We study social behavior and communication in treehoppers (Membracidae), which are small relatives of cicadas. Treehoppers are well known for their amazing morphological diversity, especially in the form of their pronotum, whose shape ranges from elegant to bizarre. Treehoppers are also famous for the diversity of their social behavior: some species are relatively solitary, but most live in groups for part or all of their lives. The major determinants of their form of social life include first, the extent to which there is maternal care; and second, whether they form mutualisms with ants or other Hymenoptera.

 

Treehoppers are also highly communicative: as adults when searching for mates; and as immatures (and sometimes as adults) when making collective decisions about foraging, movement or defense.

 

In the thornbug Umbonia crassicornis, females lay a single clutch of eggs and spend the rest of their lives defending it, first from egg parasitoids and later, once the nymphs hatch from the egg, from predators. Communication within mother-offspring groups is dynamic and sophisticated. When a predator appears, nymphs near the threat begin to produce signals, every few seconds (Cocroft 1996, 1999a, 1999b). Gradually more and more other individuals join in, in a process of positive feedback, until there is a collective signal that is highly effective at alerting the female to the predator’s presence.

 

Furthermore, female defenses work only over short distances (1-2cm), so the female has to find the predator to drive it away (Cocroft 2002). The way that signaling behavior spreads within the group allows the female to work out which end of the group has been attacked (Ramaswamy and Cocroft 2009). Once the female drives away the predator, she begins signaling, and her signals (in a rare example of negative feedback in social communication) cause the nymphs to become quiet once more (Hamel and Cocroft 2012; see also Hamel and Cocroft 2019 for a study of communication in the related Platycotis vittata).

 

Currently we are investigating the evolutionary history of maternal care and offspring-parent communication in the Hoplophorionines. In a parallel with the ancient offspring distress signals of mammals, the offspring distress signals of Hoplophorionines are highly conserved and appear to date back some 25 mya (Cocroft, Michael & Lin, in prep).

 

Groups of immature membracids in other species communicate during cooperative foraging (Cocroft 2001; Cocroft 2005; Cocroft and Hamel 2010), and we are investigating the dynamics and evolution of cooperative communication about feeding sites in the Membracinae.