Life Inside an Acorn

I made my way out to a nice stand of northern red oaks, planning on looking for bud galls but got distracted by other things. Today's focus ended up being finding things living inside of acorns. As I was walking along the trail I noticed that some of the acorns were just staring to sprout and send out their radicles. This reminded me of a little inquiry project I did last year when it was getting pretty late in the spring and all of the viable acorns had sent out their radicles (some were even starting to leaf out), so it was obvious which ones were not viable. I assumed that the main culprit was an acorn weevil (Curculio sp.), but decided to cut open ten acorns to see why they didn't germinate. Of those ten, eight of them had obvious weevil damage (usually the larva was still present). To my surprise two of them had cryptic cells of a gall wasp in the genus Melikaiella! With this in mind, I was curious what else I might find inside of these acorns.

The most charismatic find of the day were acorn ants (Temnothorax sp.). These ants will find acorns with small holes in them and make it their home. Luckily for them there were plenty of acorns with small exit holes from various insects in the area. Some of the acorns had hundreds of ants within them. As I was watching it seemed like some of the large colonies were breaking up to establish new ones in new acorns. Many species of Temnothorax ants are parasitized by slavemaker ants (often other species of Temnothorax) which will steal pupa from the colony to enslave as workers in their own colony. Maybe I actually saw a slavemaker abducting the pupa, who could say! It seems that there are a few possible candidates for which species of ant I saw today. It also looks like each Temnothorax species that establishes their own colony are parasitized by a handful of slavemaker species, along with some less detrimental inquilineous species. I'll have to try and find some keys and take a closer look to try and ID any species I come across and see if I can see and understand these interesting behaviors. These dynamics - one species of ant hosting many closely related species of parasitoids/inquilines are very similar to what I've seen rearing gall wasps. Many of the rose galls (Diplolepis sp.) I have reared host at least one inquiline (closely related Periclistus sp.) and sometimes up to 13 different parasitoids (Chalcid wasps). Diplolepis seems to be especially susceptible to parasitoids and inquilines, but other wasps such as Diastrophus sp. and cynipids also boast pretty high numbers. I still cannot believe the hyper-biodiversity with such specific niches found in such a small area in these gall communities. Although to a lesser extent, it seems like acorn ant colonies also host a great deal of biodiversity. Of all things, when I was comparing these ant colonies with gall wasps, they both made me think of the role of beavers in the ecosystem. Beavers are considered a keystone species, largely because of their role in shaping the landscape as an ecosystem engineer. I think of gall wasps and these ants as micro-keystone species and micro-ecosystem engineers. They fit both the definition of keystone species (significantly supports the food web of an ecosystem) and ecosystem engineer (significantly modifies habitat), but just on a small scale. 

Temnothorax sp. inside of an acorn

A close up

Moving pupa to form a new colony?

I also saw many other species in the acorns I looked at, although my camera died shortly into the walk so I didn't get pictures of everything. But, I think I might have just scratched the iceberg today. I plan on going out and collecting some acorns and trying to rear out whatever I can to study some of these species more closely.

Woolly catkin gall wasp (Callirhytis quercusoperator) 

The ants were really abundant in many of the acorns with openings from this species

Acorn moth (Blastobasis glanduella)

I didn’t get a picture but some of these had parasitoid exit holes 

Melikaiella sp. cryptic galls

I also saw some cocoons that may have been from the acorn moth and weevil larva, but didn't get a photo of either. Last year I assumed that all these unviable acorns that didn't sprout were mostly due to weevil damage. Now I know that there are at least a handful of species causing this damage and countless others supported by those species. Many people would consider these insects "damaging" these acorns as pests, causing harm to the forest. It's all just part of the ecosystem and this biodiversity leads to stability. I'm sure the birds, squirrels, bears and anything else that might eat acorns enjoys the extra fat and protein in these insects when they eat the acorn. 

I've been finding some interesting things in bur oak acorn caps recently as well. I was back home about two weeks ago and collected a few handfuls of the cryptic acorn cap galls. Adults have started to emerge from them, so far it's only been inquilines. These galls have been so fascinating that I've only been looking at the caps, who knows what I might have missed going on inside the acorns. Red group oaks and white group oaks support quite a different community of galls and probably other types of insects as well. They have slightly different natural histories with white oak acorns maturing in one year and sprouting in fall and red oak acorns maturing in two years and sprouting in spring. With this extra time red oaks would be more susceptible to predation, but luckily they have much higher tannin levels to deter animals from eating them. I'm guessing that if I were to look closely inside some of the white oak acorns they would support many species as well. I wonder which one supports more, white oaks with less defenses and less time, or red oaks with more defenses and more time. 

Undescribed cryptic acron cap gall 

Bisected to show larva

The most common inquiline to emerge is a Synergus sp.

Another inquiline a Ceroptres sp? or maybe a Synergus sp?

On the walk back I made two interesting finds. It was my first sighting of the gall wasp Biorhiza caepuliforme which forms galls on the stems of northern red oak right at ground level. It was somewhat abundant here, but I only saw it on relatively old trees that were cut down at ground level and sent out stump sprouts. I'd guess that the stems it was on were all 3-5 years worth of growth. I also saw a (presumably) undesribed species of Marmara moth making mines on a quaking aspen. 

Biorhiza caepuliforme 

Marmara sp.