noun
explosive increase; severe outbreak; upsurge of pests
Used especially for a sudden, large-scale increase in pest populations such as insects or rodents.
今年はカメムシが大発生して農作物に大きな被害が出た。
This year there was an explosive increase in stink bugs, causing major damage to crops.
大量発生 is a broader term for mass occurrence of any organism, while 大発生 strongly implies a sudden, severe outbreak, typically of pests.
Compound of 大 (large, great) and 発生 (outbreak, occurrence).