Read the given passage and answer the following questions:
Biocontrol refers to the use of biological methods for controlling plant diseases and pests. In modern society, these problems have been tackled increasingly by the use of chemicals – by use of insecticides and pesticides. In agriculture, there is a method of controlling pests that relies on natural predation rather than introduced chemicals. The very familiar beetle with red and black markings – the Ladybird, and Dragonflies are useful to get rid of aphids and mosquitoes, respectively. An example of microbial biocontrol agents that can be introduced in order to control butterfly caterpillars is the bacteria Bacillus thuringiensis (often written as Bt ). Because of the development of methods of genetic engineering in the last decade or so, the scientists have introduced B. thuringiensis toxin genes into plants. Such plants are resistant to attack by insect pests. A biological control being developed for use in the treatment of plant disease is the fungus Trichoderma. Trichoderma species are free-living fungi that are very common in the root ecosystems. They are effective biocontrol agents of several plant pathogens. Baculoviruses are pathogens that attack insects and other arthropods. The majority of baculoviruses used as biological control agents are in the genus Nucleopolyhedrovirus. These viruses are excellent candidates for species-specific, narrow spectrum insecticidal applications. They have been shown to have no negative impacts on plants, mammals, birds, fish or even on non-target insects. This is especially desirable when beneficial insects are being conserved to aid in an overall integrated pest management (IPM) programme, or when an ecologically sensitive area is being treated.
Bacillus thuringiensis is available in sachets as dried spores, which are mixed with water and sprayed onto vulnerable plants such as brassicas and fruit trees. Where is the Bt toxin released so that the caterpillars are killed?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → In the gut of the insect larvae that eat the spores
Bacillus thuringiensis (Bt) is a bacterium used to control butterfly caterpillars. Its spores are sprayed on the plants, but the toxin does nothing until the larvae eat the spores.
Option (1) On the leaf surface, as soon as the spores are sprayed — Incorrect. The spores only lie on the leaf after spraying. The toxin is not released there.
Option (2) In the soil around the roots of the sprayed plants — Incorrect. Bt is sprayed on the plants the caterpillars feed on, not on the soil around the roots.
Option (3) In the gut of the insect larvae that eat the spores — Correct. The toxin is released only inside the gut of the larvae that eat the spores, so it kills the leaf-eating caterpillars.
Option (4) Inside the plant cells, after the leaves absorb the spores — Incorrect. Leaves do not absorb the spores. Plant cells make the Bt toxin only in Bt plants, which carry Bt toxin genes put in by genetic engineering.
NCERT Extract:
Microbes as Biocontrol Agents – “… An example of microbial biocontrol agents that can be introduced in order to control butterfly caterpillars is the bacteria Bacillus thuringiensis (often written as Bt ). These are available in sachets as dried spores which are mixed with water and sprayed onto vulnerable plants such as brassicas and fruit trees, where these are eaten by the insect larvae. In the gut of the larvae, the toxin is released and the larvae get killed. The bacterial disease will kill the caterpillars, but leave other insects unharmed.”