ZOYSIAGRASS (ZOYSIA SPP.) RESISTANCE TO FALL ARMYWORM (SPODOPTERA FRUGIPERDA): POLYPHENOLS AND FLAVONOIDS ¿ COMPONENTS OF RESISTANCE

Abstract

Leaf tissue collected from six zoysiagrass (Zoysia spp.) cultivars was analyzed for polyphenols and flavonoids by high performance liquid chromatography (HPLC). The experiment investigated the effects of sampling date and N fertilization rate on green leaf chemistry of zoysiagrass and their relationship to fall armyworm (FAW) [Spodoptera frugiperda (J. E. Smith)] resistance. Six zoysiagrasses cultivars {>Crowne= (Z. japonica Steud.), >Palisades= (Z. japonica Steud.), >El Toro= (Z. japonica Steud.), >Meyer= (Z. japonica Steud.), >Cavalier= [Z. matrella (L.) Merr.], and 'Emerald' (Z. japonica Stued. × Z. tenuifolia Willd. ex Trin.)} were planted in a randomized split-plot design with 4 replicates using cultivars as the main plot and 12.2 and 48.9 kg of N ha month [(NH4)2SO4] as the split plots. Flavonoid concentration decreased as the amount of applied N increased. Flavonoid concentration was greatest in June and steadily declined throughout July and August. Two unidentified flavonoids (luteolin-glycosides) were consistently associated with FAW mortality when analyzed by stepwise regression techniques. Unidentified luteolin 3 had an inverse relationship with mortality, while unidentified luteolin 9 was positively correlated with mortality. Additional research is needed to determine if the concentration of luteolins found in resistant zoysiagrasses are sufficient to cause biocidal activity in FAW. INTRODUCTION The fall armyworm (FAW) [Spodoptera frugiperda (J. E. Smith)] is a major pest of turfgrasses in the Americas and the Caribbean Basin. This pest is particularly damaging on turf in large contiguous areas such as pastures, sod farms, recreational parks and on golf courses. Larvae feed on above ground plant parts moving in mass across the landscape. Most established warm-season grasses will recover from FAW damage if properly cared for after an infestation. However, newly established turf is particularly susceptible to FAW damage due to its limited root system and carbohydrate reserves. Some cool-season grasses, including bluegrasses and fescues, can be permanently damaged if the meristematic region of the plant is consumed (Brandenburg and Villani, 1995). Many genotypes of turf and forage grasses have been reported to have varying levels of resistance to FAW as summarized by Reinert et al. (2004). Resistance to FAW in bermudagrass, (Cynodon dactylon Pers. var. ‘Tifton 292’), was first reported by Leuck Proc. II IC on Turfgrass Eds.: J.C. Stier et al. Acta Hort. 783, ISHS 2008 508 et al. (1968). Tifton 292 was reported to have high levels of non-preference and antibiosis to FAW (Lynch et al., 1983; Chang et al., 1985a). Chang et al. (1985b) reported wing pad deformities in FAW pupae and/or serious lesions in adult wings when Tifton 292 bermudagrass leaf extracts were incorporated into diet mixtures and fed to neonate FAW larvae. However, Jamjanya and Quisenberry (1988) found only moderate resistance in Tifton 292. Leuck and Skinner (1970) reported that mortality of FAW was greater on Georgia accession No. 239 bermudagrass than on >Coastal= bermudagrass. Resistance to FAW has also been documented in ‘Common’ centipedegrass [Eremochloa ophiuroides (Munro) Hackel] (Wiseman et al., 1982). Chang et al. (1985a) reported that the order of preference of FAW larvae for six grasses tested was ‘Tifton 10’ bermudagrass > Coastal bermudagrass > common centipedegrass > ‘C-181’ centipedegrass > Tifton 292 bermudagrass > zoysiagrass (no cultivars identified). In no-choice feeding experiments, Reinert (1994, 1997, 1998) determined that FAW larvae had higher mortality levels when fed excised leaf tissue of zoysiagrasses (Zoysia spp.) cultivars, ‘Cavalier’, ‘Meyer’, ‘El Toro’ and ‘Emerald’ compared to ‘Crowne’ and ‘Palisades’. Plants produce a wide array of naturally occurring chemicals that are believed to provide defense against herbivore and pathogen attack. Two groups of these naturally occurring chemicals that have been shown to have biocidal properties against FAW in laboratory bioassays are caffeic acid derivatives, primarily chlorogenic acid, and flavonoids such as maysin, luteolin, rutin and isoorientin. Research studies conducted on the interaction of these chemicals and insect biology vary in their results. Chlorogenic acid and flavonoid-glycosides added to diet media reduce FAW larval weights and increase FAW mortality in laboratory bioassay (Wiseman et al., 1982, 1992; Johnson et al., 2002). Wiseman et al. (1990) reported that chlorogenic acid, maysin and other luteolin-glycosides are the major factors for the antibiotic resistance of centipedegrass to FAW. Silks of Zapalote Chico, an exotic corn cultivar, contain high levels of maysin that severely reduced corn earworm [Helicoverpa zea (Boddie)] larval growth and development (Waiss et al., 1979). Maysin and other luteolins were reported to be responsible for resistance of teosinte, an ancestor of corn, to FAW (Gueldner, 1991). Cole (1984) reported that high levels of isochlorogenic acid in lettuce (Lactuca sativa L.) increased resistance to the root aphid [Pemphigus bursarius (L.)]. Johnson and Severson (1989) reported that the weights of larvae of the tobacco budworm [Heliothis virescens (F.)] were depressed by chlorogenic acid. However, Lindroth and Peterson (1988) reported that chlorogenic acid had no effect on southern armyworm [Spodoptera eridania (Cramer)]. The purpose of our study was to determine if previously observed variability in FAW resistance was associated with the concentrations of polyphenol and flavonoid in the zoysiagrass cultivars. Secondly, we wanted to determine the impact of N fertilization on the concentration of polyphenols and flavonoids in these cultivars. MATERIALS AND METHODS Plant Material The study was conducted at the Texas A&M University Field Laboratory, College Station, TX on a fine, montmorillontic, thermic Vertic Albaqualfs. Six zoysiagrasses cultivars {>Crowne= (Z. japonica Steud.), >Palisades= (Z. japonica Steud.), >El Toro= (Z. japonica Steud.), >Meyer= (Z. japonica Steud.), >Cavalier= [Z. matrella (L.) Merr.], and 'Emerald' (Z. japonica Stued. × Z. tenuifolia Willd. ex Trin.} were planted by sprigging on 7 May 1996 in a randomized split-plot design with 4 replicates using cultivars as the main plot and N treatments as the split plots. Grasses were grown for about 2 years prior to the initial tissue sampling for this study. Whole plots were 3.6 m wide and 5.5 m in length. N was applied as (NH4)2SO4 at 12.2 and 48.9 kg of N ha month to the respective split plots during the growing season beginning at planting and continuing throughout the duration of the experiment. The plots were irrigated twice each week during the growing season to replace 70% of evaporation from a Class A Pan to prevent