Graph shows that writing about varroa, a disease in honey bees, started around 1970.
What does the Ngram Viewer do?
When you enter phrases into the Google Books Ngram Viewer, it displays a graph showing how those phrases have occurred in a corpus of books (e.g., "British English", "English Fiction", "French") over the selected years.
Have a go - ask your own question
My question, which produced the graph above was "When did we start talking about varroa?". It's a bit of fun. Try it, run your own little mini-research using Ngram. Maybe you will gain an unexpected insight.