ReviewYellow sticky traps for decision-making in whitefly management: What has been achieved?
Highlights
► Yellow sticky traps (YST) were compared with direct monitoring methods of whiteflies. ► YST-based economic thresholds for whiteflies in cucumber and tomato were deemed possible. ► Required research for improving YSTs as a whitefly monitoring tool is presented.
Introduction
The fact that many insects show preference to particular light wavelengths has lead entomologists and researchers involved in plant protection to develop monitoring tools and control strategies against many insect pests exploiting this behaviour. One good example of such an approach is the use of coloured traps. Yellow sticky traps (YSTs) in particular have been a subject of research for many decades, and incorporated in management programmes of various pests such as whiteflies, thrips, leaf mining flies, shore flies and fungus gnats in a number of crops. In greenhouses, they have become a key component of IPM programmes of several greenhouse pests (Steiner et al., 1999; Kaas, 2005; Park et al., 2011a). YSTs serve as a tool for early detection, identification of hotspots as well as for estimating relative abundance and monitoring dispersal activity of adult whiteflies, including those occurring in greenhouses, namely Trialeurodes vaporariorum (Westwood) and Bemisia tabaci (Gennadius) (Gillespie and Quiring, 1992; Heinz et al., 1992; Naranjo et al., 1995). In addition, YSTs have the potential of suppressing adult populations alone or in combination with other control strategies such as biological control (Yano, 1987a; Gu et al., 2008 and references therein) or trap crops (Moreau, 2010; Moreau and Isman, 2011).
The goal of using traps for insect monitoring is to predict insect densities that cause crop damage or yield reduction or commodity losses so that timely control actions can be taken. Pest densities are monitored indirectly from the crop, using trap catches as an indication of pest density on the plant. Therefore, determining the relationship between trap catches of the pest with its numbers in the crop and related yield losses are critical to make correct control decisions. In spite of the fact that YSTs have been widely used with several advantages for growers such as low cost and low training demands, the development of decision-making tools based on YSTs (e.g. sampling protocols and economic thresholds (ET)) has been rather limited. Nevertheless, in some greenhouses, whitefly control decisions are commonly guided by adult densities on traps (Kaas, 2005).
The evaluation of sampling techniques (e.g. precision and efficiency) should be one of the first steps towards the development of sustainable sampling plans (Buntin, 1994). In spite of this, only a limited number of studies have compared YSTs (or other colour sticky traps) with other sampling techniques in different pest/crop systems (Table 1). YSTs have been regarded by some as a poor and inconsistent tool for estimating pest densities on plants, and therefore, not suitable for decision-making purposes in field conditions (Palumbo et al., 1995; Naranjo et al., 1995, 2010). According to Ekbom and Rumei (1990), however, YSTs can be considered in greenhouses as one of the most efficient sampling techniques, and they should be very useful as decision-making tools, once their optimal placement sites are determined for a given species and crop. It is possible that in greenhouse environments, YSTs are a more powerful tool than in the field for decision-making purposes, either alone or in combination with other techniques. There are several encouraging studies, especially for whiteflies, suggesting the potential use of YSTs in greenhouse conditions (Gillespie and Quiring, 1987; Yano, 1987b; Kim et al., 2001; Park et al., 2011a), and further development of this method holds promise.
This review aims to clarify the potential of YSTs as a monitoring tool, and the feasibility of designing YST-based sampling protocols for the purpose of decision-making for pest management actions. The review's logic is guided by assessing to what extent YST insect-counts comply with the four criteria of effective sampling of reliability, representativeness, relevance and practicality as described by Binns et al. (2000). Based on these criteria, we identify gaps of knowledge in the effective use of YSTs as decision-making tools that deserve further investigation. The review focuses on whiteflies and greenhouse vegetables, particularly tomato (Lycopersicon esculentum Mill) and cucumber (Cucumis sativus L.) in both seasonal and year-round production systems practiced in Northern latitudes, as these are the target crops of our whitefly management studies in the greenhouse cluster of Finnish Ostrobothnia (Vänninen et al., 2011; Vänninen, 2012). However, examples from studies of other systems are also included whenever feasible.
Section snippets
Reliability of yellow sticky traps
According to Binns et al. (2000), reliability assumes that the results are not influenced by the person collecting the data, or by exogenous, uncontrolled variables such as weather or possible diurnal behaviour of the pest. Because YSTs rely on the behavioural responses of whiteflies, many physiological characteristics, environmental conditions as well as intra- and interspecific biotic interactions occurring in the greenhouse can affect the number of whiteflies ending up on the traps. In
Future research
YSTs were introduced as a monitoring tool in the 1980s and are now used for several flying pest species in field and greenhouse crops. Sticky traps are available and sold commercially in all parts of the world, but despite that, no explicit data are available on the quantitative extent of their use. There are also qualitative differences in their use, i.e. in some firms systematic data are collected from monitoring, whereas in others the traps are checked irregularly without doing a systematic
Acknowledgements
We would like to thank to Dr. Les Shipp from the Agriculture and Agri-Food Canada for critically reading the manuscript. We would like to thank to Dr. Kwang-Ho Kim and Dr. Taek-Joon Kang from the Rural Development Administration, South Korea for their assistance in translating the Korean literature, and Ms Peng Luo for translating the key Chinese papers. The Finnish Ministry of Agriculture and Forestry is acknowledged for financial support. D.M.P.Z. is currently supported by the CNPq Process
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