CHAPTER 6: Feasibility studies on the use of near-infrared reflectance
6.1 Introduction
There are over 1,500 insect species that attack sugarcane worldwide (ul-Hussnain et al., 2007). Stalk borers are some of the most serious insect pests of crops such as sugarcane (Saccharum L), maize (Zea mays L.) and sorghum (Sorghum bicolor L.
Moench) (Kfir et al., 2002). In South Africa Busseola fusca Fuller (Lepidoptera:
Noctuidae) and Chilo partellus (Swinhoe) (Lepidoptera: Crambidae) are the two major stem borers on maize and sorghum, while Eldana saccharina Walker
(Lepidoptera: Pyrialidae) and Fulmekiola serrata (Thysanoptera: Thripidae) (thrips) are the most serious pests of sugarcane (Kfir et al., 2002). Extreme changes in climate and increasing global trade results in the spread of pests and diseases more easily and causes them to establish in new, previously unaffected countries (Goebel
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and Sallam, 2011). The potential for an invasion by the borer Chilo sacchariphagus Bojer (Lepidoptera: Crambidae) from Mozambique into South Africa poses a great risk to the South African sugarcane industry. Although it has not yet entered South African territory, studies on climatic conditions suited for the pest show that the
coastline of KwaZulu-Natal and neighbouring river valleys, particularly northwards are suitable for the pest’s establishment (Goebel, 2006; Bezuidenhout et al., 2008). C.
partellus has adapted to sugarcane in North Africa and is present in the South
African sugarcane agro-ecosystem (Assefa et al., 2009). C. partellus may represent a threat similar to the one once posed by E. saccharina before it added sugarcane to its list of host plants. C. sacchariphagus and C. partellus larva feed on the whorl of the plant before becoming top borers, while all life stages of F. serrata take place in the leaf spindle suggesting that similar resistance mechanisms may act against them.
Among the few South African sugarcane varieties with known resistance or susceptibility to C. sacchariphagus, there appears to be a correlation between F.
serrata and C. sacchariphagus rankings (Figure 1.9; Chapter One). With an increasing number of potential pests of sugarcane in South Africa, the need for a rapid and less costly method to screen varieties increases in importance.
The development of new varieties can take up to 15 years and is a resource intensive process (Purcell et al., 2010b). Screening for pest and disease resistance can only take place in much later selection stages when plant numbers are less, due to space and cost limitations (Rutherford, 1998; Purcell et al., 2010b). The use of field trials is also difficult due to variable pest populations, or lack of infection (Purcell et al., 2005).
Near-infrared reflectance spectroscopy (NIRS) is a rapid, non-destructive and reliable technique that can be used in the field. NIRS will allow for earlier screening of
varieties which will have a positive effect on the number of clones being brought forward to later stages in a selection programme, a reduction in the need for field trials, which will in turn allow for better resource management and generation of reliable data that can be used in future research projects (Purcell et al., 2005).
Calibration of near-infrared spectrometers involves acquiring spectra of
representative samples, reference analysis of samples using laboratory or traditional methods and model building using chemometrics (Table 6.1) (Blanco and Villarroya, 2002; Chen et al., 2002).
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Table 6.1 The basic steps involved in near-infrared reflectance (NIR) model construction and their associated purposes (adapted from Blanco and Villarroya, 2002)
Step Purpose
1 Select calibration samples Should be representative of the component of interest, with a good range
2. Obtain reference data (traditionally/wet chemistry)
Obtain a value for the component of interest in an accurate manner
3. Obtain spectral data Scan samples in a reproducible manner 4. Averaging and pre-treatments of
spectra
To reduce unwanted effects such as scatter effects and particle size on spectra
5. Constructing a model (calibration) using multivariate methods
To determine the relationship between predicted data and reference data
6. Validation Using independent samples to ensure the model accurately predicts the property of interest
7. Real time use in the industry Predict unknown samples
NIRS has been applied in numerous industries, including the likes of the sugarcane industry. Rutherford and Van Staden (1996) developed near-infrared (NIR)
techniques to predict for E. saccharina resistance by building a stepwise linear multiple regression model using surface stalk waxes. Purcell et al. (2003, 2004) also analysed sugarcane surface waxes using gas chromatography (GC) and
spectroscopy and were able to differentiate between plant properties. In an
experiment conducted by Purcell et al. (2010b), 31 sugarcane samples were used for a validation trial. NIRS was used to obtain the spectra from stalk bud tissue which were then pre-treated and analysed using chemometrics. The smut ratings based on NIR were compared to ratings from field trials. The results obtained were promising and showed good potential for using NIRS as an early screening method for
resistance in sugarcane to smut (Purcell et al., 2010b). NIRS has also been used to scan intact sugarcane leaves in order to predict pre-challenge constitutive resistance to Fiji Leaf Gall Virus (Purcell et al., 2009). Pest resistant eucalypt species have also
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been identified using NIRS, and it has been found that plant resistance to pests can be due to physical, chemical, or ecological components of plants (Henery et al., 2008). Physical traits may be leaf toughness or the presence of hairs, while chemical traits can be components within the plant tissues which act as toxins or antifeedants that effect insects (Purcell et al., 2005). Young C. partellus larvae feed on the leaf whorl during the seedling stage, after which the adult larvae leave the whorl to bore into the stalk of the plant (Kumar et al., 2006). Therefore, observed differences in sugarcane varieties in terms of resistance may in part be due to biochemical and physical characteristics of the leaf. Wax components on the surface of sugarcane stalks have been shown to be involved in resistance to E. saccharina using NIR (Rutherford and Van Staden, 1996). However sub-surface characteristics can also be probed using NIR, shown in a study whereby NIRS was used to relate spectra
obtained from foliage samples of Eucalyptus grandis to damage caused by the beetle Paropsis atomaria (Coleoptera: Chrysomelidae) (Henery et al., 2008). It has been shown that NIR can penetrate up to 2.5 mm into plant material which suggests that NIR spectra should represent the biochemical and structural differences of the leaf (Purcell et al., 2010a).
With regards to the success of the above studies in constructing inferential methods to predict results from laboratory or traditional analysis conducted on sugarcane, we hypothesized that the same methods could be used to predict the performance of sugarcane varieties with respect to pest resistance. The main objective of this study was to develop fibre-optic NIR methods to predict resistance of sugarcane varieties to Chilo spp. and F. serrata using reflectance/transflectance spectra from intact leaves. Such a method would allow for rapid screening of sugarcane varieties for resistance to these pests, without the need for artificial infestation or the use of field trials.
6.2 Materials and methods