When a certain plant is introduced into an area, the population of a certain insect species declines. What can be concluded from this?

It’s possible that the plant was toxic to the insects, or that it drove out another plant they fed on, or even that there was no connection between the introduction of the plant and the insects’ decline. There may even be other possibilities beyond these. Based on this observation it’s premature to make any conclusions about cause and effect. To determine which, if any, of these explanations is true, it would be necessary to conduct further investigations, including controlled experiments.