For this month’s “Notes from the Lab,” we’re going to highlight the worst-kept secret in honey bee research for the past year. If you’re a beekeeper who watches YouTube videos (which is pretty much every beekeeper I’ve ever met, including myself), you’ve probably seen Sammy Ramsey’s award-winning “Three minute thesis” video on Varroa, which can be viewed here:
https://www.youtube.com/watch?v=Fyfyj-2O47Q. As I write this, the video has been viewed 14,411 times.
If you’ve ever met Sammy, you’ll know the video is spot-on at capturing his dynamic oratory skills. But what about the science underneath the exciting rhetoric? Well, the work that Sammy describes in the YouTube video was finally published in a peer-reviewed paper last month. So, for our seventeenth “Notes from the Lab,” we highlight “Varroa destructor feeds primarily on honey bee fat body tissue and not hemolymph,” written by Sammy Ramsey and colleagues and published in the journal Proceedings of the National Academy of Sciences of the United States [116:1792-1801 (2019)].
Ramsey and colleagues’ study was inspired by several astute observations. First, while it may make sense for blood-feeding parasites of humans to get their nutrients from blood (think about ticks latching onto your leg and engorging themselves), it actually makes little sense for parasites of insects to feed on hemolymph. Why? Because human blood is rich in nutrients, but insect hemolymph is generally quite poor in nutrients. Indeed, several previous studies of insect parasites have found they do not use hemolymph as their main source of nutrients.
Second, the initial work determining that Varroa feeds on hemolymph was conducted over 40 years ago and used outdated techniques that are known to be flawed. Essentially, the radioisotopes that were used to test where Varroa obtained its nutrients while feeding on different honey bee tissues don’t work as well as we thought they did in the 1970s.
Third, if you look at where Varroa typically latches onto honey bees, it’s not in an area with easy access to hemolymph. Instead, it’s in close proximity to the fat body. Furthermore, if you look even closer at Varroa, its digestive system and mouthparts are organized in such a way that would make it difficult to feed on liquid (such as hemolymph) vs. semi-solid tissue (such as the fat body).
Finally (and this is a key point for why Ramsey and colleagues’ study is more than just interesting biological trivia) many promising varroacidal drugs that have been developed for honey bees over the years haven’t worked. However, several of these drugs have been developed to utilize hemolymph — the tissue that Varroa was ….