Associative olfactory learning in the desert locust, Schistocerca gregaria

Abstract

SUMMARYLocusts can learn associations between olfactory stimuli and food rewards, and use the acquired memories to choose between foods according to their nutrient requirements. They are a model system for both the study of olfactory coding and insect nutritional regulation. Previous studies have used operant paradigms for conditioning freely moving locusts, restricting the study of the neural mechanisms underlying the acquisition of olfactory memories, which requires restrained preparations for electrophysiological recordings. Here we present two complementary paradigms for the classical conditioning of olfactory memories in restrained desert locusts (Schistocerca gregaria). These paradigms allow precise experimental control over the parameters influencing learning. The first paradigm is based on classical (Pavlovian) appetitive conditioning. We show that opening of the maxillary palps can be used as a measure of memory acquisition. Maxillary palp opening in response to odour presentation is significantly higher in locusts trained with paired presentation of an odour and a food reward than in locusts trained either with unpaired presentation of food and odour or the odour alone. The memory formed by this conditioning paradigm lasts for at least 24h. In the second paradigm, we show that classical conditioning of an odour memory in restrained locusts influences their decisions in a subsequent operant task. When locusts that have been trained to associate an odour with a food reward are placed in a Y-maze, they choose the arm containing that odour significantly more often than naïve locusts. A single conditioning trial is sufficient to induce a significant bias for that odour for up to 4h. Multiple-and block-trial training induce a significant bias that lasts at least 24h. Thus, locusts are capable of forming appetitive olfactory memories in classical conditioning paradigms and can use these memories to modify their decisions. Supplementary material available online at

A long-latency aversive learning mechanism enables locusts to avoid odours associated with the consequences of ingesting toxic food

“…Aversive olfactory conditioning was designed to work against the locusts' naive preference for vanilla over lemon odour (Simões et al, 2011). Therefore, the conditioned stimulus (CS) was 25l of pure vanilla extract (Dr Oetker, Thorpe Park, Leeds, UK) on 1cm 2 of filter paper inside a plastic tube connected to an air pump.…”

“…The two alternative test odours were presented in the two decision arms: vanilla extract (the CS), and lemon extract (Holland & Barrett, Nuneaton, Warwickshire, UK), the untrained odour. The arena, odour delivery setup and test procedure are described elsewhere (Simões et al, 2011). Half of the animals were tested with vanilla odour in the right arm and the other half with vanilla in the left.…”

“…Half of the animals were tested with vanilla odour in the right arm and the other half with vanilla in the left. A locust was placed inside a holding tube positioned on the stem of the Y-bar (Simões et al, 2011) and left undisturbed for a few seconds before the air flow inside the Y-maze was turned on. Each locust was allowed 5min to exit the tube and make a choice.…”

“…Generalist herbivores, like locusts, are known to use non-associative as well as pre-and post-ingestive associative learning mechanisms to evaluate food and decide whether to ingest it (Szentesi and Bernays, 1984;Bernays and Lee, 1988;Lee and Bernays, 1990;Chapman et al, 1991;Behmer et al, 1999;Simões et al, 2011). Pre-and postingestive learning processes may involve separate neuroaminergic pathways, suggesting they are mechanistically distinct (Wright et al, 2010).…”

“…Using a Y-maze paradigm (Simões et al, 2011), we show that the olfactory preference of locusts can be aversively conditioned by pairing a naively preferred odour with a nicotine hydrogen tartrate (NHT)-enriched diet. A single trial was sufficient to produce a robust aversive memory that lasted for 24h.…”