CHAPTER 3: Evaluation of constitutive resistance in sugarcane to Chilo
3.4 Discussion
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Table 3.5 Effect of leaf powders in artificial diet from 20 different sugarcane varieties on Chilo partellus survival percentage. (NCo376 was used as a control variety for each batch - colour coded). Mean larval survival number was analysed using GLMM analysis and the Sidak test (F pr. < 0.001, Appendix 3.3).
Variety Survival %
NCo376* 67.82
Control 3 55.8 ± 4.552a
N24 59 ± 4.506ab
N26 61.67 ± 4.457abc
Control 1 61.7 ± 4.457abc M1135/64 62.5 ± 4.438abc
N31 63.3 ± 4.418abcd
Control 2 65 ± 4.372abcd
R 570 65.8 ± 4.348abcd
R 576 68.3 ± 4.264abcd
Co1287 70 ± 4.201abcd
R 568 70.8 ± 4.167abcd
N32 71.7 ± 4.131abcd
N28 71.7 ± 4.131abcd
M861/60 74 ± 4.054abcd
Co6505 75.8 ± 3.877abcd
Control 4 77.5 ± 3.828abcd
R 572 77.5 ± 3.828abcd
Control 5 79.1 ± 3.723bcd
N22 79.1 ± 3.723bcd
N21 80 ± 3.416cd
N25 80.8 ± 3.608bcd
M1025/70 80.8 ± 3.608bcd
N27 82.5 ± 3.483cd
R 573 85 ± 3.273d
Mean 72.41
SD 8.3189
CV% 11.49
Mean ± SE, values in a column followed by the same letter are not significantly different using the Sidak test (P = 0.05)
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similar study conducted by Kumar et al (2006), differences in larval survival, larval weight, pupal weight and other larval parameters were also observed when
incorporating leaf powders of different sorghum varieties into a diet for C. partellus.
Sugarcane varieties M1135/64, R570, N26 and N24 gave consistently lower larval weights and larval survival when they were incorporated into the artificial diet of C.
partellus. This suggests that these sugarcane varieties do not promote larval growth and therefore have a higher constitutive resistance to C. partellus larvae when compared to the other sugarcane varieties. Conversely, varieties M1025/70, R573 and N25 gave higher mean larval weights and larval survivals when incorporated into the diet, which suggests that they are more susceptible varieties, which have little to no constitutive resistance against C. partellus. This indicates that the more
susceptible varieties meet the nutritional requirements required by the larvae to ensure the larval growth can be completed in a shorter time frame (Arabjafari and Jalali, 2007). The mechanisms in plants accounting for the negative effects on larval survival and growth are non-preference and antibiosis (Davis et al., 1999). The antibiosis resistance mechanisms present in resistant sugarcane varieties may be because of secondary plant substances in the leaves or also due to low nutritional quality of the food (Kumar et al., 2006). In a study conducted by Kumar (1997), to determine the effect of dry leaf powders and fresh leaf juices on C. partellus larval development, it was suggested that non-nutritional factors (allelochemicals) in plant tissues are lost during the preparation of the diet, due to the heat involved. Therefore, the survival and development of larvae was directly related to nutritional factors in the plant tissues. C. partellus resistance in sorghum is associated with low sugar content and an increased level of amino acids, tannins, total phenols, neutral detergent fiber (NDF), acid detergent fiber (ADF) and lignins in the plant (Kumar et al., 2006). Plants may contain large amounts of preformed chemicals produced through secondary metabolism, such as various phenolics, terpenoids and steroids. In some tissues, these chemicals may be in high concentrations, which can be very toxic to insects (Keen, 1999). In artificial diet bioassays conducted by Kumar et al. (1993a, b), it was shown that C. partellus larvae displayed feeding non-preference, indicated by the low amounts of food ingested by resistant maize varieties compared to a susceptible variety.
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Not all of the sugarcane varieties showed a correlation between larval weight and larval survival. Varieties N27 and N21 gave high survival percentages of 82.5% and 80% respectively, whereas their mean larval weights were relatively low compared to other varieties (0.07227 g and 0.07825 g respectively). This could be due to more competition between larvae in vials, resulting in less available food for individual larvae to feed on. Another explanation could be that antibiosis had an effect on the larvae, stunting their weight, however this is more unlikely as antibiosis also reduces survival of the insect (Sharma, 1997; Padmaja et al., 2012). On the other hand, N26 and N31 had low larval survival percentages (61.67% and 63.3% respectively), whereas their larval weights were high. The lower number of larvae could have reduced competition for food within the vials and therefore more food was available for individual larvae.
When comparing some of the results from this study to the ratings shown for C.
sacchariphagus in a study conducted by Conlong et al (2004), some interesting observations could be made. N25 gave a fairly high Chilo leaf feeding index (CLI) and C. sacchariphagus rating (the higher the rating the more susceptible the variety) compared to the other varieties (Table 3.1), which correlates with the results from this study, where N25 gave high larval weight and larval survival for C. partellus
compared to other sugarcane varieties, indicating its lower constitutive resistance.
Moreover, F. serrata numbers obtained for N25 were fairly high (Table 3.1) as well.
This backs the concept of insect surrogacy, where similar feeding mechanisms of C.
sacchariphagus, C. partellus and F. serrata result in plants having similar resistance mechanisms acting against them. However, N26 was shown to have a very high CLI and C. sacchariphagus rating by Conlong et al (2004), as well as high F. serrata numbers, whereas in this study C. partellus had a fairly low larval survival (61.67%) and an intermediate larval weight (0.08124 g) when reared on diet incorporating cane tops from N26. This could indicate that N26 has a poor constitutive resistance but a strong induced resistance against C. partellus, if the concept of insect surrogacy for C. partellus and C. sacchariphagus is used. According to the Chilo leaf feeding index and ratings obtained for the variety N28 (Conlong et al., 2004), it was shown to be the most resistant variety to C. sacchariphagus. However, a high larval weight and larval survival for C. partellus was observed when larvae were reared on the diet incorporating N28 cane leaf powder. This could suggest that the mechanism of
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resistance for N28 is not that of the constitutive type, but rather that of antixenosis present on the surface of the leaves, or that of induced resistance, which requires the insect to come into contact with the plant, which in turn results in compounds such as phytoalexins or pathogenesis related (PR) proteins being produced (Ahman, 2006).
R570 is fairly resistant to C. sacchariphagus according to Nibouche and Tibere (2009); and in this study, C. partellus gave a relatively low larval survival and mean larval weight in the diet containing R570, indicating that it has strong constitutive resistance against C. partellus.
There seemed to be a correlation between pupal number and mean larval weight, where the diets resulting in higher larval weight also resulted in a higher number of pupae being found and vice versa. This suggests that the diets with high larval
weights and pupal numbers result in faster growth of the borer, while those diets with low larval weights and pupal numbers reduce larval growth. Antibiosis is also shown by reduced pupal weight and reduced pupation and adult emergence (Kumar et al., 2006).
The incorporation of cane leaf powder of different sugarcane varieties into the artificial diet of C. partellus is useful in overcoming the variation of infestation that would be seen under field conditions and allows one to test different sugarcane varieties under uniform conditions (Kumar et al., 2006). The use of an artificial diet to establish mechanisms of resistance to borers can be used, but should not replace conventional field trials for screening germplasm. The resistance level of sugarcane varieties established using this technique may not conform to those in the field due to the absence of strong genotype x environment interactions (Kumar, 1997). The results from this study suggest that specific sugarcane varieties do contain some degree of constitutive antibiosis as a resistance mechanism against C. partellus larvae. However, it is a complex interaction of factors that determine the resistance or susceptibility of sugarcane varieties to stem borers, which needs to be explored further. Sugarcane varieties, such as Co1287, M1135/64, R570, N26 and N24, which show some form of constitutive antibiosis resistance to C. partellus in this study, should be tested further, and have the potential to be used in resistance breeding programmes to increase the diversity of resistance to C. partellus, and ultimately C.
sacchariphagus.
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