Scale Insects on Plum, with Ants

2010 September 18

Around June 6, S_ noted that one of our small plum trees [1] had a significant infestation of these scale insects:

Yes, I know that they don’t look much like insects, but trust me on this. I think they are probably one of the Lecanium Scales, possibly European Fruit Lecanium Scale, Parthenolecanium corni. Which, in spite of the name, actually seem to be a native North American species that is also found in Europe. It isn’t clear to me whether they were transported to Europe early on, and then found there before an entomologist described them here, or whether they were always present all around the northern hemisphere.

These vary a lot in appearance, so it is hard to make a definite identification, but they are well-known pests of plum trees among many other hosts. The newly-hatched scale insects have legs and are referred to as “crawlers”, and these are the ones that spread around. When they mature, they drive their mouthparts deep into the plant, fasten themselves down, and then harden their shell for protection.

If you look closely, you can see an ant there where the branches all come together. I think she’s probably one of the Formica species, at any rate she is pretty typical of our larger ants around here. There were a lot of ants patrolling the area, here’s a better picture of one:

Edit: James Trager said on BugGuide that these ants were in the Formica rufa species group. And, based on the visible setae (hairs) on the hind tibia, he thinks that they are either Formica obscuripes or Formica obscuriventris.

The ants aren’t there by accident. A lot of the immobile plant-juice-sucking insects, including scale insects, secrete “honeydew”, which is basically sugar syrup. Ants really like this, so they come around and lick off the honeydew. The benefit for the insects secreting the honeydew, is that the ants also then prowl around and eat, kill, or drive off the many things that want to eat the immobile insects, things like ladybug larvae, lacewing larvae, parasitic wasps, and other predators.

I had picked one of the scale off to get a picture of the underside, but it isn’t terribly illuminating – it looks like a hollow shell.

It is quite possible that this one was dead, and all I had was a vacant shell. In the first picture on this page, you can see that some of the scale insects are smooth, dark, hard domes, while others are lighter-colored and have a rougher surface. It could be that the lighter-colored ones are still alive while the darker ones are dead. I couldn’t confirm this, though, because after noting that the one I had first might have been empty, I went back out to the tree in mid-August to get another specimen – and they were gone! Where once there had been hundreds of them, there were now only one or two visible, and they were the dark, hard type (and may therefore have been dead). And, the tree looked none the worse for wear.

Reading up on them, I see that the European Fruit Lecanium lifecycle only has one generation a year, and the time when I photographed them was right about the time the females were laying eggs and dying. By August, the eggs would have hatched out into “crawlers” that would have been on the leaves, not the stems. Now that summer is over, any surviving crawlers would be moving back down to the stems and getting set to overwinter right about now, but last time I checked I didn’t see anything. So, I think they might be pretty much gone at this point. The crawlers would have been vulnerable to many different predators (like the lacewings from a couple of weeks ago, perhaps). It is quite likely that they just got eaten, and are no longer a concern here in the Land of the Living.

Which just goes to show that, sometimes, if you ignore a problem long enough, it will solve itself [2].

[1] It has been a small plum tree for a long time, and is about 2-3 feet tall. At least, we think it is still a plum tree. See, we had bought a plum sapling from a local nursery, which was doing just fine until the accursed deer came and ate most of the leaves, followed by the top part dying back over the winter. It then came back up from the roots. Unfortunately, this means that the actual plum variety that had been grafted onto it is now dead and gone, so all we have left is whatever the nursery used as grafting root stock[3]. Root stocks are generally very hardy trees closely related to whatever is being grafted onto the top, so with any luck it is some type of plum tree itself. However, it could be something related to plums, but not actually a plum. So, we just have to wait and see whether it ever has any fruit before we find out whether it is an edible plum, or some nasty, gnurly, vaguely plum-like fruit.

[2] In this case, it probably solved itself because the tree was outside and fully exposed to all predators, and I never did anything to it that might have interfered with the natural controls. If it had been a houseplant, that would not have been a workable strategy due to a lack of predators, and scale on houseplants is a big problem. In particular, their waxy coating makes it difficult to kill them with pesticides. For example, KT Cat has been having some difficulties with scale on a Gerber daisy. His past poor results with insecticides lead him to consider simple asphyxiation as a solution. The device used for this turned out to be more of a potted plant sauna, which appears to actually have been quite effective – it looks like scale cannot tolerate heat as well as plants can. I think he’s onto something, hopefully KT can update us on how it all came out in the end.

[3] A problem with most fruit trees (apples, cherries, plums, pears, etc.) is that if you plant the seeds, you don’t get the same variety as the parent tree. What you get instead is a crapshoot, where the resulting fruit will normally be some small, sour thing that closely resembles the ancestral, pre-domestication tree. Although sometimes, you luck out and get a good one. So, once you get a good variety that you want more of, you need to have some way of propagating it other than planting seeds. Some plants you can just cut off a branch, stick it in some soil, and water it, and it will root and turn into a new plant, but that doesn’t work very well for the common fruit trees. So instead, what you do is plant a lot of the seeds to get saplings that can serve as “root stock”. You don’t care what kind of fruit the root stock would make, because the next thing you do is cut off their tops, and graft on a branch from the tree that you actually want to propagate. The graft then fuses to the root stock, and voila! Grafting is actually pretty easy even for amateurs, my mother has been doing it with her trees for some time with good success. The problem from my point of view is that the graft is a weak point, and the grafted branch is a bit more susceptible to being killed by cold over the winter than the root stock is. And, since our winters are long and severe, a graft that would be perfectly fine further south just doesn’t make it in our yard. We’ve had a number of purchased trees die back to below the graft over the winter, so what is growing now is root stock, not the grafted variety. So, in those cases, we might as well have just planted seeds. What I would really like to do, is use rooting hormones to get cuttings from good trees to root directly rather than having to worry about grafts. Then, we can avoid the problem with the weakness due to grafting altogether. The only thing I’m not sure about, is whether rooting hormone works on fruit trees. I suppose I need to get some and try it out.

8 Responses
  1. September 18, 2010

    I really love this blog. Keep up the good work! I have never heard of scale insects, and am going to be looking much more closely at our fruit trees.

  2. September 18, 2010

    As I read the article, I became more and more impressed at how many new things I was learning, from about the insects to tree grafting (which is a back-burner project of mine).

    Excellent stuff here. And I particularly liked the closing quote. I should try that technique more often!

  3. Della3 permalink
    September 19, 2010

    I remember having problems with scale insects on both indoor and outdoor plants. I can’t remember the solution that worked best, so I’ll give all three:

    1) Buy a small container of horticultural oil (sometimes called pesticide oil). This is a very fine oil that is usually sprayed on plants to protect from future insects or kill current insects. It does not contain any poisons. It is just an oil. It asphyxiates existing bugs and new bugs often won’t bother a plant coated with it. Follow the dilution directions on the bottle to make a small amount in a cup. Then use a cuetip to brush a coat of the stuff on the offending insects. You can wait a while or keep brushing until the insects start to rub off of the plant. I’m pretty sure this stuff worked on the scale insects, and it is the safest for people and plants.

    2) Mix a week solution of rubbing alcohol and water. Again, use a cuetip and dab a very small amount on the bugs.

    3) Mix a week solution of bleach and water. Use the cuetip.

    I’m not sure about the results of items 2 and 3. They probably worked, and I probably used them before I found out about the horticultural oil. However, they can damage your plant as well, so you need to be much more careful with them. A few plants with more porous surfaces can be damaged by the oil, but hard stems and hardy leaves are usually not affected. For instance, this is a common pest and mildew control for eco-friendly rose gardeners, so I have used a lot of it. I have many rose bushes, and most can tolerate the oil just fine. But one bush that is particularly disease and pest prone also cannot tolerate the oil. The leaves are just too delicate, and will turn brown and die when they are coated with anything. So try a product on a very small spot first and see what happens before coating a large area.

  4. Della3 permalink
    September 19, 2010

    Rooting hormones work well on most plants. You can also get horticultural wax to coat the branch and protect it from drying out before it grows roots. Or you can put a small greenhouse tent (sticks and plastic or some other device) over the branch. Peeling a tiny bit of the bark off of small spots on the bottom of the branch (the part that will be in the soil) and dusting those places with the hormone will also encourage roots from those spots. You just want to peel off the hard outer layer and expose the (usually green) active growth layer. If you go too deep, you’ll just get the main artery where all of the nutrients and water are sucked up. Be patient. It takes a long time for a hardwood plant to grow roots and sprout new growth.

  5. September 19, 2010

    Thanks for all the links! My daisy seems to be back in full health, but I really need to get out there and take some more photos for comparison sake. I have to say I was really surprised that these guys have but one generation a year. Ours seem to run wild 24/7/365.

  6. September 20, 2010

    These insects infest the trees along the canal by work. I always seem to find them at the dead or dying stage too; I’ve never seen the insect proper.

    Had I caught this year’s batch early enough I’d have run an infested twig though our Xray microtomography scanner for some real macro pictures…

    Here is one example :

  7. September 20, 2010

    Mike and Andy: Thanks!

    Della3: I appreciate the rooting tips. I gather than right now is a good time to try rooting cuttings, so I should give it a whirl – maybe starting with our mulberry bushes and plum tree.

    KT: That’s one advantage of living way up here in the frozen north. While it may be harder to grow things, and there are a lot of things we just can’t cultivate, on the other hand we don’t have anywhere near the same pest problems that people have in the south. The winter really does help to keep down the pests. It’s as if we have a giant RESET button that gets pressed every year, which keeps the pests from really getting ahead of us. The really noxious species seem to be mostly tropical.

    David: that X-ray microtomograph of the oak apple gall is great, I had no idea that X-ray tomography could manage that kind of resolution!

  8. September 20, 2010

    We mainly use the XMT to look at teeth and bones and various minerals, but it is always fun to look at something more interesting. There is a rendered video from the same oak apple online too :

    At least four wasps are seen developing in this gall.

Comments are closed.