Subject: How Drosophilas and oranges can mantain a stable evolutionary relationship?

I've been doing a project about how citrus oils induce on viables Parental and 
Second generation of Drosophilas. I was able to prove that it does have its 
influence, but I couldn't prove or even understand why I found this 
statement: " If organges had no defenses against fruit flies, the oranges would 
be wiped out. If the defense mechanisms of oranges against flies are so severe, 
then all the fruit flies die or would avoid oranges". This proved me that they 
have a stable evolutionary relationship, but I'm not able to undersand why. 
Thank for your time and patience

Re: How Drosophilas and oranges can mantain a stable evolutionary relationship?