Cellular encapsulation in the eastern subterranean termite, Reticulitermes flavipes (Isoptera), against infection by the entomopathogenic fungus Metarhizium anisopliae Thomas Chouvenc a, * , Nan-Yao Su a , Alain Robert b,1 a Department of Entomology and Nematology, Ft. Lauderdale Research and Education Center, University of Florida, Institute of Food and Agricultural Sciences, 3205 College Ave. Davie, FL 33314, USA b UMR 5548 Communication chimique et développement chez les insectes, Université de Bourgogne, 6 Bd. Gabriel, 21000 Dijon, France article info Article history: Received 11 March 2009 Accepted 14 May 2009 Available online 20 May 2009 Keywords: Termite Metarhizium Disease resistance Cellular encapsulation Hemocytes abstract Reticulitermes flavipes workers were topically inoculated with 10,000 conidia of the entomopathogenic fungus Metarhizium anisopliae. After being kept in groups of 20 individuals for 1–9 d, histopathological examination showed that termites had an individual immune reaction. The nodule formation at the point of entrance of the fungal hyphae was identified as a cellular encapsulation and the different steps in the nodule formation are described. The relative number of hemocytes per termite increased 24 h after fungal exposure and remained high in the hemolymph for at least 3 d before decreasing back to pre-exposure levels. The role of an individual immune cellular reaction in social insects is discussed. Ó 2009 Elsevier Inc. All rights reserved. 1. Introduction Reticulitermes flavipes (Isoptera: Rhinotermitidae) is a subterra- nean termite species with a large economical impact as a structural pest (Su and Scheffrahn, 1998). It usually lives in soil that is a favorable habitat for microbial growth and eventually infests buildings. For this reason, the entomopathogenic fungus Metarhiz- ium anisopliae has been considered by many for biological control of subterranean termites (Lai, 1977; Milner and Staples, 1996; Culliney and Grace, 2000), but despite multiple promising labora- tory studies, there has been limited success in using M. anisopliae as a biological control agent against subterranean termites in the field (Lai, 1977; Rath, 2000). A successful epizootic in an insect population depends on optimum host-pathogen-environment interactions and, it is necessary to understand the factors respon- sible for insect susceptibility or resistance to a pathogen if the pathogen is intended for use as a biological control agent (Hajek and St. Leger, 1994). Termite defense mechanisms against fungal pathogens have attracted greater interest in recent years, espe- cially those involving behavioral processes and social interactions such as repellency (Rosengaus et al., 1999a; Myles, 2002a; Wang and Powell, 2004), allogrooming (Rosengaus et al., 1998b; Shimizu and Yamaji, 2003; Yanagawa and Shimizu, 2007) and necrophagy (Jones et al., 1996; Chouvenc et al., 2008). Several reviews have re- cently emphasized the complex interaction of defense mechanisms in social insects, including some chemical and immunological fac- tors (Cremer et al., 2007; Feldhaar and Gross, 2008; Wilson-Rich et al., 2009). Recently, significant progress has been made in gen- eral knowledge about insect immunity (Ferrandon et al., 2007), especially in dipteran and lepidopteran insect models such as Dro- sophila melanogaster, Anopheles gambiae, Bombyx mori, Hyalophora cecropia, and Manduca sexta. Various aspects of the innate humoral and cellular immunity mechanisms against various bacterial and fungal pathogens were described at the molecular level in these species (Gillespie et al., 1997; Trenczek, 1998; Vilcinskas and Götz, 1999; Asano and Ashida, 2001; Bulet et al., 2002; Hoffmann and Reichhart, 2002; Hoffmann, 2003; Cerenius and Söderhäll, 2004; Schmid-Hempel, 2005; Stanley, 2006; Feldhaar and Gross, 2008). Although the mechanisms of innate immunity are well docu- mented in Diptera and Lepidoptera, the understanding of individ- ual immunity in Isoptera remains fragmentary because most of the previous studies mainly focused on collective immune defense mechanisms (Cremer et al., 2007). Cellular encapsulation and nod- ule formation processes against fungal entomopathogen infection have been described in many other insects (Vey and Götz, 1986; Gunnarsson, 1988; Lavine and Strand, 2002). However, cellular encapsulation in Isoptera is still poorly documented. Bao and Yendol (1971) and Kramm and West (1982) studied the histopa- thology of entomopathogens in Reticulitermes, but did not mention the occurrence of cellular encapsulation. More surprisingly, Sajap 0022-2011/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jip.2009.05.008 * Corresponding author. Fax: +954 475 4125. E-mail address: tomchouv@ufl.edu (T. Chouvenc). 1 Present address: Centre IRD France Nord, 32 avenue Varagnat, 93143 Bondy, France. Journal of Invertebrate Pathology 101 (2009) 234–241 Contents lists available at ScienceDirect Journal of Invertebrate Pathology journal homepage: www.elsevier.com/locate/yjipa