How to prove that insects play an important role in pollination of plants?

in many plants, seeds and fruits are formed as a result of cross-pollination (the transfer of pollen from the stamens of a flower of one plant to the stigma of the pistil of another. This is done by insects – pollinators. Approximately 80% of wild and cultivated plants need insect pollinators. (beetles, butterflies, bees, etc.) .) if there are enough pollinators, then the plants give a good harvest of fruits and seeds.