Effect of Switchgrass Ecotype and Cultivar on Establishment, Feeding, and Development of Fall Armyworm (Lepidoptera: Noctuidae)

Plant Resistance Journal of Economic Entomology, 112(1), 2019, 440–449 doi: 10.1093/jee/toy292 Advance Access Publication Date: 22 October 2018 Research Effect of Switchgrass Ecotype and Cultivar on Establishment, Feeding, and Development of Fall Armyworm (Lepidoptera: Noctuidae) Marissa K. Schuh,1,2,5 Christie A. Bahlai,3 Carolyn M. Malmstrom,4 and Douglas A. Landis1 Subject Editor: John Trumble Received 13 June 2018; Editorial decision 31 August 2018 Abstract As interest in production of second-generation biofuels increases, dedicated biomass crops are likely to be called upon to help meet feedstock demands. Switchgrass (Panicum virgatum L.) is a North American native perennial grass that as a candidate biomass crop, combines high biomass yields with other desirable ecosystem services. At present, switchgrass is produced on limited acres in the United States and experiences relatively minor insect pest problems. However, as switchgrass undergoes breeding to increase biomass yield and quality, and is grown on more acres, insect pest pressure will probably increase.To investigate how currently available switchgrass ecotypes and cultivars may influence herbivory by generalist insect herbivores, we performed feeding trials using neonate and late-instar fall armyworm [Spodoptera frugiperda JE Smith (Lepidoptera: Noctuidae)]. No-choice feeding experiments were used to explore how switchgrass varieties influence larval establishment, consumption levels, and life-history traits in contrast to a preferred host, corn (Zea mays L.). Neonate S. frugiperda consumed greater amounts of corn than switchgrass and increased amounts of upland versus lowland ecotypes. Late-instar larvae, which do the majority of the larval feeding, exhibited lower consumption of lowland ecotypes, which led to increased development time and reduced pupal weights. The exception to these trends was the upland cultivar ‘Trailblazer’, which unexpectedly performed similarly to lowland cultivars. These results suggest that both switchgrass ecotype and cultivar can influence feeding damage by a common generalist herbivore. These findings can be used to help inform current switchgrass planting decisions as well as future breeding efforts. Key words: bioenergy cropping systems, switchgrass herbivory, plant breeding Switchgrass (Panicum virgatum L.) is a perennial C4 grass native to much of the central and eastern United States (Casler 2012). An ecologically important prairie plant, switchgrass has also been bred for forage production and, more recently, as a model bioenergy feedstock (Wright et al. 1993). Switchgrass has several advantages as a biomass crop including its large native range (Parrish and Fike 2005), effective use of nutrients (McLaughlin and Walsh 1998), and high biomass yields, even on marginal lands (Fike et al. 2006, Schmer et al. 2008). Another benefit is that switchgrass is generally resistant to insect herbivory, although when planted as monocultures grasses may become more susceptible to pest outbreaks (Tscharntke and Greiler 1995). Indeed, as more switchgrass has been planted as part of breeding efforts in the United States, several insect pests of switchgrass have been identified (Prasifka et al. 2010; Calles Torrez et al. 2013, 2014). Moreover, as high-yielding cultivars of switchgrass are developed, selection for increased growth rate may lead to reduced plant defenses (Herms and Mattson 1992). Insects that feed on switchgrass and may emerge as pests include thrips (Thysanura; Gottwald and Adam 1998), grasshoppers (Orthoptera; Casler et al. 2004), aphids (Aphididae; Bradshaw et al. 2010, Schrotenboer et al. 2011, Burd et al. 2012), leafhoppers (Cicadellidae; Holguin et al. 2010), and lepidopteran (Lepidoptera) larvae (Prasifka et al. 2009, 2011b; Calles Torrez et al. 2013). An important factor in switchgrass breeding is consideration of parental ecotype, a term used to differentiate between forms in a species adapted to different environmental conditions (Gregor 1944, © The Author(s) 2018. Published by Oxford University Press on behalf of Entomological Society of America. All rights reserved. For permissions, please e-mail: . 440 1 Department of Entomology, Michigan State University, 578 Wilson Road, Room 204, East Lansing, MI 48824, USA, 2Current address: Michigan State University Extension, 1040 South Winter Street, Suite 2020, Adrian, MI 49221, USA, 3Department of Biological Sciences, Kent State University, 1275 University Esplanade, Room 209, Kent, OH 44243, USA, 4Department of Plant Biology and Graduate Program in Ecology, Evolutionary Biology, and Behavior, Michigan State University, 612 Wilson Road, Room 166, East Lansing, MI 48824, USA, and 5Corresponding author, e-mail: Journal of Economic Entomology, 2019, Vol. 112, No. 1 441 well known. Prasifka et al. (2009) showed that fall armyworm survived through pupation on switchgrass at rates similar to that of Bermuda grass, a host on which fall armyworm is a known economic pest (Leuck et al. 1968, Prasifka et al. 2009). Dowd and Johnson (2009) showed that across six upland cultivars, neonate fall armyworm mortality during initial establishment was minimal; however, subsequent leaf consumption varied significantly by cultivar. Here, we use fall armyworm as a model to investigate how differences among switchgrass cultivars may influence the survival, feeding patterns, and development of a generalist insect herbivore. We compared rates of neonate establishment and herbivory on corn (Zea mays L.), a known host plant for fall armyworm, versus 16 switchgrass cultivars representing a range of ecotypes and breeding histories. We further compared developmental rates and survival of late-instar fall armyworm on a subset of eight switchgrass cultivars. Based on ecotype differences in leaf traits, we hypothesized that southern lowland switchgrass, which have tougher, more structurally defended leaves, would be less favorable hosts for fall armyworm than northern upland cultivars/populations. Specifically, we predicted that on southern lowland switchgrass fall armyworm would consume less leaf area, develop more slowly, and experience greater mortality. Materials and Methods Plants Switchgrass cultivars for all tests were established from seed and transplanted into the field in spring of 2014 (Table 1). Seeds were cold stratified in distilled water at 4°C and germinated in plug trays (X-72PS, Landmark Plastic Corp, Akron, OH) under high light conditions (16:8 [L:D] day length) in a plant growth chamber at 26°C for 2 wk. Trays were then moved to a greenhouse until seedlings were approximately 0.3 m tall and then acclimated to outdoor conditions in an outdoor courtyard for 1 wk. In mid-June 2014, plants were transplanted to a field (alfisol soil, Marlette sandy loam) at the Michigan State Entomology Farm (East Lansing, MI). The experiment was established as a randomized complete block design with each cultivar represented by two pla (...truncated)