Abstract
Adult moths need energy and nutrients for reproducing and obtain them mainly by consuming flower nectar (a solution of sugars and other compounds). Gustatory perception gives them information on the plants they feed on. Feeding and food perception are integrated in the proboscis extension response, which occurs when their antennae touch a sugar solution. We took advantage of this reflex to explore moth sugar responsiveness depending on different parameters (i.e., sex, age, satiety, site of presentation, and composition of the solution). We observed that starvation but not age induced higher response rates to sucrose. Presentation of sucrose solutions in a randomized order confirmed that repeated sugar stimulations did not affect the response rate; however, animals were sometimes sensitized to water, indicating sucrose presentation might induce non-associative plasticity. Leg stimulation was much less efficient than antennal stimulation to elicit a response. Quinine prevented and terminated sucrose-elicited proboscis extension. Males but not females responded slightly more to sucrose than to fructose. Animals of either sex rarely reacted to glucose, but curiously, mixtures in which half sucrose or fructose were replaced by glucose elicited the same response rate than sucrose or fructose alone. Fructose synergized the response when mixed with sucrose in male but not female moths. This is consistent with the fact that nectars consumed by moths in nature are mixtures of these three sugars, which suggests an adaptation to nectar perception.
Highlights
Moth reproduction and its regulation by sex pheromone have been largely studied to find ways to control their populations, as their larvae are important crop pests (Cook et al, 2007; Naranjo et al, 2015; Evenden, 2016)
The sugar used was usually sucrose, but fructose, glucose, and various mixtures of these sugars were tested. We focused on these three sugars as they are the main constituents of flower nectar, the main food of adult moths: typically 10–15% of the fresh mass for each sugar, possibly up to 30% for sucrose (Lüttge, 1977; Gottsberger et al, 1984; Pacini et al, 2003; De La Barrera and Nobel, 2004; Roy et al, 2017)
For male moths (Figure 1A and Supplementary Figure S1A), significant differences were observed for all sucrose solutions of 1% and higher (χ2 or Fisher’s exact test, p ≤ 0.004): overall, fed animals were less responsive than unfed ones, irrespective of their age, while 5-day-old moths unfed for 5 days were consistently among the highest proboscis extension response (PER) rates
Summary
Moth reproduction and its regulation by sex pheromone have been largely studied to find ways to control their populations, as their larvae are important crop pests (Cook et al, 2007; Naranjo et al, 2015; Evenden, 2016). Gustatory perception is important for moths to assess food quality and discriminate what is edible from what is toxic (Glendinning, 2002; Adler and Irwin, 2005; Tiedeken et al, 2014) When their legs, antennae, or proboscis contact a sugar solution of sufficient concentration, moths extend their proboscis and use it to imbibe the solution. Sugar-induced PER has been used in various moth species (Hill and Pierce, 1989; Winkler et al, 2005; Jørgensen et al, 2007; Zhang et al, 2010; Minoli et al, 2012) using different sucrose concentrations and animals in different physiological conditions; the impact of the physiological state on sugar responsiveness is hardly known in these insects
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