over 14 equally spaced Kromekote cards placed across the flight line). The FP treatment had faster droplet velocities than water alone and SA, especially at 0.5 g/L concentration (10% faster than water alone). In summary, the results of this study indicate that SA and FP have the potential to mitigate off-target drift from drone-based aerial pesticide applications. Compared to SA, FP has the added advantage of providing slightly more on-target coverage, which is desirable for improving pesticide application efficiency. The observed better drift reduction potential of FP could be attributed to, at least in part, a combination of factors, including its ability to increase viscosity; the presence of inhomogeneities (its partial solubility in water); and, to a lesser extent, a decrease in the surface tension. Our study set the foundation for further testing these plant-based polymers in row crops (such as corn, cotton, and soybean) with different densities and varied topographies.”