During the past 55 years, methyl bromide (MeBr) has been the most effective soil fumigant for controlling soil pests in California high-value crops (Johnson et al., 2012). MeBr is a highly effective biocide that has a consistent and broad pest control spectrum and short plant-back periods (Jacoby, 2016). MeBr is a gas that has a low boiling point and high vapor pressure; such properties allow MeBr to move through soil unimpeded, resulting in high pest control efficacy. However, in the early 1990s, MeBr was identified as a stratospheric ozone depletor (Butler and Rodriguez, 1996); therefore, the use of MeBr was phased out in the United States (Butler and Rodriguez, 1996; Downing, 2016; Jacoby, 2016; Johnson et al., 2012). Although MeBr was used on many crops, California strawberries were the most economically impacted by the phase-out, and this helped strawberry farmers win a critical use exemption that allowed small quantities of MeBr to be used until 2016 (Downing, 2016).
Research showed that the soil fumigants that are alternatives to MeBr are generally less effective pest control agents. These alternatives to MeBr include chloropicrin (CP), 1,3-dichloropropene (1,3-D), methyl isothiocyanate (MITC) generators such as metam sodium (Met-Na), metam potassium (Met-K), and dimethylformocarbothialdine (DMTT), and synthetic allyl isothiocyanate (AITC) (Gao et al., 2012; Klose et al., 2008). Additionally, research has shown that to obtain acceptable pest control levels, a combination of more than one fumigant must be applied. For example, 1,3-D controls nematodes and some soil arthropods, but it provides little control of fungi and weeds, whereas Met-Na controls weeds, nematodes, and some fungi. CP controls insects and fungi, but it has less activity against nematodes and weeds (Ajwa and Trout, 2004). The most promising MeBr alternatives are chloropicrin and Met-Na (Gao et al., 2012; Jacoby, 2016; Klose et al., 2008; Yates et al., 2002); therefore, more than 7000 tons of chloropicrin were used in 2011 (Nelson et al., 2013) in California.
Although Met-Na is an effective fumigant (Nelson et al., 2002), the use of Met-Na is strongly regulated due to the excessive soil fumigant release into the atmosphere (Goodhue et al., 2016; Saeed et al., 1997). Because of this excessive release into the atmosphere, Met-Na, Met-K, and DMTT labels require wide buffer zones and specific measures to protect people from off-target movement (Guthman and Brown, 2016). Furthermore, Met-Na, Met-K, and DMTT degrade in soil to MITC, which is classified as a toxicity I category pesticide (Gao et al., 2012; Klose et al., 2008).
Recently, the United States Environmental Protection Agency (EPA) approved the registration of synthetic AITC as a preplant soil bio-fumigant (Isagro, 2016). AITC is a natural product produced by Brassica plants to defend against insects and diseases (Ahuja et al., 2010). Brassica plants naturally produce glucosinolate, which is converted to AITC by the enzyme myrosinase when plants are attacked or mechanically damaged. Several reports showed that AITC performed as well as MeBr and Met-Na when applied as a preplant soil treatment (Bangarwa et al., 2011; Devkota and Norsworthy, 2014). Moreover, research showed that AITC is effective for controlling select weeds (Bangarwa et al., 2011; Devkota and Norsworthy, 2014), nematodes (Wu et al., 2011), insects (Noble et al., 2002), and plant soil pathogens (Gilardi et al., 2000).
Synthetic AITC is not considered a restricted use pesticide; therefore, it differs from all other fumigant alternatives. AITC can be applied to all crops during the preplant period, has a maximum buffer zone of 7.5 m, has no limit regarding the number of hectares that can be treated per day, and has no restrictions regarding the type of covering film used with it (Isagro, 2016). AITC is considered generally regarded as safe (GRAS) by the Food and Drug Administration, and it is a food additive approved for direct human consumption as a synthetic flavoring substance and adjuvant (FDA, 2018).
It is difficult to find a chemical with biological, chemical, and physical properties similar to those of MeBr that will pass regulations and that is relatively easy to transport and apply. MeBr has a boiling point of 4.5 °C and a vapor pressure of 218.6 kPa; however, for MITC, CP, and AITC, the boiling points are 119, 112, and 150 °C, respectively, and vapor pressures are 1.73, 2.44, and 0.49 kPa, respectively (Downing, 2016; Jacoby 2016; Nelson et al., 2013). The high boiling point and relatively low vapor pressure of MeBr alternatives do not allow these fumigants to exist in a gas phase in their original state. Liquid fumigants need to convert to a volatile or semi-volatile state to control soil-borne pests; therefore, the dispersal rate and the ability of a fumigant to move in soil at high concentrations will determine the effectiveness of the fumigant (Nelson et al., 2013).
In California, MeBr alternatives are usually applied to the soil through drip irrigation (also known as chemigation); however, in many other places, shank applications are still a common application method for fumigants. During drip irrigation, polyethylene plastic mulch is installed before the application of the fumigants, and this application method is gaining popularity with growers due to limited worker exposure (Jacoby, 2016; Nelson et al., 2013). Although Met-Na and other isothiocyanate generators are popular and widely used as MeBr alternatives, previous research suggested that soil-borne pest control with Met-Na is inconsistent because application methods seldom achieve optimum distribution of Met-Na (Gilreath et al., 2008; Rodriguez-Kabana, 2002). This is especially important in sandy soils in which lateral water movement is limited (Papiernik et al., 2004; Yates et al., 2002). Met-Na principle derivative (MITC) is not very mobile in soil, and AITC has less mobility in soil than MITC (Jacoby, 2016).
Multiple reports have shown that the efficacy of isothiocyanate generators could be improved by increasing the residence time of the fumigant in soil (Nelson et al., 2013). As a result, several researchers investigated different application methods that could provide better fumigant soil distribution by physical means such as multiple drip lines, using totally impermeable film, and adjusting water application rates and volumes (Nelson et al., 2013; Papiernik et al., 2004; Yates et al., 2002). Such methods have indirectly led to improving molecular diffusion of fumigants and their persistence in soils (López-Fernández et al., 2016).
It has been reported that the efficacy of Met-Na was improved when applied with the soil surfactant Integrate (triblock co-polymer and glucoethers) (Santos et al., 2013). Furthermore, Santos et al. (2013) observed a significant reduction in the number of propagules of purple nutsedge when a soil surfactant was added to Met-K compared with Met-K applied alone. Furthermore, the use of surfactants to influence pesticide movement in soil has been reported. Sanchez-Camazano et al. (1995) described how the addition of surfactants influenced nonionic pesticide mobility in soil. Moreover, Katagi (2008) discussed how surfactants modify the environmental behavior of pesticides, and Mao et al. (2015) described how surfactants can be used to remediate soils by influencing the solubility of pesticide residue in soil. However, all reports agree that the ability to enhance fumigant efficacy with surfactants depends on the type of surfactant (anionic, nonionic, or cationic) used and soil properties because soil surfactant generally alters the bonding properties of soil–water interactions. Although the effects of soil surfactants on the pest control efficacy of Met-Na have been well-studied, questions remain regarding whether the type of surfactant affects the pest control efficacy of other fumigants such as AITC and CP. Therefore, the objective of this study was to characterize and quantify the effects of different surfactants on AITC mobility and CP mobility and their distributions in different soils.
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