If you’ve been keeping up with our gall hunting updates then you might remember the spongy oak apple gall, Amphibolips confluenta, that was co-opted into a nest by a leafcutter bee. We were concerned that the bees wouldn’t emerge until spring, but, in late July we had 7 bees emerge(!) — almost a month after the gall was collected.

All of the bees that emerged were the species Megachile mendica. This species is widely distributed throughout the United States with higher prevalence in the east coast. All of the individuals that emerged happened to be male, which was unexpected since other Megachile species nests are known to yield a mixture of male and female bees. M. mendica is known from trap and soil nests, so it being found in a gall is pretty neat! The bees were ID by Shelby Kilpatrick, a Ph.D. candidate in the Entomology department who has created the Checklist of the Bees of Pennsylvania. All of the sources used by Shelby can be found at the end of this post.

Thanks to Dr. Urban who took time out of her busy fieldwork season, we were able to get the gall X-rayed. As a result, we were able to see the route the bee took, excavating a continuous tunnel in the gall to place the larvae inside. We also found that the gall still contained an unemerged bee as seen below. This unexpected find actually aided us in interpreting the images we obtained. 

Resources used by Shelby to ID the specimens:

• Sheffield, C.S., Ratti, C., Packer, L. and Griswold, T., 2011. Leafcutter and mason bees of the genus Megachile Latreille (Hymenoptera: Megachilidae) in Canada and Alaska. Canadian Journal of Arthropod Identification, 18, pp.1-107. DOI: 10.3752/cjai.2011.18 Available online here.
• Mitchell, T.B. (1962) Bees of the eastern United States. II. Technical bulletin (North Carolina Agricultural Experiment Station), 152, 1-557. Available online here. 

 

Social distancing regulations may be preventing some individuals from conducting research, but gall hunting has continued in full force. These outings have mainly been a solo activity — observing and collecting galls on oaks in Centre County. Leading to amass approximately 400 galls(!), all of which are being reared from a makeshift lab space in my living room apartment. This isn’t the ideal set up for observations, but once we went into lockdown, I moved all of the galls I was monitoring into my apartment to reduce possible COVID-19 exposure. This summer, I’ve bumped into a few particularly interesting galls on red oaks that I’d like to highlight.

The first being Amphibolips nubilipennis, commonly known as ‘Translucent Oak Gall Wasp’. This gall earned ‘Observation of the Day’ on iNaturalist back in June! Although many commented on its bubblegum appearance, its firmness and heftiness reminded me more of a bouncy ball. However, not all translucent galls belonging to this species share that bright pink coloration. A different A. nubilipennis gall that I was able to rear was actually much smaller and clear and had a parasitoid wasp emerge instead. This clearly illustrates how the salivary secretions from A. nubilipennis are what signals the gall to grow bigger and pinker.

Another interesting observation I made also occurred back in June. While out collecting, I noticed a spongy oak apple gall, Amphibolips confluenta, that contained two exit holes, which seemed unusual. The first exit hole was likely from a gall wasp at only 1mm in diameter, while the second one was considerably bigger at 7 mm. Since the presence of a gall wasp size exit hole meant I likely couldn’t rear a gall wasp from the gall, I decided it was worth opening it up to check out why it had such an odd, larger opening. I discovered that a leafcutter bee had co-opted the inside of the gall to make its nest. It appears like the bee excavated the inner spongy structure in order to lay her eggs. I’m currently trying to rear the larvae so we can identify the species of leafcutter bee and compare this nest to its normal nesting material since they generally use straight structures for their nest. In the meantime, we are arranging to get the gall x-rayed so we can see the inner modifications of the gall without disturbing the larvae.

Last but not least, our most recent exciting observation was of the asexual generation of Callirhytis quercusoperator. While looking for pip galls under the red oaks in front of the Frost Museum, I noticed a bunch concentrated on an ant colony. Pip galls resemble small seeds that grow between the acorn and the cap and easily detach when the acorn falls from the tree. After some inspection, we noticed that it was actually a combination of whole and empty gall shells. We can only speculate that the ants collected them and are using everything inside of the gall for nutrition — plant tissue and wasps alike — since we didn’t find any discarded gall wasp larvae near the nests! It was definitely surprising to see this type of gall predation since ants are never listed among common gall wasp predators. On the contrary, they are actually known for having a mutualistic relationship with nectar‐secreting galls.

On a final note, I was recently invited to talk about gall wasps in the Arthro-pod podcast. Check it out!

I’ve cycled through many hobbies already, during this time in isolation – throwing darts, writing letters, building a chicken coop, an ill-fated attempt to get in the sourdough game … My current project is to set up a pitfall trap array on our property, mostly using construction scraps from a recent renovation. 

I haven’t really done much pitfall trapping in my career, as the insects I usually study are better collected in pan traps and Malaise traps or reared from galls. I do have a growing interest in spiders, though. I also want to grow our collection of mygalomorphs. Maybe I can get some wandering males? Our lone Sphodros rufipes male is from a pitfall trap near Harrisburg.

This review of pitfall trapping strategies by Brown & Matthews (2016) was a great starting point, especially when thinking about what to use as a preservative. For the traps themselves I constrained myself to whatever we had on hand—plastic drink cups, pieces of PVC pipe, wood scraps from our old deck, leftover tiles, and glass jars. I read a bit more about propylene glycol in Thomas (2008) and trap spacing in Ward et al. (2001). Brown & Matthews (2016) found that many publications did not report sufficient details on trap design, so I made a point of documenting all aspects of my design and will draft an SOP (#25, soon to be available on our policies and procedures page).

Here’s what I came up with. The parts:

Here’s how the trap looks in situ:

I set up more than a dozen of these pitfalls on a hillside, in the forest. I’m using RV/Marine antifreeze (Splash brand -100F; Mfr. Model # 919626-G35; Elliott Auto Supply Co., Inc.) as my preservative, diluted a bit with water (1.2 L water: 0.7 L antifreeze: 2.5 mL biodegradable dish soap). The traps are arranged more or less haphazardly—basically wherever my rocky soil would let me dig. I can’t wait to see how well they work and to share what I found! If any of you pitfall experts have ideas for how this design can be improved please add them in the comments below.