Which of the following pests does Bacillus thuringiensis primarily control?

Bacillus thuringiensis (Bt) is a naturally occurring bacterium that serves as a biological insecticide, particularly effective against a specific group of pests. The primary mode of action is through the production of protein crystals that are toxic to the larvae of certain insects upon ingestion. The bacteria produce these crystals, which contain proteins that are activated in the alkaline environment of the insect gut, leading to the insect's death.

Caterpillars, which are the larvae of moths and butterflies, are particularly susceptible to these toxins. When caterpillars ingest Bt, the spores and toxins disrupt their digestive systems, ultimately leading to mortality. This specificity makes Bt an invaluable tool in integrated pest management, particularly for targeting lepidopteran pests while being relatively safe for beneficial insects, humans, and other wildlife.

The other options relate to pest types that are not effectively controlled by Bacillus thuringiensis. Most beetles, aphids, and mites do not have the right digestive systems to activate the toxins from Bt, which is why they aren’t affected by this biological control agent. Thus, focusing on its effectiveness against most caterpillars provides a clear understanding of Bt's role in pest management strategies.