REVIEW:INSECT-SYMBIONT INTERACTIONS Endophytic Fungus–Vascular Plant–Insect Interactions A. RAMAN, 1 W. WHEATLEY, 2 AND A. POPAY 3 Environ. Entomol. 41(3): 433Ð447 (2012); DOI: http://dx.doi.org/10.1603/EN11317 ABSTRACT Insect association with fungi has a long history. Theories dealing with the evolution of insect herbivory indicate that insects used microbes including fungi as their principal food materials before ßowering plants evolved. Subtlety and the level of intricacy in the interactions between insects and fungi indicate symbiosis as the predominant ecological pattern. The nature of the symbiotic interaction that occurs between two organisms (the insect and the fungus), may be either mutualistic or parasitic, or between these two extremes. However, the triangular relationship involving three organisms, viz., an insect, a fungus, and a vascular plant is a relationship that is more complicated than what can be described as either mutualism or parasitism, and may represent facets of both. Recent research has revealed such a complex relationship in the vertically transmitted type-I endophytes living within agriculturally important grasses and the pestiferous insects that attack them. The intricacy of the association depends on the endophytic fungusÐ grass association and the insect present. Secondary compounds produced in the endophytic fungusÐ grass association can provide grasses with resistance to herbivores resulting in mutualistic relationship between the fungus and the plant that has negative consequences for herbivorous insects. The horizontally transmitted nongrass type-II endophytes are far less well studied and as such their ecological roles are not fully understood. This forum article explores the intricacy of dependence in such complex triangular relationships drawing from well-established examples from the fungi that live as endophytes in vascular plants and how they impact on the biology and evolution of free-living as well as concealed (e.g., gall-inducing, gall- inhabiting) insects. Recent developments with the inoculation of strains of type-I fungal endophytes into grasses and their commercialization are discussed, along with the possible roles the endophytic fungi play in the galls induced by the Cecidomyiidae (Diptera). KEY WORDS chemical ecology, fungal endophytes, gall induction, infection, insects Insect association with fungi has a long history (Shear et al. 1984). Superimposition of established phyloge- netic trees (Kristensen 1981) and feeding habits of diverse groups of insects (Gillott 1980) reveal detri- tivory as the ancestral condition (Jolivet 1998), which, with time, evolved into carnivory and phytophagy, through microphagy (Labandeira 1997). We know today that many of the larvae belonging to Diptera (e.g., Drosophilidae; Romoser, and Stoffolano 1998) are microphagous and not detritivorous. Modern Co- leoptera display diverse feeding behaviorsÑfrom obligatory herbivory to obligatory carnivoryÑand they too have evolved from a microphagous (my- cophagous, to be exact) habit (Leschen 2000). Among extant insects, the Coleoptera, Diptera, Hemiptera, Hymenoptera, and Isoptera are the principal orders that bear an intimate association with fungi. The in- sect-associated fungi, however, are mostly Ascomyce- tes within Hypocreales (e.g., Beauveria, Metarrhizium, Fusarium), the ambrosia fungi (e.g., Ophiostoma, Cera- tosystis), species of Laboulbeniomycetes, and Saccha- romycetes (Bourtzis and Miller 2003). Historically, insects obtained sugars from plant rusts (e.g., Uredi- nales); modern species of Uredinales not only provide insects with a nectar reward, but also lure them through various volatiles with strong diffusing odors (Stoffolano 1995). Volatiles produced by fungi, in gen- eral, attract female insects toward oviposition (Honda et al. 1988). Mycophagous insects rely on fungi as a key source of proteins and fat derivatives. Xylem-feeding Cerambicidae (Coleoptera), Siricidae (Hymenop- tera), and Termitidae (Isoptera) accomplish cellulose digestion aided by the preingested fungal cellulase that remains active in their guts (Barbehenn et al. 1999, Bignell 2000). Mutualism emerges as the predominant ecological pattern in the interactions between insects and fungi. Symbiotic yeasts and unicellular fungi (and bacteria) facilitate phytophagous sap-feeding insects (e.g., Hemiptera) to digest plant food consumed by the insect and even activate the synthesis of some of the nutritional supplements such as sterols, vitamins, and amino acids, which the principal host plants provide This review is an invited paper. 1 Corresponding author: Charles Sturt University & E H Graham Centre for Agricultural Innovation, Orange, New South Wales 2800, Australia (e-mail: araman@csu.edu.au). 2 Charles Sturt University, Orange, New South Wales 2800, Aus- tralia. 3 AgResearch Limited, Hamilton, New Zealand. 0046-225X/12/0433Ð0447$04.00/0  2012 Entomological Society of America