Question

Which insect was used to check the spread of cacti?
(a)Colias eurytheme
(b)Cactoblastis cactorum
(c)Neodiprion sertifer
(d)Periplaneta americana

Answer 1

Insects that are used to check the control of a weed or any invasive plant come under biocontrol or biological control agents. This process makes use of predation to keep the prey in check. The insect used to check the spread of cacti was a moth.

Complete answer:
The role of predators in an ecosystem is very important. They help keep the prey population in check. When prey populations are not kept in check, they increase to such large numbers that they exhaust the resources and eventually become extinct from that place.
This is what happened in the case of the prickly pear cactus which was introduced in Australia in the 1920s. It was brought from South America and thus had no natural predators in Australia to keep it in check. Thus, they soon became an invasive species by rapidly spreading into millions of hectares of rangeland.
This invasive cactus was finally brought under control only after a cactus-feeding predator called Cactoblastis cactorum from its natural habitat was introduced into the country.

Additional Information: Predators also keep the ecosystem running smoothly by:
-Keeping the intensity of interspecific competition in check: In the rocky intertidal communities of the American Pacific Coast, the starfish Pisaster is an important predator. This was elaborately proved in a field experiment when all the starfish were removed from an enclosed intertidal area, more than 10 species of invertebrates became extinct within a year, as they all fought for the same resources.
-By being prudent: If a predator is too efficient and over exploits its prey, then the prey might become extinct, and following it, the predator will also become extinct for lack of food. Thus predators in nature act prudently.
So, the correct answer is ‘ Cactoblastis cactorum ’

Note: Prudent means careful, wise, and thinks about the future.
Examples of other invasive species include water hyacinth, Lantana camara , Parthenium, African catfish, etc. Water Hyacinth (Eichhornia crassipes) was so invasive in Bengal that it is known as the terror of Bengal.