Kairomone Involvement in the Host Specificity of the Egg Parasitoid Trissolcus Basalis (Hymenoptera: Scelionidae)
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Eur. J. Entomol. 103: 311–318, 2006 ISSN 1210-5759 Kairomone involvement in the host specificity of the egg parasitoid Trissolcus basalis (Hymenoptera: Scelionidae) GIANANDREA SALERNO1, ERIC CONTI1, EZIO PERI2, STEFANO COLAZZA2 and FERDINANDO BIN1 1D.S.A.A. – Department of Agricultural and Environmental Sciences – Entomology, University of Perugia, Borgo XX Giugno, 06121 Perugia, Italy; e-mail: [email protected] 2Dep. S.En.Fi.Mi.Zo., University of Palermo, Viale delle Scienze 13, 90128 Palermo, Italy Key words. Scelionidae, Trissolcus basalis, egg parasitoid, Pentatomidae, kairomone, host selection, host specificity Abstract. This paper reports the results of a comparative laboratory analysis of the behavioural responses of the egg parasitoid Tris- solcus basalis (Wollaston) (Hymenoptera: Scelionidae) to semiochemical cues from four species of pentatomid bugs, Nezara viridula (L.), Eurydema ventrale Klt., Murgantia histrionica Hahn. and Graphosoma semipunctatum F. (Heteroptera: Pentatomidae). In a Y-tube olfactometer, T. basalis was attracted by volatile chemicals from N. viridula, but not from other pentatomid species. In an open arena, the parasitoid reacted to chemical trails left on filter paper by all the species but most intensely to those left by N. viridula. However, upon encountering pentatomid eggs, T. basalis examined more intensely and probed more frequently the eggs of G. semipunctatum than those of the other species. The parasitoid only parasitized and emerged from eggs of G. semipunctatum and N. viridula; those of the other species were unsuitable. Therefore N. viridula is semiochemically confirmed to be a coevolved host (old association) of T. basalis, whereas G. semipunctatum may be a potential non-coevolved host (new association). The utility of these tests for defining a parasitoids’ host specificity and in assessing the risk of non-target effects in biological control is discussed. INTRODUCTION host specificity tests, although a fundamental tool for the Insect parasitoids are important indicators of species practitioner, do not explain the mechanisms that deter- richness and are also often used as models for studies on mine host range. In contrast, studies of the chemical cues behavioural and chemical ecology. Female parasitoids involved in host/prey selection behaviour (Vet & Dicke, forage for host insects to provide food for their offspring. 1992; Vinson, 1998) provide a quantifiable characteriza- Female parasitoid foraging behaviour has been divided tion of host specificity. into a series of hierarchical steps: habitat preference, host Trissolcus basalis (Wollaston) (Hymenoptera: Scelioni- community location, host location, host recognition and dae), an egg parasitoid of the southern green stink bug, acceptance, and among the different ecological and Nezara viridula (L.) (Heteroptera: Pentatomidae), physiological factors involved, chemical cues influence responds to volatile chemicals and the chemical trails left all of these steps (Vet & Dicke, 1992; Vinson, 1998; by adult hosts (i.e., chemicals remaining on the substrate Steidle & van Loon, 2002). When dealing with host – egg where the host has walked) during host searching parasitoid associations, stimuli indirectly associated with (Colazza et al., 1999), even though adults are not the the presence of the host (Nordlund, 1994; Schmidt, 1994; stage it parasitizes. In addition, T. basalis responds to Vinson, 1998; Powell, 1999) and coming from the plant – volatile plant cues systemically induced by N. viridula host complex (Hilker & Mainers, 2002; Hilker et al., oviposition and feeding (Colazza et al., 2004a, b). The 2002) play a fundamental role. host-induced plant volatiles and adult volatiles are Recent research has shown the importance of host probably used to locate a potential host community, semiochemicals in determining host specificity in parasi- whereas the chemical trails left by adults and nymphs toids (Meiners et al., 2000; Conti et al., 2004). Such elicit parasitoid arrestment and stimulate searching. studies suggest new ways of evaluating and predicting the Finally, the process continues with host recognition, possible adverse side effects of biological control agents which is mediated by contact chemicals present in the on non-target species (Conti et al., 2004). Quantifying the ovariole secretion used by the host as an egg adhesive specificity of host-parasitoid associations is necessary for (Bin et al., 1993; Conti et al., 2003). Comparable cues are safe and successful biological control (Hågvar, 1991; used in other Trissolcus spp. – Pentatomid associations Nechols et al., 1992; FAO, 1995; van Lenteren, 1997; such as Trissolcus simoni (Mayr) – Eurydema ventrale Legner & Bellows, 1999; van Lenteren et al., 2003). Kolenati (the European harlequin bug) (Conti et al., 2003; Basic information on the level of specificity of a natural Colazza & Bin, pers. observ.) and Trissolcus brochy- enemy can be obtained from host records in the literature menae (Ashmead) – Murgantia histrionica Hahn (the (Nechols et al., 1992; Gordh & Beardsley, 1999) and American harlequin bug), where the parasitoid also sequential specificity tests (van Lenteren et al., 2003). responds to volatile and chemical trails from nymphs and However, host records are often poor and/or unreliable short-range volatiles from the host’s eggs (Conti et al., (Nechols et al., 1992; Gordh & Beardsley, 1999), whereas 2003). 311 According to the literature, T. basalis is associated with trionica. Water was provided weekly in soaked cotton wool at least 51 species of pentatomid bugs (Salerno, 2000). In balls. spite of its broad host range, this parasitoid has been used T. basalis was collected from fields in the area of Perugia, on a world-wide scale, with different degrees of success, Italy, using N. viridula sentinel egg clusters; i.e., laboratory laid egg clusters were exposed on plants of French bean, maize and for biological control of N. viridula (Jones, 1988; Jones et fava bean in the field. The parasitoid was reared in 85-ml glass al., 1996), a serious pest of more than 30 different crops tubes (30 mm diameter × 150 mm length) and kept in an incu- (Todd, 1989). bator (25 ± 1°C, 80 ± 5% RH, 15L : 9D photoperiod). Adult However, although many crops are also attacked by wasps were fed on a specific parasitoid diet (Safavi, 1968). other pentatomid bugs, T. basalis may not parasitize all Freshly laid (0–24 h old) host eggs were exposed to parasitoid species. For instance, on cabbage and other Cruciferae females for 48 h. Females were then removed and the eggs were this parasitoid parasitizes the southern green stink bug but stored for incubation. After emergence, male and female parasi- not the European harlequin bug (N.F. Johnson & L. toids were kept together to allow mating. Twenty-four h before Musetti, Hymenoptera On-Line Database, http://iris.bio- the experiments were started, 2–5 d old mated females were iso- sci. ohio-state.edu/hymenoptera/; Salerno, 2000). In the lated in small glass vials (10 diameter × 25 mm length) con- taining a small drop of the Safavi diet and kept in an incubator USA, however, T. basalis is occasionally recorded from (25 ± 1°C, 80 ± 5% RH, 15L : 9D photoperiod). the American harlequin bug (Buschman & Whitcomb, 1980). Conversely, although T. basalis has never been Parasitoid response to volatile chemical cues in a Y-tube olfactometer recorded from Graphosoma semipunctatum (F.) (Heter- optera: Pentatomidae) (N.F. Johnson & L. Musetti, The response of T. basalis females to volatile chemicals from Hymenoptera On-Line Database, http://iris.biosci. pentatomid bugs was evaluated in a Y-tube olfactometer, which consisted of a polycarbonate plate (15 mm thickness) with a ohio-state.edu/hymenoptera/; Salerno, 2000) in the field, Y-shaped space milled into the centre (stem 90 mm long; arms this parasitoid can be successfully mass reared on G. 80 mm long at 130° angle; internal section 15 × 15 mm) sand- semipunctatum eggs (Kartavtsev et al., 1975). Therefore, wiched between two glass plates. A 10 mm diameter hole was the question to be addressed here is: could such aspects of drilled through the block into the end of each arm to allow air host specificity be explained in terms of semiochemical tubes to be connected and the test organisms to be introduced. cues (host kairomones) influencing the different steps in Compressed air of medical-grade, regulated by flowmeters, ran host selection? through both arms creating an air stream of 144 ml/min per arm. A series of experiments were designed to verify the To humidify the air before it passed into the olfactometer, the host records reported in literature and understand how an flow was bubbled through a water jar. The olfactometer was illuminated by two 22-W cool white fluorescent tubes located incomplete host selection sequence may interfere with a above the device, which to minimize possible cues from the given host-parasitoid association. The behavioural room was surrounded by a black curtain. The temperature in the responses of T. basalis to chemical cues from four penta- bioassay room was maintained at |25°C. tomids: (1) the co-evolved host, N. viridula, (2) a non- During host location, the parasitoid T. basalis uses cues from host such as E. ventrale, (3) an occasional allopatric host different host instars, but shows a stronger response to gravid such as M. histrionica and (4) a laboratory host such as females (Colazza et al., 1999). Therefore, such pentatomid G. semipunctatum, were compared. In particular, T. females, which can be recognized by their enlarged and slightly basalis’ sequential responses to chemical volatiles and bloated abdomens, were bioassayed. For each experiment, one trails produced by gravid females (host location), and to female was caged in a small brass mesh box (25 × 15 mm) and chemical and physical factors associated with the host placed close to the air orifice at the end of one arm, randomly assigned. An empty box was placed at the end of the control eggs (host recognition) and with the accepted host (host arm. Each species (N. viridula, E. ventrale, M. histrionica or G. suitability) were determined.