Scientists in Australia have shown for the first time that when insect larvae follow a leader to forage for food, both leaders and followers benefit, growing much faster than if they are in a group of only leaders or only followers. The work gives new insight into why such social relationships evolve in insects, and why they are maintained.

The study, which was published in the journal Proceedings of the Royal Society, looked at larvae of the iconic Australian steel-blue sawfly (Perga affinis), which are also known as “spitfires.” Sawfly larvae can grow up to seven centimeters long, and they remain in the larval stage for seven months. Caterpillars forage nocturnally in eucalyptus trees, forming large groups that can strip all of the leaves from a tree in a few days.

Sawfly societies operate democratically, with leaders and followers co-operating to decide on group movements. The team observed the insects’ natural behavior for two weeks, individually marking each sawfly to understand which behaved as leaders or followers when foraging for food, noting their weight at the start and end of the fortnight.

Once leaders were established by their position at the front of the group, the scientists generated experimental groups of larvae which contained either all leaders, all followers, or a mixture of the two, and they measured their weight again over two weeks.

This video shows sawfly larvae foraging for food. Video by Lisa Hodgkin.

“Our field experiments revealed no clear individual benefit to being a leader, but all individuals in groups with a mixture of leaders and followers gained more weight than those in groups of only followers or only leaders,” said Lisa Hodgkin, a PhD candidate at the University of Melbourne. “In many types of animals, the dominant leaders in a group are larger and stronger because when they forage or hunt, they take more of the food resources. But we found no difference in the weight gain between sawfly leaders and followers. We see that leaders only benefit from being leaders if they have followers, and that followers only benefit if they have leaders. There is no use being a shepherd without sheep or sheep without a shepherd.”

Study co-author Professor Mark Elgar said that while leaders do not differ in growth rates or weight, they may acquire other benefits such as lower predation or enhanced immune function.

“The next stage of our research is to find out how certain larvae become the leaders in a group and how they are communicating directions and encouragement to their followers,” he said.

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– Leaders benefit followers in the collective movement of a social sawfly