Conventional sampling plan for thrips in tropical soybean fields
Introduction
Soybean, Glycine max (L.) Merr., is the most cultivated legume in the world (Fried et al., 2019). It is used in human and animal feed, as well as in the production of biofuels, textiles, vegetable oils, paints, and plastics (Chen and Hua, 2000; Swain et al., 2004; Singh et al., 2008; Araujo et al., 2010; Chen et al., 2012; Liu et al., 2000). Soybean crops occupy an area of 125 million hectares, with an annual production of 349 million tons worldwide (FAO Statistical Databases [FAOSTAT] 2018).
Thrips (Thysanoptera: Thripidae) are emerging pests in soybean cultivation. Thrips can decrease the photosynthetic rate of soybean plants by 45%, causing a reduction in yield of more than 15% (Gamundi et al., 2005; Gamundi and Perotti, 2009). In recent years, increasing densities of thrips have been observed in soybean fields, resulting in more crop damage (Silva, 2012; Agrocampo, 2019; Engel and Pasini, 2020).
Some thrips species have been reported causing damage in soybean plants, including species from Caliothrips and Frankliniella genus (Gamundi and Perotti, 2009). Caliothrips genus includes 21 species, most of them reported in Americas (Cavalleri et al., 2018). Caliothrips phaseoli species has achieved the potential economic importance status because is usually associate to damage in Fabaceae plants (Gamundi and Perotti, 2009). In the case of Frankliniella genus, it includes 160 species worldwide distributed and has Franklineilla schultzei as an important quarantine pest (Cavalleri et al., 2018).
Thrips are polyphagous insects that attack several species of cultivated plants and weeds (Palmer, 1990; Yudin et al., 1998; Lima et al., 2000). They can damage plants both directly and indirectly. Thrips cause direct damage to stems, leaves, flowers, and fruits by sucking cellular contents and introducing toxins (Palmer, 1990; Mouden et al., 2017). They can also cause indirect damage by acting as tospovirus vectors (Ullman et al., 1997; Riley et al., 2011). The attacked parts of the plant initially appear whitish, then turn brown, and finally become necrotic (Gamundi and Perotti, 2009).
Adopting a sampling protocol with an acceptable level of precision and logistically feasible may reduce the unnecessary insecticides applications (Araújo et al., 2020). Conventional sampling plans serve as the starting point for the establishment of decision-making systems for integrated pest management (IPM) programs (Lopes et al., 2019). A conventional sampling plan establishes the methodology for assessing pest populations, the number of samples, and the time and cost required for sampling (Bacci et al., 2008; Lopes et al., 2019; Silva et al., 2019). The methodology established in the conventional sampling plan is used to assess economic damage, which is the main basis of control decisions in IPM programs (Gusmão et al., 2005). Furthermore, conventional sampling plans are used as standards for validating sequential pest sampling plans (Silva et al., 2017; Lopes et al., 2019). Sampling plans must be simple, representative, accurate, quick to implement, and low-cost (Gusmão et al., 2005; Lopes et al., 2019). In addition, they must enable decision making in fields of different sizes and at any phenological stage of the crop (Lopes et al., 2019).
Due to the damage thrips cause to tropical soybeans fields, there is an urgent need to establish IPM programs for this group of pests. Accordingly, this research aims to fill part of this gap by developing a conventional sampling plan practicable for evaluating thrips populations in soybean fields. We sought to (i) identify the best sampling technique, (ii) determine the optimal number of samples, and (iii) calculate the sampling time and costs required for commercial soybean fields of different sizes.
Section snippets
Experimental conditions
This research was carried out from 2017 to 2019 in 29 commercial soybean fields in the municipalities of Formoso do Araguaia (11°47ʹ48ʺ S, 49°31ʹ44ʺ W, 240 m altitude) and Gurupi (11°43ʹ48ʺ S, 49°04′08ʺ W, 287 m altitude) in the state of Tocantins, Brazil. The variety used was M8808 IPRO. Plants of this variety have a 140-day growing cycle, are resistant to lodging, and are genetically modified with Intacta RR2 PRO technology that gives them glyphosate tolerance and resistance to Anticarsia
Species found
During field monitoring, two species of thrips were found attacking soybean plants: Caliothrips phaseoli (Hood) and Frankliniella schultzei (Trybom). In this study, it was observed similar densities of both species over the soybean fields.
Evaluation of sampling techniques
For all phenological stages of soybean plants (vegetative, flowering, and fruiting), the highest thrips densities were obtained by beating the plants on a plastic tray or using a shake-cloth (P < 0.05) (Fig. 2A). The thrips densities recorded using the plastic
Discussion
In this study, thrips density in soybean crops was monitored by taking samples from the apical part of the plant. The best area to sample is usually the area where the highest pest densities are found (Gusmão et al., 2005; Silva et al., 2017). In this context, plant-sucking insects, such as thrips, tend to occur at higher densities on the apical part of the plant, as the tissues in this area have a finer cuticle, higher water content, and better nutritional quality; moreover, flowers are
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We thank the ‘Conselho Nacional de Desenvolvimento Científico e Tecnologico’ (CNPq), the ‘Coordenação de Aperfeiçoamento de Pessoal de Nível Superior’ (CAPES), Finance Code 001, and the ‘Fundação de Amparo à Pesquisa do Estado de Minas Gerais’ (FAPEMIG) for financial support. We are also grateful to 'Universidade Federal de Tocantins' (UFT) and 'Universidade Federal de Viçosa' (UFV) for their continuous support throughout this research.
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