JAE zyxwvutsrqponm 121 (1997) zyxwvutsrqpo -. J. Appl. Ent. 121. 261-269 (1997) GSN 093 1-2048 1997, Blackwell Wissenschafts-Verlag, Berlin zyxwvutsr Host-finding behaviour of zyxwv Prostephanus fruncatus (Horn) (Col., Bostrichidae): primary attraction or random attack? D. Scholz, A. Tchabi, C. Borgemeister, R. H. Markham, H.-M. Poehling and A. Lawson International Institute of Tropical Agriculture, Plant Health Management Division, Cotonou. Republic of Benin Abstract: Host-finding behaviour of zyxwvutsrq Prostephanus truncatus was evaluated in a four-choice olfactometer for reactions to odours of maize, cassava chips, wheat, cowpea (a non-host plant), and woody plant species in which reproduction has been observed, and of organisms associated with maize stores. P. truncatus reacted in general to odours from starchy commodities and to some of the woody plants. Beetles did not respond to volatiles from cowpea or organisms associated with maize stores, but did react to the aggregation pheromone produced by single male P. truncatus (secondary attraction). P. truncatus raised on cassava and emigrants from maize responded as strongly to maize odours as beetles raised on maize, whereas P. truncatus trapped with pheromone traps showed reduced or no reactions. Stored commodities seem to elicit short-range arrestment, but not primary attraction at long range. However, stored products might only be attacked facultatively by P. truncatus, co-evolved primary attraction could possibly exist to the native host-plant complex. 1 Introduction The larger grain borer, Prostephanus truncatus has long been known as a sporadic pest of stored maize in Central America and Mexico (GILES and LEON, 1974). Its acci- dental introduction to East and West Africa in the late 1970s and early 1980s (DUNSTAN and MAGAZINI, 1981; KRALL, 1984) and its rapid spread (HODGES, 1994; ADDA et al., 1996) prompted numerous research activities to control this destructive pest of stored maize and dried cassava by chemical and biological means (MARKHAM et al., 1991). However, the successful implementation of integrated pest management strategies requires a tho- rough understanding of a pest’s population dynamics and especially of its host-selection behaviour (HODGES, 1994; MARKHAM et al., 1994) Secondary attraction is well documented for P. trun- catus. On a suitable food source and in the absence of females, male P. truncatus produce an aggregation pheromone (SMITH et al., 1996), which attracts male and female conspecifics (OBENG-OFORI and COAKER, 1990). The two main components of the pheromone have been identified (CORK et al., 1991) and are mainly used for monitoring purposes (RICHTER and BILIWA, 1991; TICAR et al., 1994). However, factors influencing the initial attack have not been investigated satisfactorily yet. P. truncatus responded to odours emitted from maize grains and dried cassava in electroantennogram studies and in short-distance bioassays (PIKE et al., 1994), but neither the single nor the mixed attractive components identified for maize grains nor whole maize grains have been found to be attractive in wind tunnel trials (FAD- AMIRO, 1995). Maize cobs used as baits in field trials did not attract any P. truncatus (WRIGHT et al., 1993; M. RAMIREZ-MART~NEZ, pers. comm.). P. truncatus belongs to a family of wood-boring beetles, can be caught with pheromone traps in non-farmland, both in the neo- tropics and in Africa (REES et al., 1990; NANG’AYO et al., 1993; GILES et al., 1995) and has been found to reproduce in certain woody plant species in laboratory experiments (NANG‘AYO et al., 1993) and in the field (RAM~REZ-MARTINEZ et al., 1994; F. NANG’AYO, pers. comm.). It is therefore likely that the beetle is only facultatively associated with stored commodities like maize and cassava. We were therefore interested in verifying whether the behavioural responses of P. truncatus to maize odours are influenced by the beetle’s origin or physiological state and if its reactions are plant specific. We also evaluated the effects of other odour sources associated with maize stores in coastal West Africa, as well as odours from suspected woody host plants. 2 Material and methods 2.1 Insects Initial laboratory cultures of P. truncatus and Sitophilus zea- mais Motsch. (Col., Curculionidae) were set up with indi- viduals collected in the Mono District of south-western Benin in 1992. Cultures of the predator Teretriosoma nigrescens Lewis (Col., Histeridae) were started with specimens of the same geographical origin, being progeny of the population which was released in Togo from 1991 onwards in the course of a classical biological control programme (BOEYE et al., 1994). Basic cultures were mixed several times with insects from the same region. The insects were reared on 350 g maize grains (cv. TZSR-W) in 1000 ml glass jars covered with brass gauze at 30 zyxwv f 1°C and 75 f 5% r.h. under a L12:D12 pho- toperiod. The maize was infested with 200 unsexed adult P. truncatus or zyxwvu S. zeamais, all of which were removed after 2 weeks to obtain a synchronized F, . T. nigrescens cultures were set up with 500 adult P. truncatus, and 10 days later 50 adult T. nigrescens were added, which were removed after 2 weeks. Mussidia nigrivenella Ragonot (Lep., Pyralidae) was reared u. S. Copyright Clearance Center Code Statement: 0931-2048/97/21054261 $ 14.00/0