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 my 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, who recently published a beginners book, has previously covered this topic a couple times, 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 on the interwebs about whether or not these treatments work, I think the more interesting challenge is in realizing you have root aphids, 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. If this isn’t happening, then consider popping it 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 but I would certainly not hesitate with a 5 year old, especially if the rootball is solid.
I’ve had some trouble distinguising 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.
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.
Have some experience treating root aphids on bonsai successfully? Leave a comment below!