The Journal of Experimental Biology 1692 © 2014. Published by The Company of Biologists Ltd | The Journal of Experimental Biology (2014) 217, 1692-1700 doi:10.1242/jeb.091843 ABSTRACT We examined gustatory responses of the larval parasitoid Microplitis croceipes to determine whether the adults discriminate among common sugars, including fructose, glucose, maltose and sucrose, found in plants. When given single sugar solutions of sucrose, glucose, fructose and maltose at concentrations of 0.008–2.0 mol l −1 , the estimated concentrations at which 50% of wasps initiated feeding ranged between 0.054 and 0.085 mol l −1 for sucrose, glucose and fructose, which was significantly lower than for maltose. Wasps showed a strong decrease in feeding time for maltose or fructose following a brief exposure to other sugars, suggesting that wasps can distinguish maltose and fructose from the other sugars tested. The higher acceptance threshold and short feeding time in the case of maltose appears adaptive in light of the relatively poor nutritional quality of the sugar in the longevity trial. The pronounced feeding inhibition seen for fructose following exposure to other sugars is not linked with lower nutritional performance. This feeding inhibition was even seen in wasps that had fed on glucose at the lowest acceptance threshold (0.031 mol l −1 ) and persisted for 24 h. This study is the first to show feeding inhibition of otherwise phagostimulant sugars such as maltose and fructose after gustatory stimulation on other sugars. KEY WORDS: Feeding inhibition, Gustatory responses, Sugar feeding INTRODUCTION There is a large variation in sugar composition among natural sugar sources, such as nectar and honeydew (Percival, 1961; Kunkel and Kloft, 1977; Baker and Baker, 1983; Nemec and Starý, 1990; Wäckers, 2005). Floral nectars have been categorized according to their nectar composition as sucrose-rich or hexose (glucose/fructose)- rich (Baker and Baker, 1983), and insect groups often show a tendency to visit nectars of a certain carbohydrate composition (Baker and Baker, 1982). However, in many groups of nectar feeders, it remains unclear whether the observed insect–flower associations are directly driven by nectar sugar preferences, or whether this is based on other factors, such as flower architecture (Jervis, 1998; Olson et al., 2005; Wäckers and van Rijn, 2012). Parasitoid feeding responses to different nectar and honeydew sugars can vary widely; while several sugars fail to elicit any feeding RESEARCH ARTICLE 1 Graduate School of Bioresources and Bioenvironmental Sciences, Kyushu University, Fukuoka 812-8581, Japan. 2 Crop Protection and Management Unit, USDA-ARS, Tifton, GA 31793-0748, USA. 3 Centre for Sustainable Agriculture, Lancaster Environment Centre, Lancaster University, Bailrigg, Lancaster LA1 4YQ, UK. 4 Faculty of Agriculture, Kyushu University, Fukuoka 812-8581, Japan. *Present address: Nong Lam University, Ho Chi Minh City, Vietnam. ‡ Author for correspondence (takasu@brs.kyushu-u.ac.jp) Received 5 June 2013; Accepted 22 January 2014 response in food-deprived parasitoids, other sugars stimulate feeding at concentrations as low or lower than 0.016 mol l −1 (Wäckers, 1999; Beach et al., 2003). At equal concentrations, adult parasitoids consume larger quantities of high-quality sugars such as glucose, fructose and sucrose as compared with low-quality sugars (Wäckers, 2001; Beach et al., 2003; Williams and Roane, 2007). Because the parasitoid feeding response is often correlated with the nutritional suitability of the carbohydrates tested (Wäckers, 2001), the variation of parasitoid feeding responses to different sugars could be explained by optimal foraging theory, which predicts that foragers should choose diet items to maximize their net energy intake or net fitness benefits (Charnov, 1976; Stephens and Krebs, 1986; Sih and Christensen, 2001). Parasitoids are known to adapt their preference for hosts of different quality in accordance with their host encounter rates and their own physiological states (van Alphen and Vet, 1986). As natural sugar sources are vital for their survival and reproduction, parasitoids can be expected to have a preference for high quality of sugars and adapt their preference in accordance with feeding experiences. While parasitoid gustatory responses to individual sugars have been tested, the impact of prior experience and the relative preference for individual sugars remains unknown. The preference of nectar feeders for various nectar components has been investigated in several vertebrates and invertebrates. The methods used to assess such preferences vary among studies. The most direct way to investigate the ability and propensity of an animal to discriminate and select foods is to present it with two or more types of food simultaneously, observe its behavior and/or measure the quantity of food eaten (Dethier, 1976). These methods require that the test organism shows repeated feeding bouts, and an inclination to sample the food source. In the case of social Hymenoptera, both of these conditions are met, as the foragers continuously collect food to cover the needs of the entire colony. This has facilitated the study of food preferences in ants and honeybees. The tests used to determine feeding preferences in honeybees typically assess the relative consumption in choice experiments (Wykes, 1952; Waller, 1972; Inouye and Waller, 1984; Alm et al., 1990). An alternative method makes use of the fact that bees exhibit a typical ‘bee dance’ to recruit additional foragers (von Frisch, 1934). As bees communicate food source quality through the turning frequency, this parameter can also be used as an indicator of how bees valuate a previously visited food source (Waller and Bachman, 1981; Barron et al., 2009). In the case of ants, the relative intensity with which scouts recruit nestmates to food alternatives is also used as a method to establish preferences in choice experiments. These studies typically count the number of ants visiting the different food alternatives (Lanza and Krauss, 1984; Lanza, 1988; Lanza et al., 1993; Völkl et al., 1999). While these methods are effective in establishing preferences in social Hymenoptera, they are unsuitable for the study of food An acquired distaste: sugar discrimination by the larval parasitoid Microplitis croceipes (Hymenoptera: Braconidae) is affected by prior sugar exposure J. K. Makatiani 1 , H. K. Le 1, *, D. M. Olson 2 , F. L. Wäckers 3 and K. Takasu 4,‡