Infestation of Miniature Birch Tree
what has attacked my flourishing miniature birch tree near the end of June (2019). Found the leaves covered with small blisters, humps, bumps . I walk the garden almost everyday so was surprised to see what had been a happy healthy tree. There were a few 1-2" thin caterpillars crawling around and eating the leaves. I hand-picked them (maybe 15-20) off. Seemed to be mostly on the new growth. The bumps were on top part of leaf and as time went by they turned from the green color of the leaf to a bright yellow color, grew in size, and got a tough thick shell. Were hard to scrape or pick off when I tried. They continued to grow for rest of the season but I never saw anymore caterpillars. Was it insects, a virus?? What can I do to prevent it next year if it was insects? Is there any help if it is a virus? I have pictures which I developed but lack the ability to download to my computer to sent them. Need a tutorial on that, but until I do I am also mailing this question and the photos by mail to your Ellicott City address. Is that helpful, do you accept pictures by mail??
Baltimore County Maryland
Sounds like you are describing symptoms of spiny witch hazel gall aphid. This is a common pest. Here are good photos: https://bygl.osu.edu/node/719
Many native beneficial insects feed on and control this aphid. They could have been the "caterpillars" you saw. Here is our page about it: https://extension.umd.edu/hgic/topics/shade-tree-galls with some links, also. The IPM link has color photos of the beneficial predator insects you want to be sure to conserve.
Underside of leaf looks like hickory leaf stem gall rather than the river birch gall although it is aFox Valley Dwarf River Birch. And the galls were not arranged in a straight pattern or corrugated. they were more random like the one in the maple illustration in your response. I could never see any insects other than the caterpillar‘s that had wide apart black stripes on them even though I looked it over and over day after day. Can I still use horticultural oil in the Spring before leaf emergence?? I feel like your information has helped me. But I am 89% sure it does not look like witch hazel/ river birch gall. Is it at all possible that it’s the maple gall? Would just like to have my curiosity satisfied. Thank you.
If the photos are on a phone, usually a cord connecting it to the computer (typically into a USB port) will allow the copying of the picture files onto the computer. Alternatively, you might be able to upload the photos via the phone directly if you are using the Ask an Expert submission form on the phone itself.
The caterpillars may have been one of the species of sawflies that feed on birch. Sawflies are cousins of wasps but not wasps themselves, and their larvae look like caterpillars; often, thew chew on foliage just like caterpillars do. Physical removal as you have done is the easiest, fastest, and most environmentally sound technique for treatment. Otherwise, insecticidal oils, insecticidal soaps, and possibly some formulations containing spinosad would have minimal impact on beneficial insects and should still provide some control if and when these caterpillars reappear.
The galls do sound different than the typical gall-forming aphid found on birch, so if photographs do reach us, we'll see if they look familiar. (What address did you send the samples to?) Most gall-formers are very host-specific (either to certain species or a group of closely-related species); a few are more generalized. Based on the description of the galls, they do not sound familiar enough to suggest an ID, particularly with River Birch as the host. (Are you sure the tree was correctly labeled and no other species sapling is growing among the birch stems?)
A virus is unlikely, though plant viruses can be transmitted to plants via sap-feeding insects. No treatments exist for such a virus and the plant would need to be removed and discarded (in other words, not composted). Pruning tools can also transmit pathogens like viruses if not cleaned and sterilized between plants. Viruses also tend to be host-specific, however, and other unrelated landscape plants, even if they harbor a plant virus, are unlikely to have infected the birch.
If it is one of the routine gall-forming insects (or mites), no control sprays would be recommended as they are either ineffective or impractical as the deformities do not harm the overall health of the tree. Any particularly unsightly leaves could always be picked off if they are within reach.
You can check out this page for River Birch on BugGuide.net to see if any of their photos look familiar. (Nothing jumped out to us regarding the description of its galls.) https://bugguide.net/index.php?q=search&keys=betula+nigra&search=Search
I sent the question with two photos to the address in Elliott City, MD. I will try to get them to you from my phone but have limited technical skills.
We viewed the photos. This looks like a type of leaf gall caused by a small insect such as a midge, wasp, or mite. We cannot say which one.
Chemicals produced by the egg laying and feeding activities cause most insect and mite galls. The chemicals cause the affected plant cells to swell. Once the galls are brown, the insect is gone. The galls will not harm the tree and no control is needed.
If this can not be tolerated, you can pick the leaves off when the galls are green and discard in the trash. However, you do not have to do this.
These insects can be cyclical and you may not have a problem next season. Here is our page on shade tree galls https://extension.umd.edu/hgic/topics/shade-tree-galls