Rapid means of screening for resistance to pests in a sugarcane plant breeding selection programme.
Chilo partellus (Lepidoptera: Crambidae) and Chilo sacchariphagus (Lepidoptera: Crambidae) are two stem borers which pose a threat to the South African sugar industry at present. The reliable supply of good quality insects for host-plant resistant studies is vital. The techniques used at the South African Sugar Research Institute (SASRI) for establishing and maintaining C. partellus colonies were described because these insects are vital in host-plant resistance research. Sugarcane agro-ecosystems in KwaZulu-Natal were surveyed for C. partellus, and species confirmation took place using cytochrome oxidase I subunit barcoding. A neighbor-joining tree showing Chilo phylogeny supported the concept of using C. partellus as a surrogate insect for C. sacchariphagus for host-plant resistant screening studies in South Africa. Artificial diets were developed to optimize insect growth and reproduction and to meet or exceed the nutritional requirements of the target insect. Experiments were conducted to test different diets, with the incorporation of various ingredients, and the use of different inoculation and rearing methods. Vials that were inoculated with two neonate larvae each gave greater mean larval weights and larval survival percentages compared to the multicell trays and plastic jars. An improved artificial diet for rearing C. partellus was established incorporating non-fat milk powder (2.35% m/v) and whole egg powder (1.75% m/v). This diet gave higher mean larval survival percentages and mean larval weights than other diets tested. A version of this diet was developed with an increased content of cane leaf powder (from 2.5% to 6.5% m/v), so that better discrimination between leaf powders from different sugarcane genotypes would become possible in diet incorporation bioassays. Stalk borers can have detrimental effects on crops such as sugarcane, maize and sorghum in sub-Saharan Africa. C. partellus and C. sacchariphagus are serious pests on a number of hosts and pose a serious threat to the sugarcane industry in South Africa. The use of host-plant resistance is extremely important in controlling these pests, and the breeding of new and improved varieties are important in maintaining good yields. Constitutive antibiosis resistance to C. partellus larvae was explored in a diverse collection of 20 sugarcane varieties, by incorporating crushed dried leaf whorl powder into an artificial diet. There were significant differences in larval weight, total C. partellus weight and larval survival in diets incorporating leaf powder from different sugarcane varieties. Varieties M1135/64 and N24 gave consistently lower larval weights and larval survival, whereas varieties M1025/70, R573 and N25 gave higher larval weights and larval survival when incorporated into the diet, which suggests that they have little or no constitutive resistance against C. partellus. The concept of insect surrogacy was also explored, whereby known field resistance ratings of specific sugarcane varieties to C. sacchariphagus were compared to the results obtained for resistance to C. partellus from this study. Some correlations were observed for specific sugarcane varieties, such as N25 and R570, with respect to C. partellus and C. sacchariphagus resistance. However, further investigations will be required using different resistance screening methods to determine the different components of resistance of sugarcane varieties. Chilo partellus was used as a surrogate insect for C. sacchariphagus in ovipositing studies on sugarcane because C. sacchariphagus is not yet present in South Africa. Both pests belong to the same family and have the same feeding mechanisms, therefore similar defense mechanisms in plants may operate against them. The concept of ovipositional antixenosis behaviour of insects is based on the theory that female insects will choose their hosts in a hierarchal manner, laying most of their eggs on the preferred plant. This could be due to characteristics of the plant that either fail to provide ovipositing behaviour-including stimuli (attractants), or contain ovipositional-inhibiting stimuli (repellents). In this study, differences with respect to ovipositing by C. partellus moths were investigated on 20 selected sugarcane varieties. Two experiments were conducted, whereby the 20 sugarcane varieties were planted into 98 well seedling trays in a completely randomized design, replicated five and ten times for Experiments One and Two, respectively. Individual trays were placed into BugDorm® rearing tents when plants were still in their seedling stage, and moths were put into the tents for ovipositing to take place. No statistically significant differences were found between sugarcane varieties for both egg and batch number for both experiments (F pr > 0.05). A direct correlation was seen between egg number and batch number, with R2 values of 0.696 and 0.899 for Experiments One and Two, respectively. C. partellus and C. sacchariphagus initially feed on the leaf whorl of their hosts before boring into the stalk, which results in destruction of the growing point and extensive stalk damage. Host-plant resistance plays a pivotal role in controlling such pests, and therefore it is important to identify sugarcane varieties that could potentially have resistance against them. Glasshouse trials conducted in pots were used to compare 21 sugarcane varieties for their resistance against C. partellus. The whorls of the plants were inoculated with 10 neonate larvae, and after 30 days sugarcane varieties were assessed for various damage parameters. Results from these trials give preliminary indications as to whether Fulmekiola serrata (Thysanoptera: Thripidae) and Chilo sacchariphagus resistances are correlated; whether C. partellus or F. serrata could serve as surrogates in assessing resistance to C. sacchariphagus; and whether C. partellus itself poses a threat to sugarcane varieties. There was a significant difference between sugarcane varieties for the mean number of shotholes/lesions, mean number of borings, and mean number of larvae recovered. Sugarcane varieties N32 and M1135/64 showed the highest levels of resistance against C. partellus, which concurs with C. sacchariphagus ratings obtained from previous studies. With respect to F. serrata numbers, sugarcane varieties N21, R568, R574 and R572 had the highest F. serrata numbers in a trial conducted by the South African Sugar Research Institute. All four of these varieties were shown to be susceptible to C. partellus in the pot trials conducted in this study. Eldana saccharina Walker (Lepidoptera: Pyralidae) and F. serrata are considered serious pests of sugarcane in South Africa. The potential for an invasion by the borer C. sacchariphagus from Mozambique poses a great risk to the South African sugarcane industry, and C. partellus may represent a threat similar to the one once posed by E. saccharina before it adapted to feed on sugarcane. F. serrata, C. partellus, and C. sacchariphagus all feed on the whorl of their hosts, and therefore similar plant resistance mechanisms may act against them. Rating of sugarcane clones for damage caused by these pests and the selection of resistant genotypes can be difficult and expensive, and it can take up to 15 years before new varieties are released. A study was made to develop a rapid, non-destructive, on-site technique for predicting sugarcane resistance to pests such as Chilo spp. and F. serrata. The technique was based on near infrared reflectance spectroscopy (NIRS), which can also be used to examine the interaction between sugarcane and its attackers. Differences in resistance of sugarcane varieties to these pests may be in part due to biochemical and structural differences within the leaf. NIRS can penetrate up to 2.5 mm into plant material which infers that NIR spectra should represent the constitutive structural and chemical composition of the leaf that could be related to sugarcane resistance to pests such as C. partellus and F. serrata. Therefore, spectral data was obtained from intact leaves of 21 selected sugarcane varieties using a portable NIR spectrometer. Correlations between NIR spectral data and reference data obtained for C. partellus and F. serrata were developed using partial least squares (PLS) regression with full cross validation. Validation plots were useful in discriminating between sugarcane varieties for either constitutive or induced resistance based on predicted and actual values of reference data. Test validation was conducted on selected reference material using a validation set of five samples. Test validations gave better results than cross validations, with the best predictive model for the mean number of shotholes per variety (R2 of 0.75, SEP of 8.1). Larger calibration and validation sample sets, with equal numbers of resistant, intermediate and susceptible varieties, are required for models to have an improved predictive capability. Reference data such as the number of shotholes per variety, which are directly related to leaf characteristics, gave better model performance than reference parameters not directly related to the scanned leaves, such as boring length in the stalk.
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