Prey-Predators systems 
Another application
In the late nineteenth century, to control a plague of cochineals that was destroying the orchards in California, a species of ladybird that feeds on these insects was introduced into the ecosystem.
Although this initiative did not exterminate the cochineals, it moderated their growth, keeping them within certain reasonable limits. Why?
In the 1940s, with the appearance of insecticides, it was believed that a technique for the elimination of this pest had been achieved, which was then attempted through the use of DDT. The action of this product corresponds, in this context, to fishing the two species. However, contrary to what the farmers expected, the cochineals did not die out, and there was even an increase in their numbers. This unexpected result led to a more intense application (fishing), which only increased the problem. Remember what we have studied and give an explanation for what has been observed here.
(Click on the figure to observe the effect of increasing DDT on the
mean values.)
