Complex sentence: Natural enemies are already adapted to the habitat and to the target pest, and their conservation can be simple and cost-effective, as when nectar-producing crop plants are grown in the borders of rice fields.
Split into simpler sentences: Natural enemies are already adapted to the habitat and to the target pest, and their conservation can be simple and cost-effective. An example that has shown such cost-effectiveness is growing nectar-producing crop plants in the borders of rice fields.