There are 9 "a Spiroc Harvester(s)" that spawn on the island, sort of in a loop and around the southern shore and then north on the ridge of the rock. About 8.5 minute spawn timers.
They do cast SoW and run away extremely fast, but it is the 4th buff they cast after spawning; so if you keep up with the cycle you won't have to worry about them running to the invisible wall rectangle in the middle of Timorous Deep (actually happened to me).
It took me 7 complete cycles (sans the last spawn), so 61 kills total to get the "Tropical Melon." Indeed a Rare to Very Rare drop rate.