Which type of nematode can attack insects and is commonly used in insect controls?

Parasitic nematodes are the correct choice because they are specifically known for their ability to target and infect insect hosts. These nematodes enter the insects' bodies, often leading to the insects' death. This biological control method is employed by pest management professionals to reduce pest populations without relying on chemical pesticides, making it an environmentally-friendly option.

Beneficial nematodes, while generally considered helpful to gardens and agricultural areas, are a broader category that includes parasitic types. Microscopic roundworms can refer to many types of nematodes, not all of which are parasitic or beneficial; they might inhabit a variety of ecosystems. Plant-parasitic nematodes, on the other hand, primarily feed on plant roots and do not have a role in controlling insect populations, further clarifying why they are not the answer in this context.

Focusing on the specific attributes of parasitic nematodes highlights their unique role in pest control strategies.