I got a call a couple days ago; hornets, bees, flies, and wasps "of all kinds" were descending on someone's burr oak. The caller told me that they were witnessing a phenomenon that they "had never seen before in their life". Entomologists (at least extension entomologists) get these kinds of calls with some regularity. The caller was reporting to me that their neighbor's burr oak tree had millions of kinds of Hymenoptera and Diptera moving about all over the acorns of the tree.
Back in the lab... We now have a number of things happening; sugar beet harvest, sunflower rating, dry bean host resistance assays, potato harvest, wheat planting... you get the point. Yet, the caller was a little dismayed when I refused to drop everything and make the 20-minute trip out to his property to look at a burr oak full of hornets. After getting a short lecture about how so-'n-so back in the day would drop everything to come out whenever he called and how he, as a farmer, has the luxury of doing what he wants when he wants because he is his own boss, I requested that he bring a branch off of the tree into my office. He then remarked that he couldn't because he had wheat to plant... He also didn't seem to amused when I pointed out the irony of his apparent mantra of "doing what he wants when he wants". But I told him that I would look into it.
Yesterday... One of my two very observant technicians came in to my office and mentioned that he had seen a number of burr oaks in town exhibiting the same phenomenon. He also mentioned that we had a burr oak in the front of the PREC, again in the same situation. So, I grabbed my camera, headed out, and the learning began. I saw the same thing that the caller had described -- wasps and hornets and flies roaming all over the "acorns" on the tree. So I did what any self-respecting entomologist would do and picked one of the "acorns" and licked it. Yep, sweet. I had mentioned to the caller that what he was observing on his burr oak was likely due to an infestation of insects (e.g., aphids or scales) secreting honeydew somewhere on the tree. I was partially correct.
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| Yellow jackets frantically combing over cynipid galls on oak. |
The investigation revealed that... The "acorns" that were observed, are not acorns at all but galls. These particular galls are produced by the tree around a bud that is infested with a tiny cynipid wasp. The wasp larva develops inside of the gall and as it feeds it secretes a honeydew that eventually oozes its way to the surface of the gall. In the fall of the year (after many of the flower plants in the area have finished blooming) many hornets in particular are attracted to the crystalline sugars on the surface of the green galls. If you split a gall open, inside you will find a single, tiny wasp larva. Apparently, the hornets and bees do an exemplary job of defending the galls from gall parasitoids (even tinier wasps, sort of like this one) that attack the cynipid larva. In fact, a relatively recent study (I think in eastern Colorado) demonstrated that in areas where hornets and kin are less common, the parasitoids have a higher success rate at attacking the galls; therefore, the burr oaks are likely to succumb to the galling cynipids.
Once the investigation ended... I contacted the caller. He was pretty anxious to know whether I had come out for a visit. I said that I had not, but that I had found other burr oaks in the area exhibiting the same thing. He was amazed that his situation was not unique and inquired as to what was causing it. I said, "aliens". He gave a half-hearted chuckle to my lame pun and then I went on to explain to him all of the details above. He was (again) amazed the diversity of the insect world and the wonders of nature.
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| How many hornets do you see in the photo above? Leave your answer as a comment under the link posted in Facebook. (Click on the photo to see an enlarged view.) |
