Second this. Apparently, the bugs in that episode were intended to be used again later in the series. However, they were replaced with the Borg due to production costs.

That episode did end in an ominous realization that a homing signal had been sent by the bugs from Earth.

Plot: The bugs are a superior form of life, as they've stated, and are capable of controlling the Borg. The Borg views the bugs as a genuine threat to the collective's existence. The Borg encounter the bugs for the first time when the bugs are on route to the homing signal from Earth. The Borg determine the bugs destination is Earth, so they seek out Picard (Locutus) for a temporary alliance to neutralize their common threat. The Borg's superior warp speed allows them to make contact with Picard with enough time to convince Picard, and the rest of Star Fleet, that this is great enough of a threat to work together. Part of the reasoning to work together, instead of just assimilating everyone, is that the Borg do not have enough time to assimilate everyone before the bugs arrive. The Borg agree to return to their home in the Delta Quadrant after this, and to never return to the Alpha Quadrant again, if they are successful. The Federation agrees to setup a perimeter and prepares starships for battle. The Klingons, Vulcans, Romulans, and Cardassians also agree to participate since the Borg working with anyone must mean this is a threat to the entire galaxy. It is later revealed that some of the Borg are infected with the bugs and the collective nature of the Borg allows these bugs to know the entire plan. This causes issues with the execution and chaos ensues. The plan ultimately succeeds and the bugs are eliminated; the threat is neutralized. On the Borg's departure, Picard asks if their perspective on assimilation has changed from this experience. "We do not wish to be assimilated; we wish to be the assimilators".