The Tree Center


Written by davethetreecenters • September 06 Some Harmless (but interesting) Garden Pests

I’m sticking with it, but looking back at the title I just wrote for this blog, I see right away a classic oxymoron – if they are ‘harmless’ they can’t be ‘pests’. But for many (most?) gardeners, anything that doesn’t seem to belong, and looks like it could be hurting their valuable garden plants, is most definitely a ‘pest’ until proven otherwise.

In the days when I was presented with a stream of samples of the unknown, brought in, or sent, by worried gardeners, it sometimes seemed that the answer was either ‘fatal’ or ‘harmless’, with ‘harmless’ greatly outweighing all other answers. So I thought that today we would take a closer look at a random selection of oddball things that turn up in gardens, causing concern, but that are actually harmless curiosities.

For at least some of us they are also fascinating examples of the diversity and complexity of life on this blue marble – and more reasons why we need to take seriously our responsibility to protect as much as possible of it. Hopefully knowing a little more about these ‘harmless but interesting’ finds will help nurture that attitude, and keep the old marble rolling around in a decent state for a while longer at least.

Gall Insects

You don’t garden for long without meeting greenflies (which can also be black or red). These sap-suckers are often first noticed when the new leaves start to look puckered and deformed. The damage caused by their feeding creates new shapes, and although greenfly damage is pretty random, other insects have mastered the art of creative plant shaping to engineer perfect homes for themselves, and curious effects that top the list of ‘what is happening to my plant?’ questions.

These structures are called ‘galls’, and they vary from slender, delicate green columns to nut-sized hard lumps, with lots of variety in between. We know almost nothing about the mechanisms that trigger their creation in particular plants, but they are fascinating, mostly caused by small insects, and completely harmless. Let’s take a look at some:

Maple Galls

Galls of Maple Trees in Ohio | Ohioline
Spindle Galls

Sugar, silver or red maples are among the most common trees in much of the country, so it makes sense that their galls are often causes for concern. The most common are all found on leaves, varying in their abundance from year to year, tree to tree and even branch to branch. All of them are caused by eriophyid mites – tiny relatives of spiders and scorpions that are only about 1/100th of an inch long, with four little legs and sucking mouth-parts – like the ‘rust mite’ at the top of this blog. (Although relatives, these are not like the well-known and true-pests, spider mites.)

Maple gall mite - Vasates quadripedes - BugGuide.Net
Bladder Galls

These critters spend the winter sleeping in crevices on the bark, or in the scales around buds. When new leaves begin to develop they head out and start feeding on the developing leaves. The combination of the plants natural growth hormones and chemicals injected by the mites produce unique structures on the leaves that make nice, protected homes for mites, where they lay eggs and then die, leaving the young to carry on. The pictures show the most common galls found on maple trees. Once the young mites have developed, and the leaves are mature and so cannot make more galls, they migrate back to the bark, and wait for spring to return. . .

Rose Galls

Robin's pincushion | The Wildlife Trusts

We all agree roses are beautiful, and sometimes they produce unique objects that might also be seen as beautiful, or at least interesting. Sometimes called ‘robin’s pin cushion’, these can be a few inches across, and are a round, fuzzy ball of thin threads, light green, usually with red tips. They are created by a tiny wasp, no  more than 1/5th of an inch long, that lays an egg in a leaf bud in early summer. The egg and larva secrete chemicals that distort the normal growth to produce this gall. The insects spends the winter in the gall, and comes out the next year to do it all over again.

Sumac Galls

Sumac galls - Melaphis rhois - BugGuide.Net

We have had galls produced by mites, and by wasps, so how about one produced by aphids? Staghorn Sumac is a popular native shrub, with big leaves divided into segments like a fern. In summer you can sometimes see clusters of green ‘bladders’ an inch or more across, hanging on the leaf stems. These are created by an aphid, which spends the winter in moss on the ground and in spring flies up to the sumac, laying eggs that produce the galls, inside which the aphids reproduce asexually. In fall the bladders split open and the aphids fly back to the moss for their winter vacation.

Oak Apples

Oak trees seem to live with more insects than just about any other plant. Two or three hundred insect species (almost all harmless) have been recorded growing on oaks in forests, and garden oaks can have a bunch too. One of the most common is the Oak Apple, a round, woody gall up to 2 inches across, usually found on the undersides of leaves. There are several different insects – all tiny wasps like those that cause the Fuzzy Rose Gall – causing these, but it takes an expert to tell them apart. Often one called the Oak Marble is more common in fact. The tiny female wasp lays an egg on a leaf bud, which then grows the gall, protecting the grub inside. They spend the winter inside the gall (if not extracted by a woodpecker!).

Oak Apples are particularly interested because from Roman times, through the Middle Ages and into the beginning of the 20th century they were the most common source of ink for writing. The galls contain high levels of tannin, and when this is reacted with iron sulfate a dark color forms. Gum Arabic (the sap of species of mimosa, Acacia) to make it sticky enough to stay on the paper.

So, Don’t Reach for the Sprayer

Often, the more bizarre and curious, the less likely something strange is to be a problem. Most plants die from hidden causes, like fungi inside their water-conducting tissues, and strange objects are rarely problems. So treat them as curious looks into the amazing diversity of life, and let them be. We all need to learn to tread more lightly on the planet.