Adding Insult to Injury: An Oak’s Struggle

(Above: Adult female Oak Rough Bulletgall Wasp on twiggall. Credit: Whitney Cranshaw, Colorado State University, Bugwood.org)

Port Stanton, Ontario lies 20 km north of Orillia on Sparrow Lake, the southern boundary of the Canadian Shield. Typical cottage country in appearance, it is upland forests, rolling hills, bare rock outcroppings and undulating land where we find numerous wetlands of various types.

We have found a beaver wetland complex just west of the Bayview Wildwood Resort on the southwestern shore of Sparrow Lake. This wetland has the appearance of a fen: no trees with fairly consistent patches of heath-like plants and sedges dotting the open water. But it is large and contains a few bare rock islands. The largest island, in the centre, is ringed with small alders and has some White Pines, White Birch, three small White Oaks, junipers and various other shrubs growing on it. My visit to it in early February of 2020 found the island mostly snow covered.

The three White Oaks might be described as shrub size. Two of them showed heavy signs of previous browsing by beaver and both of these plants had many suckers from the previous year’s growth. And every sucker had numerous Oak Bullet Galls on it. This February there was no evidence of browsing and the suckers flourished, reaching a metre in height.

Left: One of the two browsed White Oaks with tall suckers and galls on top.
Right: Base of White Oak showing previous Beaver cuttings. (Photos: Bruce Mackenzie)

The suckers were big and they did not resemble at all the normal growth of White Oaks twigs (the size and shape of them reminded me more of suckers from aspen trees). However, it is not uncommon to find suckers growing very aggressively on plants where browsing has taken place. The tips of all of this year’s suckers appeared withered, perhaps a factor of the numerous galls found along the stem just below the tip. The sheer number of galls would have taken a lot of energy from the top of the sucker.

The Oak Bullet Galls are caused by wasps from the family Cynipidae and the species may have been Disholcaspis quercusmamma (Walsh). These wasps have a very complex lifecycle. (A good description of their biology can be found here.) There are two generations each year and the galls I found this winter were previously the homes for the second generation, which is made up of only females. In each gall there had been one larva and it emerged from the gall as an adult in late fall. They are small wasps, about 6mm in length. They would have had but one function before winter and that was to lay eggs in dormant buds. The next generation, involving both sexes, will appear in the spring from these eggs.

A cluster of Oak Bullet Galls
(Photo: Bruce Mackenzie)

The Oak Bullet Galls that appear in the summer start with a rough surface and they exude a honeydew type substance. This honey dew attracts a number of species of wasps and ants which feed on the sugary liquid. Meanwhile, the larva is inside the gall growing and then later going through a pupae stage inside the gall.

The larvae of gall wasps release chemicals into the plant that cause the plant to grow a structure around the developing larvae. This structure not only houses the larvae but supplies the larvae with food. The gall, its controlled growth and its actions, is a function of chemicals released by the larva.

Common galls that are more familiar to many naturalists are the individual galls found on goldenrods that house another species of wasp. The Oak Bullet Gall is usually found in masses. I only found the Bullet Galls on these two White Oak plants. The other White Oak growing on the Island, which showed no signs of browsing, did not have galls on it.

On the lands surrounding the wetland I found many young White Oaks growing and did not observe any galls on them. I found them looking like a normal young White Oak sapling. I did observe some young White Oak and Red Oak trees that had been cut down by beavers but did not see any signs of suckering from those stumps.

The White Oak generally reaches its northern limits at the southern edge of the Canadian Shield. In this area it is usually found growing where bedrock is near the surface and where soils are thin. Around Port Stanton it was growing as well in deeper soils in a mixed forest of White Pine, Sugar Maple, Red Maple and Large-toothed Aspen.

I was quite surprised by the large number of White Oak saplings here. I do not remember finding an area in my Ontario travels with this amount of White Oak regeneration. The larger White Oaks in the area were generally found associated with rock outcroppings.

One is left with the question, why were these two plants subject to continued beaver browsing and why did they sucker so much? I am going to go out on a limb here, but I suspect the reason may be micronutrients. The thin soils on this isolated island will be very different than the soils in the neighbouring upland forests. The soils would be dryer and warmer in the summer. The chemistry, the microfauna and microbial life in the soils would be different as well. Could something about the soil nutrient regime on the island make these White Oaks more attractive to the beavers? Could something about the growing regime encourage these trees to sucker when browsed? The small oaks were growing in full sun.

As to the presence of so many galls I wonder if the continued browsing and unusual growth of the suckers made the stems attractive to the wasps. Something was different as it was only the oaks that were browsed that had galls on them. As to the other young White Oak on the island that was not browsed was it simply missed by the beavers or was it different in some way from the other two? Some references note that galls are not usually found on healthy oak trees. Does a parasite recognize an organism with a compromised defence system? What signals coming from the plant does the wasp search for?

It is not uncommon to come across plants of various species that for some reason seem to be continually browsed while others of the same species and nearby are not browsed.

Different soil regimes and climates can affect how a plant grows. One example of that is with Red-osier Dogwood growing on the Copetown Bog on the outskirts of Ancaster. On this bog the dogwood appears very different, almost unrecognizable, and is heavily browsed by White-tailed Deer. On the neighbouring upland sites in the Dundas Valley deer browsing of this dogwood species is rare. The soils of the bog and of the surrounding areas could not be more different and something result in making the Red-osier Dogwood on the bog attractive to winter deer browsing.

Now, I am not sure if I will make it back to this wetland by Port Stanton next winter, but if I do, I will want to see what has happened to the two stunted White Oaks.

This article originally appeared in the May 2020 issue of The Wood Duck.

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