Journal of Insect Physiology 45 (1999) 871–876 www.elsevier.com/locate/ibmbjip Electrophysiological responses of gustatory sensilla of Mamestra brassicae (Lepidoptera, Noctuidae) larvae to three ecdysteroids: ecdysone, 20-hydroxyecdysone and ponasterone A Charles Descoins Jr, Fre ´de ´ric Marion-Poll * INRA, Unite ´ de Phytopharmacie et Me ´diateurs chimiques, Route de Saint Cyr, 78026 Versailles Cedex, France Received 11 August 1998; accepted 23 November 1998 Abstract Specialised phytophagous Lepidoptera such as Bombyx mori and Pieris brassicae have contact chemoreceptors that perceive ecdysteroids at very low concentrations. This sensory perception allows them to feed on substrates with a high content of phytoecdys- teroids. We have evaluated if a polyphagous insect like Mamestra brassicae does possess contact chemoreceptor cells that are sensitive to these molecules. Electrophysiological recordings were performed from contact chemoreceptors located on the maxilla. These receptors were stimulated with some sugars, amino acids and salts and with three ecdysteroids. Our results demonstrate that a specific cell within the lateral sensilla responds to 20-hydroxyecdysone and ponasterone A but not to ecdysone.  1999 Elsevier Science Ltd. All rights reserved. Keywords: Ecdysteroids; Gustation; Mamestra brassicae; Electrophysiology 1. Introduction Phytoecdysteroids are secondary metabolites widely distributed in the plant kingdom. Their structures are closely related to those found in ecdysteroids involved in the regulation of the moulting process in arthropods (Lafont, 1997). Over 200 ecdysteroids occurring in plants are yet described; the most common are 20-hyd- roxyecdysone and polypodine B. Their concentrations vary over a wide range (up to 10 -3 M) and about 6% of the plants contain high amounts of ecdysteroids (Imai et al., 1969; Dinan, 1995). In plants, the role of these metabolites is not clear (Jones and Firn, 1978; Sla ´ma, 1993). The most generally accepted hypothesis is that they protect against attacks by phytophagous insects, either by disturbing development or reducing food intake (Bergamasco and Horn, 1985; Camps, 1991; Lafont, 1997). Reduction of food intake in the presence of ecdystero- * Corresponding author. Tel.: + 33 1 30 83 31 45; fax: + 33 1 30 83 31 49. E-mail address: marion@versailles.inra.fr (F. Marion-Poll) 0022-1910/99/$ - see front matter.  1999 Elsevier Science Ltd. All rights reserved. PII:S0022-1910(99)00057-8 ids has been reported only in Pieris brassicae (Ma, 1969) and Bombyx mori (Tanaka et al., 1994). These authors demonstrated that feeding inhibition was due to a specialised sensory perception rather than to a poisoning effect. Both species have specialised diets, and their host plants contain only low levels of ecdysteroids (Morus alba: Takemoto et al., 1967; Blackford and Dinan, 1997) while plants containing high level of ecdysteroids are numerous (Lafont, 1997). What happens in species with a less specialised diet and thus a greater chance of exposure to plants containing high amounts of ecdystero- ids? We have tried to answer this question by investigating the sensory perception to three phytoecdysteroids in the cabbage moth Mamestra brassicae L. This species is relatively tolerant of ecdysteroids in the diet (Tanaka and Naya, 1995). Food choice tests conducted with different dosages of 20E (Ma, 1972), have shown that P. bras- sicae larvae were avoiding food treated with 20E 2×10 -4 M (feeding ratio of treated substrate vs control: 4%). At the same dosage, feeding was much less affected in M. brassicae (feeding ratio: 50%), while higher doses of 20E yielded a maximal feeding ratio of 40%. These observations suggest that M. brassicae larvae