OIKOS 100: 65–70, 2003 Searching behaviour of the sevenspotted ladybird, Coccinella septempunctata – effects of plant-plant odour interaction Velemir Ninkovic and Jan Pettersson Ninkovic, V. and Pettersson, J. 2003. Searching behaviour of the sevenspotted ladybird, Coccinella septempunctata – effects of plant–plant odour interaction. – Oikos 100: 65–70. In the present study the main aim was to investigate the odour mechanisms affecting habitat preferences of Coccinella septempunctata (L.). In a field study, the frequency of adult C. septempunctata was higher in barley plots containing high densities of the common weeds Cirsium arense (L.) Scop. and Elytrigia repens (L.) Nevski. than in control plots with only barley. In olfactometer experiments in the laboratory, adult C. septempunctata showed a significantly more positive response to mixed odours of barley and each of the two weeds than to barley alone. Ladybirds responded differently to barley plants that were previously exposed to volatiles from the two weeds. The E. repens -exposed barley plant lost its attractivity while the C. arense -ex- posed barley plants maintained attractivity. As no aphids or pollen resources were present in the plots during the experiment, the results show that C. septempunctata responds to the botanical characteristics of the habitat even if no food resources are available. These results strongly suggest that olfactory cues and plant-plant commu- nication from diversified plant stands can be important mechanisms in predator attraction to sites with a complex botanical diversity. V. Ninkoic and J. Pettersson, Dept of Entomology, Swedish Uni. of Agric. Sci., SLU, P.O. Box 7044, SE-750 07 Uppsala, Sweden (elemir.ninkoic@entom.slu.se). It is often stated that natural enemies are more effective at controling herbivores in more diverse botanical ecosystems (Letourneau 1987, Vandermeer 1989, Andow 1991). However, the mechanisms behind this are only partly known. This is often used as one of the general explanations of reduced pest problems in mixed cropping (Root 1973, Trujillo-Arriaga and Altieri 1990). An alternative or contributing factor could be increased difficulties for the pest insect to find its host plants (Feeny 1976, Vandermeer 1989). The searching behaviour of polyphagous predators, exemplified by ladybirds (Coccinellidae), is a combina- tion of randomised movements and responses to a wide range of stimuli (Kesten 1969, Nakamuta 1984, Meiracker et al. 1990) including olfactory cues (Obata 1986, 1997, Ponsonby and Copland 1995, Zhu et al. 1999, Ninkovic et al. 2001). The searching efficiency of herbivores may be reduced in mixed plant stands as a result of the interference of non-host plant volatiles with odours from the preferred host plant (Thie ´ry and Visser 1987). To what extent this effect is relevant to the searching behaviour of predators is still poorly understood. Herbivore-attacked plants release chemical signals that serve as important olfactory cues for parasitoids (Grasswits and Paine 1993, Du et al. 1998, DeMoraes et al. 1998, Powell et al. 1998, Guerrieri et al. 1999, Van Loon et al. 2000) and predators (Vet and Dicke 1992, Drukker et al. 1995, Bruin et al. 1992, 1995, Dicke 1999, Sabelis et al. 1999, Ninkovic et al. 2001). Volatiles from an insect-attacked plant may also induce a re- sponse in neighbouring, non-attacked plants, making them less attractive to herbivores (Bruin et al. 1992, 1995, Pettersson et al. 1996, Bruin and Dicke 2001, Farmer 2001). Furthermore, some natural enemies can use volatiles emitted by undamaged plants to locate the Accepted 1 July 2002 Copyright © OIKOS 2003 ISSN 0030-1299 OIKOS 100:1 (2003) 65