How to Treat Root Aphids on Bonsai Trees

How to Treat Root Aphids on Bonsai Trees

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As repotting season winds down here in San Francisco (yes, we’re ahead of the rest of the country due to warm winter temps) I’ve seen more than a few root aphids. They seem to mostly attack Japanese black pine in my yard, normally the younger trees that are growing vigorously with sacrifice branches.

As I mentioned in a previous post, they were feeding on pines in terra cotta containers more than those in colanders, which on the surface makes little sense since the colanders have so many holes for them to enter through.

Jonas Dupuich of Bonsai Tonight, and author of a great beginners book, has previously covered the topic of drenching the rootballs either while repotting, or during mid-season, using a variety of insecticides and generally seeing little phytotoxicity while also seeing some improvements in plant health to outward appearances.

While it seems that there is some debate online about whether these treatments work, I think the more interesting challenge is in realizing that root aphids are the problem, and also in being sure that it’s not instead arbuscular mycorrhiza. In my recent post, I noted that I did not find a combination of the two in my pots, only one or the other in 6 of the 8 test subjects (two had neither.)

Check for root aphids in between repottings

Scratching around on the surface of the soil may give you some idea; I regularly check the soil around the base of pines to ensure they are getting uniformly wet after watering. In the process I sometimes see arbuscular mycorrhiza, but I also sometimes see white dots or even the blue-tinged signs of root aphids.

Since young trees frequently grow roots right out of the bottom of the container, another option is to simply hoist the pot over your head to examine what is going on underneath. Or consider popping  the tree right out of its container mid-season to take a look at things up close. I would not try this with a 20 year old bonsai but I would certainly not hesitate with a 5 year old tree, especially if the rootball is solid.

I’ve had some trouble distinguishing between root aphids and myco by sight so I recently picked up a 20X magnifying loupe with a light in it, and then as a bonus realized I could take photos through it with my iPhone.

Under 20x Magnfication, can you tell? Is it Myco or Aphids?

Treatment for Root Aphids

Once you’ve determined that you actually have root aphids, treatment is the next concern. Since these guys can easily crawl from one pot to another, be sure to treat all the trees in the vicinity. As Jonas noted, a drench using insecticide and ensuring all the soil is wet is likely the best method. Some systemic insecticides may not effectively diffuse through the vascular system of the tree from needles to roots, so you should not assume that doing a foliar spray will kill root aphids (90% moves to the leaves from the roots per the linked study).

I’m encouraged by some stories I’ve heard of beneficial nematodes and other biological treatments controlling these aphids. I found the article and product suggestions at Arbico Organics website to be particularly interesting. I plan to try treating with AzaMax since I already have some. And I learned only recently that this is an extract from Neem oil. It’s also sold under other names like AzaGuard, AzaSol.

Top: Root aphids on Japanese Black Pine. Bottom: arbuscular mycorrhiza. Both at 20x magnification

Have some experience treating root aphids on bonsai successfully? Leave a comment below!

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