Final answer:
Bacillus thuringiensis (Bt) is a biocontrol bacterium that produces toxins effective against caterpillars. By ingesting the Bt toxin, caterpillars stop feeding and eventually die. Bt is environmentally safe and integrated into organic farming but mainly targets lepidopteran pests.
Step-by-step explanation:
Bacillus thuringiensis (Bt) is a bacterium renowned for its role in biological control by producing toxins detrimental to various insect pests. Utilized in biological control strategies, Bt is particularly effective against caterpillars, making the correct answer to the student's question (a) control caterpillars. It achieves this by producing protein crystals during sporulation that are toxic to various insects, especially those affecting plants. Bt toxin must be ingested by these pests, leading to their subsequent cessation of feeding and eventual death within days.
Furthermore, genetic engineering has enabled the incorporation of Bt toxin directly into plants, allowing them to produce the toxin internally and defend against potential insect damage. This approach is not only environmentally friendly but also approved for use in organic farming, as Bt toxin is non-toxic to humans, mammals, and beneficial to the insects. However, it should be noted that the major target of Bt is lepidopteran (moth and butterfly) larvae, and the efficacy against other pests such as aphids, nematodes, or aquatic weeds is far less or non-existent.