Galls are everywhere … if you take the time to look … particularly on Oak trees. The kids and I love insects, so we’ve been actively looking for galls for years. When they learned that one of Barb’s challenges this season included a study on galls, they proclaimed that it was one they were most looking forward to doing.
Galls are caused by many organisms living on plants, including insects, mites, mistletoe, fungi, and bacteria. We discovered that the Oak is host to more galls (specifically wasps) than any other plant. This wasn’t surprising as it is on the Oak that we most often observe them.
|An “oak apple” gall caused by the California gall wasp (Andricus californicus);
observed on an Oak stem/branch in the spring 2011.
While we were in Norway, we observed the striking red spikes on the leaf pictured below. I had asked around but no one knew what they were. Months later, I posted my spotting on Project Noah, spotting #7418081. A recent clue leads me to believe these galls were formed by a mite, Eriophyes tiliae. These chemically induced galls form an erect, oblique or curved distortion rising up from the upper surface of the leaves of the common lime tree, Tilia x europaea.
|Observed on a leaf in Norway, May 2011.|
These mites move onto the foliage in the spring, having overwintered in the crevices of the bark and around the buds. The mites are less than 0.2 mm long. The chemicals released while sucking the sap from the lower leaf epidermis creates the colorful, hollow, finger-like extensions to form on the upper surface of the leaf. Before autumn, the mites, which have been actively feeding and growing inside the galls, depart from their little ‘homes’ and seek shelter elsewhere on the lime tree whereby the cycle begins anew.
|Observed on an Oak leaf on the ground in the winter 2012.|
|The collection of galls I provided the munchkins as we read about galls.|
|Buddy’s sketch – sadly, he wasn’t really in the mood and only drew the one.|
|Sweetie’s sketches … she chose to illustrate several.|
As I sat down to write up our lesson, I discovered that alien-like thing pictured below is actually a gall. I had it in our collection of seeds and cones, however, my online research revealed that it is in fact a gall caused by an aphid-like insect (Adelges cooleyi) of the order Homoptera that invades the needle-bearing branch tips.