Bugging You From San Juan Island

Key Words: masting, Curculio, Quercus, mast seeding, acorn weevil, diapause

I picked up these acorns (Quercus sp.) when I visited my folks recently in Texas. They have several oak trees on their property and there was an abundance of acorns all over the ground this year, so I brought a few home with me in a ziplock bag. I like acorns. My mom used to draw little faces on them and I had acorn “people” to play with when I was a child.

I also found a very cool bug under one of those oak trees while I was there. This strange looking creature is one of the Nut and Acorn Weevils (Curculio sp). It has an extremely long snout. When you find out how this weevil uses it to DRILL, you may be amazed. The adult female weevil will use this drill on her very long snout to make a hole in those acorns!

Keep reading. It really is amazing.

A female weevil will make a hole in the acorn so she can put her eggs inside it!

Because I like word games, I thought I’d point out to you that rearranging the letters in the word weevil will make the words “we live?” Well, those eggs hatch into baby weevils who LIVE in a little house that is an acorn (and sometimes in other nuts too). Some folks call them grubs or worms. They are actually the larvae of the adult weevil mom who selected the acorn for her nursery.

If you are assessing this situation from an agricultural perspective (say that nut is a pecan), invested in harvesting a profitable crop, you might be feeling very worried. Sometimes the worry may indeed be justified, but in many cases, like with these acorns, the tree has evolved a strategy to deal with occasional waves of weevils and other insects we consider pests. In actuality, they are just utilizing the resources of the tree like we do when WE (the humans) eat the nut or fruit.

I’m going to name the trees’ strategy of dealing with this bug FEAR NO WEEVIL. However, the actual scientific moniker for this strategy (and it applies to other pests and adverse weather stressors as well) is MASTING. The word mast has been used since way back in the Middle Ages to refer to the acorns and seeds of forest trees that drop and accumulate on the ground. It comes from Old English, mæst. Essentially, masting is an ecological term referring to the highly variable and often synchronized periodic cycles of fruiting/seeding in the reproductive processes of trees (both forest and fruit trees).

How is it a strategy for circumventing the deleterious affects of pests? Well, the idea is that in some years, bumper crops of nuts and seeds are produced in order to satiate the predators, so some are left to germinate and continue new generations. In this particular case, you could think of it as the oak trees sacrificing some of their offspring to the weevil gods. When there are more acorns than there are adult female weevils, some of those acorns will escape the weevil drill and makeover into little bug nurseries.

In reality, this relationship is much more complicated. In some cases, acorns parasitized by only a few weevil larvae will germinate, while those acorns with many larvae will not. The trees’ bumper crops of seeds and nuts will also cycle with years of low production, where resources are scarce for the weevil (and other organisms). Nature is incredibly dynamic though. Studies show some species of weevils have adapted a counter mechanism to circumvent the trees’ strategy of masting. It’s called prolonged diapause. This means these weevils are able to sleep longer (more than one year) as they develop in order to synchronize adult emergence with years when the trees’ seed/nut production is high.

If you are interested in reading more about the history and MYSTERY of masting, I encourage you to delve into the literature I’ve listed in the references below. It’s quite fascinating – especially going back in history to the link between masting and pannage. Way more than I can cover here. Check it out.

References

Baldwin, M. 2021. Pigging out in the forest: the Common of Mast (Pannage) in Britain https://www.wildlifeonline.me.uk/blog/post/pigging-out-in-the-forest-the-common-of-mast-in-britain

Bugguide 2021. Curculio. https://bugguide.net/node/view/6682

Higaki M (2016) Prolonged diapause and seed predation by the acorn weevil, Curculio robustus, in relation to masting of the deciduous oak Quercus acutissima. Entomol Exp Appl 159:338–346. https://doi.org/10.1111/eea.12444

Jefferson, R. 2006. Why Are More Acorns Falling? Excessive Drops of Nuts from Oak Trees Is Part of Normal ‘Mast Year’ Phenomenon. Scientific Times. https://www.sciencetimes.com/articles/33795/20211006/why-more-acorns-falling-excessive-drops-nuts-oak-trees-part.htm and https://youtu.be/EQ748TZcuqs

Jesse, L. No date. The dark side of collecting acorns. Plant and Insect Diagnostic Clinic. Iowa State University Extension and Outreach. https://www.extension.iastate.edu/news/2007/sep/072107.htm

Koenig WD. 2021 A brief history of masting research. Phil. Trans. R. Soc. B 376: 20200423. https://doi.org/10.1098/rstb.2020.0423 Koenig, W. D., & Knops, J. M. H. (2005). The Mystery of Masting in Trees: Some trees reproduce synchronously over large areas, with widespread ecological effects, but how and why? American Scientist, 93(4), 340–347. http://www.jstor.org/stable/27858609