In the late 1940's, scientists released a group of chemicals, pyrethroids, to kill insects in auricular crops. by the year 2000, over 100 insect species were resistant to pyrethroids and scientists had to develop a new plan to kill them. explain what happened genetically in the insect population over time as the insects were exposed to pyrethroids. a) the toxicity of pyrethroids weakened over time. b) the allele for pyrethroid resistance increased throughout the population. eliminate c) pyrethroid resistance became the dominant allele in the insect populations. d) most insects had the allele for pyrethroid resistance, but it remained masked.

The correct answer is-

B) The allele for pyrethroid resistance increased throughout the population.

As per the information in the question, pyrethroids ( a group of chemicals for killing insects in auricular crops) was given by scientist and more than 100 insect species developed resistance against it by the year 2000.

This indicates that in order to survive, certain species of insects developed resistance ( by changing their genome) and the gene variant ( allele) that was responsible for making them resistant against pyrethroids was increased throughout the population.

Thus, these insect species that developed resistance was selected by nature as it favored their survival.

Due to this, scientist had to develop a new plan to kill them.

Thus, option B) is the right answer.