We investigate the growth mechanism of axially heterostructured InGaN/GaN nanowires (NWs) as a function of the flux conditions. The InGaN heterostructure morphology critically depends on the In/Ga flux ratio affecting the local V/III ratio at the NW growth front. Locally N-rich conditions are associated with tapered island-like morphologies, while metal rich conditions, leading to the formation of a stable Indium adsorbed layer at the NW growth front, promote the growth of heterostructures with a disk-like shape. Based on experimental results and theoretical predictions, we demonstrate that this indium ad-layer acts as a surfactant inducing a modification of the InGaN heterostructure growth mode. The impact of flux conditions and strain relaxation on the Indium incorporation are also addressed. The resulting insertions present abrupt interfaces and a homogeneous In distribution for In contents up to 40%.