Of course, it's not that the strength of those with Golden Core wasn't substantial, but the breeding rate of these black insects was simply too rapid!
The black insects seemed to be able to transform the consumed Spiritual Energy into bug eggs at the fastest speed, and to say they were breeding as they ate wouldn't be an exaggeration.
With the power of a Golden Core expert, of course, one could easily eliminate the black insects over a vast area, but those tiny, ubiquitous eggs were incredibly hard to eradicate because they were too many and too small.
If the Yuanming Sect hadn't dispatched a great number of disciples and issued tasks to Loose Cultivators, countless cultivators wouldn't have been able to unite their efforts. Otherwise, more than just the territory of two clans would have been devoured by the black insects.
As for why they didn't summon the clan cultivators this time? It was because they needed to safeguard their own clans.
