Question

Insects in the Lepidopteran group lay eggs on maize crops. The larvae on hatching feed on maize leaf and tender cob. In order to arrest the spread of three such Lepidopteran pests, Bt maize crops were introduced in an experimental field.
A study was carried out to see which of the three species of lepidopteran pests was most susceptible to Bt genes and its product.
The lepidopteran pests were allowed to feed on the same Bt-maize crops grown on 5 fields (A-E).
The graph below shows the leaf area damaged by these three pests after feeding on maize leaves for five days.

Key for lepidopteran insect pest species

	Species I
	Species II
	Species III

Insect gut pH was recorded as 10, 8 and 6 respectively for Species I, II and III respectively.

(a) Evaluate the efficacy of the Bt crop on the feeding habits of the three species of stem borer and suggest which species is least susceptible to Bt toxin.

(b) Which species is most susceptible to Bt-maize, and explain why?

(c) Using the given information, suggest why a similar effect was not seen in the three insect species.

Answer 1

(a) Species III is the least susceptible.

(b) Bt toxin protoxins are converted into an active form in the gut which solubilises the toxin crystals.
The activated toxin binds to the surface of midgut epithelial cells and creates pores that cause cell swelling and lysis and eventually cause the death of the insect.

(c) Insect species I and II have alkaline gut pH which solubilises the insecticidal protein crystals of protoxin and makes them active.

Species III has an acidic and the protoxin continues to remain in an inactive form doing no harm to insect species III.