A Nutritional Perspective of Sugarcane Resistance to Stalk Borers and Sap Feeders

TWO RELATIVELY new key species in Louisiana that conform to the plant stress hypothesis are the Mexican rice borer, Eoreuma loftini (Dyar) and the sugarcane aphid, Melanaphis sacchari (Zehntner). High performance liquid chromatography differentiated insect resistant and susceptible sugarcane cultivars based on nutritional profiles involving free amino acids (FAAs). For E. loftini susceptible cultivar LCP 85384, concentrations of essential and nonessential FAAs in whole leaf tissue samples were more than twice as high as in the resistant cultivar HoCP 85-845. Similarly, M. sacchari susceptible L 97-128 exhibited more than three fold higher concentrations of essential FAAs in the phloem sap compared to the resistant cultivar, HoCP 91-555. Two essential FAAs, histidine and arginine, were detected only in the sap of L 97-128. A novel approach to evaluate insect resistant and susceptible cultivars is proposed. Introduction The Mexican rice borer, Eoreuma loftini (Dyar) (Lepidoptera: Crambidae) and sugarcane aphid, Melanaphis sacchari (Zehntner) (Hemiptera: Aphididae) conform in their herbivore-plant interactions to the plant stress hypothesis (White, 1969). In this relationship, host plants stressed from lack of water, increased salt, reduced soil fertility, or excessive damage are more suitable hosts enhancing one or more aspects of herbivore bionomics (Reay-Jones et al., 2003, 2005b, 2007a). M. sacchari and E. loftini are relatively new pests to Louisiana sugarcane (White et al., 2001, Hummel et al., 2008). In contrast, the more established invasive species, the sugarcane borer, Diatraea saccharalis (F.) conforms to the plant vigour hypothesis where the biology is enhanced by an association with vigorously growing plants (Price, 1991). Introduced from Mexico into the Lower Rio Grande Valley of Texas in 1980 (Johnson, 1985), E. loftini is the major insect pest of sugarcane representing more than 95% of stalk borer populations in that area (Reay-Jones et al., 2005a). The insect spread northward and north-eastward into Louisiana at a rate of 16.5–23 km/year (Reay-Jones et al., 2007b). E. loftini has been cited as having a potential economic impact on the Louisiana sugarcane industry of up to $220 million/annum (Reay-Jones et al., 2008). Cryptic oviposition sites on dried sugarcane leaves, larval mining of midribs and leaf sheaths, boring into stalks within 3–7 days after egg hatch, and pupation in frass-packed tunnels render E. loftini inaccessible to control by biological and chemical agents (Reay-Jones et al., 2005a). M. sacchari is the most abundant aphid species in Louisiana sugarcane. It vectors sugarcane yellow leaf virus (SCYLV). SCYLV was discovered in Louisiana in 1999, and in 2002, 48% of Reagan, T.E. et al. Proc. Int. Soc. Sugar Cane Technol., Vol. 27, 2010 ______________________________________________________________________________________ 2 fields were infected throughout the industry (McAllister et al., 2008). Disease management of SCYLV is facilitated using a seed cane certification program conducted by the Louisiana Department of Agriculture and Forestry (McAllister et al., 2008). Spread and incidence of SCYLV depends mainly on the spring migration of M. sacchari into the field (McAllister et al., 2005). This indicates potential for using resistant cultivars to prevent disease spread, since these cultivars will presumably attract fewer aphids and/or negatively affect their biology. The M. sacchari-resistant cultivar (HoCP 91-555) and experimental lines with E. loftini resistance similar to the commercial resistant HoCP 85-845 demonstrate potential for managing these two invasive pests based on host plant resistance. This paper highlights some of the biochemical relationships involving free amino acids (FAAs) associated with Saccharum spp. plant resistance to E. loftini and M. sacchari. Eoreuma loftini Screening and finding genetic sources of resistance to E. loftini is an important component of the variety development program of the Louisiana State University AgCenter. Commercial and experimental cultivars are evaluated yearly in replicated trials under natural infestations with heavy E. loftini pressure at Ganado in Jackson County, TX. Initial studies at this location conducted in 2002 helped identify E. loftini susceptible (LCP 85-384) and resistant (HoCP 85-845) cultivars that are used as standards (Table 1). Even though the older cultivar CP 70-321 demonstrated significantly better resistance than HoCP 85-845 under heavy pressure, studies at Weslaco in Hidalgo County with lighter E. loftini infestations, showed that HoCP 85-845 would make a better standard (Reay-Jones et al., 2003). Assessing resistance to E. loftini requires determination of percentage of bored internodes and per hectare moth production based on frequency of adult (moth) emergence holes (Table 1). Table 1—Injury (+ SEM) by E. loftini to six sugarcane cultivars, resultant survival (+ SEM) of older larvae inside the stalks, and moth production (+ SEM) at Ganado, Jackson County, TX, 2002 (Reay-Jones et al., 2003). Cultivar % Bored internodes Relative survival Moth emergence x10 /ha LCP 85-384 67.5 (5.7)a 0.225 (0.065)a 112.3 (37.5)a HoCP 96-540 62.5 (6.8)a 0.200 (0.030)a 105.6 (7.9)a HoCP 91-555 57.5 (3.4)ab 0.363 (0.144)a 165.1 (65.4)a HoCP 85-845 47.2 (4.9)bc 0.150 (0.039)a 62.7 (17.0)a NCo 310 36.2 (3.1)cd 0.166 (0.035)a 53.1 (13.4)a CP 70-321 28.3 (1.9)d 0.171 (0.023)a 39.1 (5.5)a