Light affects the homing ability of Solenopsis invicta (Hymenoptera: Formicidae) foraging workers

Ants have evolved a wide variety of ways to take advantage of available food resources and to distribute them to colony members. For the red imported fire ant, Solenopsis invicta Buren (Hymenoptera: Formicidae), at least 3 major stages are involved: A) foraging or scout workers leave the nest and take a looping path as they forage an area for resources; B) the scout must orient back to its nest after food discovery either with food or to recruit workers to a food source too large for a single forager to take back to the colony, and C) the food resources must be efficiently distributed to colony members (queen, workers, and brood). Fire ant recruitment has been well studied behaviorally and chemically. However, initial homing of the foraging worker back to its colony after food discovery has only been investigated relative to the sensitivity of the fire ant to geomagnetism under dark conditions, where the time-to-trail-formation was significantly increased when the magnetic field direction was reversed. Here, we focus on light in the context of a homing cue used by foraging workers above ground. We demonstrate that fire ants use light as a powerful homing/orientation cue that allows them to orient directly back to their nest after locating food, instead of retracing their previous random foraging path. The light source needs to be relatively fixed, as rotating light inhibited trail formation. Light does not have to be very bright as there was no degradation of homing ability even at 1 lux (equivalent to moonlight). We also showed time-to-trailformation under red light was indistinguishable from under dark conditions. An experiment that changed the visual cues but kept the light fixed suggested that in the presence of a light source, visual cues had no effect on fire ant homing. Our study documents the use of light in fire ant homing and contributes to our understanding of another component of the complex fire ant recruitment system.