Guangdong Province, located in southern China, has long been an important vegetable production center, exporting various vegetables to Hong Kong every year. In 2016, the vegetable-growing area in Guangdong Province reached 1.41 million hectares and ≈50% of that area was grown in leaf vegetables (Lan et al., 2018). Many leaf vegetable crops, such as flowering cabbage (Brassica parachinensis), cabbage mustard (Brassica alboglabra), and swamp cabbage (Ipomoea aquatica), are grown continuously in the same field due to the high demand, sufficient natural sources of light, heat, and water in the subtropical region, and limited arable land per farmer (Feng et al., 2015). Unsurprisingly, vegetable production areas are infested with various insect pests, plant diseases and weeds throughout the year in the region (Liu et al., 2014; Shen et al., 2018).
To ensure a good harvest, necessary control strategies, particularly chemical measures, are implemented mainly in two stages: before cropping and during the crop-growing season. For example, in the crop-growing season, spinetoram and cyantraniliprole are applied to control striped flea beetle, diamondback moth, and tobacco cutworm (Spodoptera litura) (Hong et al., 2017; Li et al., 2015), and pendimethalin and quizalofop-p-ethyl are applied to control some annual weeds (Chen et al., 2015). In addition, vegetable growers usually take some measures for PPPM (Mao et al., 2016). In Guangdong Province, farmers typically apply burn-down herbicides (e.g., paraquat and glyphosate) to manage vigorously growing weeds and stubbles and then perform shallow tillage with a rotary cultivator (Chen et al., 2015). However, some problems are inevitable with this PPPM method. First, some important insect pests, such as striped flea beetle and diamondback moth, will not be controlled, which introduces a significant pest source to the next crop. The pests require numerous applications of various insecticides per season to produce acceptable vegetables. Second, both insect pests and weeds are prone to developing resistance to chemical pesticides with repeated applications. In China, the diamondback moth has evolved resistance to many classes of insecticides, including avermectins, chlorantraniliprole, spinosad, and indoxacarb (Wang and Wu, 2012; Wang et al., 2013; Xia et al., 2014). In addition, there have been several reports concerning paraquat and glyphosate resistance in goosegrass and horseweed (Conyza canadensis) in China (An et al., 2014; Chen et al., 2017; Song et al., 2011; Zhang et al., 2015). In this context, farmers have to increase the rates of these pesticides or switch to newly developed pesticides, which increases the control cost substantially. Third, the public demands alternatives due to increasing concerns about food security and a heightened awareness of environmental impacts. Thus, new alternative pest management techniques are needed for PPPM in leafy vegetable fields.
Flaming, one of the most important nonchemical methods for crop protection, is successfully used for controlling weeds, insects, soilborne nematodes, and pathogens mainly in the production of organic crops, such as maize (Zea mays), apple (Malus domestica), and vegetables (Agnello et al., 2017; Laguë et al., 1999; Mao et al., 2016; Stepanovic et al., 2016; Vincent et al., 2003). In addition, it is a potential measure to be applied to conventional crops as the problems with chemical pesticides (increasing price, environmental pollution and pest resistance) increase (Stepanovic et al., 2016). Flame controls the pests by overheating them rather than burning them. Most flaming machines use propane or liquefied petroleum gas as fuel, and the flame temperature can exceed 1000 °C, which rapidly raises the temperature of the pest (Datta and Knezevic, 2013; Knezevic et al., 2014). The position and protection mechanism of the growing point and the ability to regrow after the flame treatment are closely related to the thermosensitivity of plants. In addition, the thermal tolerance of the pests increases with increasing body size (Ulloa and Knezevic, 2010; Ulloa et al., 2010). A propane dose of 60 kg·ha−1 provided up to 80% to 90% control of many annual broadleaf and grass species, and a propane dose of 120 kg·ha−1 is able to control most weeds in the flowering stage (Ulloa and Knezevic, 2010; Ulloa et al., 2010).
Although flaming has a wide range of uses, most reports focus on its application for the control of weeds, insects, or pathogens separately (Kanellou et al., 2017; Sivesind et al., 2012). In fact, flaming can play an important role in PPPM in leaf vegetable fields. The advantages include that 1) flaming takes effect quickly, which may save several days for preparing for the next growing season; 2) flaming kills weedy plants and insect pests simultaneously (Laguë et al., 1997), and thus reduces herbicide and insecticide use; and 3) flaming can be used to control tolerant or resistant pests (Datta and Knezevic, 2013). Research groups worldwide have developed different types of flaming equipment (Brainard et al., 2013; Raffaelli et al., 2013). However, the flaming machines and the burners reported in the literature are often not suitable or efficient for PPPM in leaf vegetable fields. Moreover, there are no reports on the effect of flaming on weed and insect controls in China. Therefore, the objectives of this study were to 1) describe two flaming machines designed for PPPM in leafy vegetable fields and 2) evaluate the performance of flame treatment on weeds and insect pests in field trials.
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