First appearance of The Insecticons, Bombshell, Shrapnel & Kickback.
The first episode not to feature Starscream.
The Insecticon origins in the show is that they were originally part of the Nemesis crew that pursued the Ark, after it launched on its mission to locate new energy sources. When the Decepticons boarded it, three Decepticons remained on board the Nemesis. Until however it plunged in to Earth, and the three escaped in an escape pod that it to crashed on the planet. Once on Earth, their bodies adapted to life by assuming the alt modes of Insects.
An interesting character trait of the Insecticons was how they were portrayed as being more as a splinter cell. Who only worked with the Decepticons when it best served their purpose. Mainly appealing to their desire to consume anything that supplied them with energy. Often however, they would then resort to stabbing the Decepticons in the back. Whether it be an opportunity to benefit themselves. Or if when things went south, they would grab whatever they could and bail on the Decepticons. Earning Megatron's wrath and his relentless pursuit. But inevitably, the two groups would back working together.
It is never made clear if the Insecticons had been active on Earth since first crash landing in their escape pod. And they had been roaming the planet for four million years, while the Autobots and other Decepticons lay dormant. Fans have drawn theories that the long isolation on a Primitive world, would likely account for their amnesiac response to encountering Megatron. And how they were always at odd ends with the Decepticons.
Yet, if Beast Wars is to be considered Canon. This would mean they were active on Earth during the Beast Wars. But never encountered the Maximals and Predacons.
Another theory amongst fans,is that during the events of the Ultimate Doom trilogy. The electrical storms caused by Cybertron's presence in Earth's orbit. Likely awoke them, while they had remained dormant in their pod, since crash landing on Earth. This would explain how their presence had gone unnoticed by human civilization. And their sudden appearance.