Something is in Deez Nuts! (Acorn Weevils)

Insects for Fun!

This week is a special listener request for Acorn Weevils! The cute little beetles that use acorns as their safe spaces during development. The focus for today's episode is on the Northen acorn weevils scientifically known as Curculio glandium.

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Transcript:

Welcome to episode 38 of Insects for Fun! Today’s episode was actually a listener request sent to me via IG by Dean F! Without further ado let’s start the episode.

The acorn weevil is the first weevil to be featured on this podcast and it is possibly the cutest weevil to ever exist. If you haven’t seen it then please give it a google or check out the IG or FB page and drop a comment letting me know what you think!

Now because this is the first time we are talking about Weevils I will now just go over the very basics. Weevils are beetles within the family curculionidae and as with other beetles, weevils go through complete metamorphosis and they can be found worldwide except for… well, you know what I’m talking about. Now what makes weevils really cool is their modified head. They all have snouts which are called rostrums and at the end of this rostrum is a functional mouth part which they use to feed on plants. These guys happen to be herbivores so there is no need to worry that they will pierce you the same way some true bugs might

like kissing bugs for example. The females also usually have longer rostrums than males do which they use to help them prepare an egg laying site.

Acorn weevils as the name suggests are weevils which feed on and use acorns for their young, but they are actually divided into two different genera. The first are referred to as long-snouted and belong to the genus curculio and the second are short-snouted weevils in the genus Conotrachelus. Not every species within these genera are restricted to acorns though which is why they are also often referred to as nut weevils.

Actually, most of today’s episode will be focused around one of the 36 species within the genus Curculio because the other genus Conotrachelus has over 1000 species and there is much we don’t know. One thing we do know is that these short-snouted weevils cannot bore holes into nuts themselves, and as a result can only use acorns and other hard nuts if they’ve been previously damaged. Most of the time these weevils will lay their eggs on soft fruits and the larvae bore holes into those and feed on the flesh of the fruit until they are ready to pupate in the ground. We also know that species within this genus can be found from Canada down to Argentina!

Curculio weevils on the other hand are found in North America and Europe but don’t have a range as far south. These ones can also actually make holes in acorns and other hard nuts using a saw-like mouth at the end of their long snouts or rostrum. This is accomplished by doing circular motions with their mouth steady on the acorn until they’ve successfully created a small hole. Once this is accomplished they can deposit one or more eggs into the acorn or other hard nut using a long ovipositor and the larva will remain feeding inside until it is ready to pupate.

If you’ve ever noticed acorns, hazelnuts, chestnuts or pecan nuts with a small perfect looking hole then you can be sure that it once or might even still have a larva inside. The cool thing is that these kinds of weevils are particular about which host plant they like. For example, one species of weevil won’t target acorns AND chestnuts. They each have their specialty or at least that’s what the literature says.

In North America, there are about 30 species of curculio weevils but there is one species in particular which is known as the acorn weevil in both North America and Europe. This would be Curculio glandium.

This species of weevil is relatively small reaching an adult length of 3/8th inch but is actually larger than other acorn specific species and they also have characteristically longer rostrums when compared to other acorn weevils as well which they use when making galleries for egg laying.

The digging behavior usually begins with a shallow hole in the acorn to see if it’s already been disturbed or used by another insect. If the acorn is fine then females begin actual drilling which involves inserting the entire rostrum into the acorn creating galleries that run along the side of the acorn but never deep into the center of an acorn. Afterwards the female will turn around to oviposit eggs within one or more galleries, but she does not place eggs in every gallery she’s made. It's actually pretty smart to not drill deeply into the acorns because it gives the acorns the opportunity to heal and re-seal the initial hole protecting the eggs and larvae from wasps and other predators.

These weevils also start mating and creating galleries in acorns later in the season compared with other acorn Curculio species. It is believed to be a matter of difference in size which allows these species to be better adapted for larger and more mature acorns. In fact a study was done comparing acorn size preference and the females of Curculio glandium always chose to lay eggs in larger acorns when given a choice. They also preferred to lay eggs in acorns which were already damaged or cracked as opposed to spending an hour or two drilling a fresh hole into an undamaged acorn.

The females of C glandium begin mating and scavenging for acorns to lay eggs in towards the 3rd week of July and then cease drilling in the end of August when acorns are too stiff for them to pierce. These insects along with other curculio species start drilling their holes within acorns prior to their fall from the tree.

The larvae for curculio weevils are C shaped and develop within the nuts rendering the nut infertile after their development. This is actually why acorn weevils are considered a pest in Oak trees. If you want to collect acorns to use as seeds it's best to collect them just as soon as they fall, but by this time they might already be infested. If you happen to find acorns with holes that have freshly fallen you can actually soak them in water up to 120 F or 48.8 C for 20min which should kill the grubs and save the acorn.

You can’t freeze the acorn because that actually kills it, unless you don’t need the seeds for planting in which case by all means because freezing kills the grubs too as these ones aren’t resistant.

The acorn weevil grubs actually leave the acorns as soon as they are ready to pupate and they bury themselves into the ground and remain there for 1 or 2 years. It really depends on the location and how long the summers are compared to winters. They don’t feed in the ground or anything like that, they sleep for the first winter in preparation for their pupation which occurs the following summer, and if they don’t finish in time before the weather gets cold again then it takes another winter.