Whether immune challenges play a role in animal migration is poorly studied, particularly for insects. Mormon crickets Anabrus simplex engage in spectacular mass migrations where millions of insects group together and walk in a common direction. Anti-bacterial activity in these insects appears to be compromised by migratory activity. This trade-off is particularly evident when Mormon crickets do not consume sufficient carbohydrates in their diet. If migration compromises anti-bacterial activity, then challenged animals should migrate more slowly as the anti-bacterial immune system is elevated. In contrast, migratory activity could increase with parasitoid activity, and a parasitoid infection might serve as a cue to migrate faster and relocate further. In order to investigate the effect of immune activation per se, non-living surrogates of bacterial, fungal, or parasitoid agents (lipopolysaccharide LPS, laminarin, and glass rods, respectively) were introduced into radiotagged Mormon crickets and the insects’ migratory activities and immune responses were measured.
Mormon crickets always migrated more slowly when challenged with lipopolysaccharide or laminarin, and in two of the three cases, anti-bacterial activity was inversely proportional to migratory speed. Hence inducible defenses affected migratory activity of the insects. The induction of anti-bacterial activity might directly reduce migratory activity, because a lipid transport protein that is freed from its activity in locomotion can serve to enhance anti-bacterial immunity. Elevation of anti-bacterial activity by both laminarin and LPS suggests that activation of the Toll pathway is not specific to bacteria in Mormon crickets. Mormon crickets encapsulating glass rods did not migrate faster than sham-treated controls. However encapsulation activity was proportional to migratory activity. Better overall physiological condition may be influencing both traits with insects able to mount a stronger defense migrating further to displace themselves from the site of attack and injury. In conclusion, the relationship between migration and immunity was dependent on the challenge. Migration was reduced with microbial-simulated challenges but it was enhanced with parasitoid-like challenges.