Do Fecal Shields and Herbivore-Caused Volatiles Match Host Plant Chemical Profiles?
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Chemoecology (2013) 23:1–11 DOI 10.1007/s00049-012-0117-7 CHEMOECOLOGY RESEARCH PAPER Making scents of defense: do fecal shields and herbivore-caused volatiles match host plant chemical profiles? Ken Keefover-Ring Received: 18 March 2012 / Accepted: 26 September 2012 / Published online: 13 October 2012 Ó Springer Basel 2012 Abstract Many plant families have aromatic species that shields and volatile emissions are influenced by the produce volatile compounds which they release when amounts and volatilities of compounds present in the host damaged, particularly after suffering herbivory. Monarda plant. The implications of these results are explored for the fistulosa (Lamiaceae) makes and stores volatile essential chemical ecology of both the plant and the insect. oils in peltate glandular trichomes on leaf and floral sur- faces. This study examined the larvae of a specialist Keywords Aromatic plants Á Herbivore-caused volatiles Á tortoise beetle, Physonota unipunctata, which feed on two Larval fecal shields Á Monarda fistulosa Á M. fistulosa chemotypes and incorporate host compounds Physonota unipunctata Á Terpenoids into fecal shields, structures related to defense. Compari- sons of shield and host leaf chemistry showed differences between chemotypes and structures (leaves vs. shields). Introduction Thymol chemotype leaves and shields contained more of all compounds that differed than did carvacrol chemotypes, Plants emit volatiles when fed upon by insects. The vola- except for carvacrol. Shields had lower levels of most of tiles resulting from herbivory for many plant species the more volatile chemicals than leaves, but more than consist of various mono- and sesquiterpenoids, heterocy- twice the amounts of the phenolic monoterpenes thymol clic compounds such as indole, and saturated and and carvacrol and greater totals. Additional experiments unsaturated six-carbon oxygenated compounds known as measured the volatiles emitted from M. fistulosa in the ‘‘green-leaf’’ volatiles (Pare and Tumlinson 1999). These absence and presence of P. unipunctata larvae and com- chemicals often mediate interactions between the plants pared the flower and foliage chemistry of plants from these that emit them and other organisms, including the attrac- experiments. Flowers contained lower or equal amounts of tion of herbivore natural enemies (De Moraes et al. 1998), most compounds and half the total amount, compared to or the deterrence (Bernasconi et al. 1998; De Moraes et al. leaves. Plants subjected to herbivory emitted higher levels 2001; Zebelo et al. 2011) and attraction of additional her- of most volatiles and 12 times the total amount, versus bivores (Kalberer et al. 2001; Heisswolf et al. 2007; Zebelo controls with no larvae, including proportionally more of et al. 2011). Different herbivores elicit varying amounts the low boiling point chemicals. Thus, chemical profiles of and types of volatiles with the highest levels released by plants attacked by chewing insects, as opposed to much lower amounts from piercing-sucking herbivores (Turlings & K. Keefover-Ring ( ) et al. 1998). Department of Ecology and Evolutionary Biology, University of Colorado, N122 Ramaley, Boulder, CO 80309, USA The volatiles emitted by aromatic plants during her- e-mail: [email protected] bivory have received less attention (Degenhardt and Lincoln 2006; Kannaste et al. 2008; Zebelo et al. 2011). Present Address: Yet, many plant families (Apiaceae, Pinaceae, Rutacae, K. Keefover-Ring Umea˚ Plant Science Centre, Department of Plant Physiology, Solanaceae, etc.) have aromatic species that produce a wide Umea˚ University, Umea˚ 90187, Sweden array of volatile compounds, usually made and stored in 123 2 K. Keefover-Ring specialized structures. Herbivore damage to tissues con- taining these structures liberates large amounts of volatiles (Degenhardt and Lincoln 2006; Kannaste et al. 2008; Zebelo et al. 2011). The Lamiaceae is a family especially well known for aromatic plants (e.g. thyme, lavender, peppermint). Most species in this family make mono- and sesquiterpenes, commonly referred to as essential oils, constitutively on the surface of leaves, reproductive parts, and stems, mostly in Fig. 1 The four major monoterpenes found in the essential oil of peltate glandular trichomes consisting of secretory cells Monarda fistulosa thymol (T) and carvacrol (C) chemotypes mounted on a stalk which produce the terpenes and secrete them into an outer subcuticular sac for storage (Turner unpublished data). Both of these compounds deter a greater et al. 2000; Croteau 2001). Any damage, including her- variety of pathogens and parasites than do other non-phe- bivory, to plant parts containing these trichomes results in nolic monoterpenes (Linhart and Thompson 1995, 1999), their rupture, content release, and subsequent volatilization. yet, P. unipunctata readily feeds on plants of both chem- Once released mono- and sesquiterpenes often act as otypes (Keefover-Ring, unpublished data). Thus, these semiochemicals, an ideal role given their volatility and beetles may benefit defensively from host chemicals that structural diversity (Gershenzon and Dudareva 2007). they incorporate into their fecal shields, but the increased Despite the relatively large amounts of secondary volatiles emitted during feeding may attract their natural compounds found in aromatic plants, some specialist enemies or additional competitive herbivores. insects have evolved the ability to tolerate these chemicals. To begin to understand the chemically mediated interac- This has certainly been the case with the Chrysomelidae tions between P. unipunctata and its host plant, M. fistulosa, family, which contains many species that not only eat the this study addresses two fundamental questions: (1) How foliage of plants containing high amounts of secondary does the chemical composition of larval fecal shields chemicals, but often also use these compounds for their compare to that of the foliage of its host plant, and; (2) own defense (Pasteels et al. 1983; Morton and Vencl 1998; What quantitative and qualitative differences exist between Vencl and Morton 1998; Becerra et al. 2001; Weiss 2006). the volatile profiles released from host plants with or The subfamily Cassidinae, known as tortoise beetles due to without feeding by the larvae of this dietary specialist? the shell-like appearance of the adults, comprise one group of Chrysomelids noted for using host chemistry for pro- tective purposes. In many species larvae accumulate a fecal Methods and materials shield, consisting of frass sometimes combined with exu- via, and often rich in host plant secondary compounds, on a Study organisms caudal fork (Go´mez et al. 1999; Vencl et al. 1999; Vencl et al. 2005; Weiss 2006; Chaboo 2007). While cassidine Monarda fistulosa L. var. menthifolia (Graham) Fernald fecal shields effectively defend larvae against natural (Lamiaceae), commonly known as wild bergamot, bee enemies (Olmstead and Denno 1993;Go´mez et al. 1999; balm, or horse mint, is a perennial labiate that occurs Vencl et al. 1999; Eisner and Eisner 2000; Bacher and throughout much of the western half of North America Luder 2005; Vencl et al. 2005, 2009), other evidence (USDA 2012). Individual plants consist of multiple ramets, indicates that the volatiles from chrysomelid fecal shields up to 1 m high, which arise directly from the ground. Each also attract natural enemies (Mu¨ller and Hilker 1999; ramet has one to several terminal capitate inflorescences Schaffner and Mu¨ller 2001). consisting of lavender-colored tubular flowers subtended The tortoise beetle Physonota unipunctata uses Mon- by leaf-like bracts. Below the capitula the stem has alter- arda fistulosa as its only host (Hamilton 1884; Criddle nating pairs of lanceolate leaves with serrated margins. 1926; Sanderson 1948). Like most labiates, M. fistulosa In Colorado, M. fistulosa has two main essential oil produces essential oils in trichomes on leaves and repro- chemotypes, with individual plants producing either thy- ductive parts (Heinrich 1973). Throughout much of its mol (T) or carvacrol (C) as their most abundant terpenoid range, plants belong to one of two chemical phenotypes (Fig. 1; Keefover-Ring, unpublished data). In addition to (chemotypes), producing either of the phenolic (six-mem- their dominant monoterpene, both chemotypes contain bered aromatic ring with an hydroxyl side group) numerous minor components, including relatively high monoterpenes thymol or carvacrol as the dominant com- amounts of c-terpinene and p-cymene (Fig. 1; Scora 1967; ponents of their essential oil (Fig. 1; Scora 1967; Weaver Weaver et al. 1995; Johnson et al. 1998; Keefover-Ring, et al. 1995; Johnson et al. 1998; Keefover-Ring, unpublished data). The aliphatic monoterpene c-terpinene 123 Making scents of defense 3 serves as a biosynthetic precursor for the aromatic p-cymene, Larval fecal shield and host plant chemistry which then undergoes hydroxylation to produce either thymol or carvacrol (Poulose and Croteau 1978a; b;R.Croteau,pers. To compare the essential oil chemistry of larval fecal comm.). shields with that of the leaves that larvae feed upon, I Monarda fistulosa has few reported herbivores (Davis extracted essential oils from both leaves and shields and et al. 1988; Wyckhuys et al. 2007). However, one specialist analyzed the resulting solutions with gas chromatography found on the species is the horsemint tortoise beetle, or (GC) with a flame ionization detector (FID). I collected one-spotted tortoise beetle,