Scotch Broom Eriophyid Gall Mite
By Jenn Fisher, Master Gardener Program Assistant
Many of you have included Scotch broom cultivars in your landscapes for their beautiful flowers in vibrant yellows, pinks and purples and as a low-maintenance, drought-tolerant shrub. If you have noticed a general decline in the health of your Scotch broom shrubs and have observed penny-sized, lumpy masses hanging in the branches, your shrubs have infestations of gall mite. These mites are so productive in killing Scotch broom, they are, in many states, seen as a biological control of invasive Scotch broom. Without the conscious assistance of humans, these mites have traveled long distances, spreading from the Pacific Northwest down to California. It is believed mites travel by wind currents and by attaching on animals, humans and equipment. Unfortunately for those of us in Nevada who like our ornamental Scotch broom, the gall mites are not so welcome.
To have success in getting rid of Scotch broom gall mites, its important to understand their life cycle. As early spring is setting in, adult females lay their eggs in last year’s stem bud where they hid during winter. During this time, the mechanical damage from their feeding and egg-laying on the plant initiates increased production of normal plant growth hormones and creates an abnormal plant structure called a gall. The gall formation usually occurs during the high-growth season of late spring and summer. Mites go through their life cycle protected in these galls. When winter is on its way, the galls will start to shrivel, signaling the adults to leave the galls to seek overwintering sites on the new stem bud.
Since galls form on last year’s growth, the mites cause stunting, reduced flowering and can kill whole shrubs if left unnoticed and untreated. As mites cannot be seen easily with the naked eye, you may not know you have gall mites until the galls have formed. At this point, if you have a minor infestation, you can remove the parts of the plant that are infested and dispose of infested tissues by burning or bagging and discarding. Pruning should be done in early spring when the tree is dormant and the mites are still in their overwintering sites. This will not completely eradicate the mites, but will help to significantly reduce the population on the new bud.
If you have a larger infestation, you may want to try chemical applications. Because the galls protect the mites from insecticides, you should treat your plant when mites leave the galls and are looking for safe hiding spots for winter. To manage mites, you can apply a late-season application of an ultrafine horticultural oil spray or other insecticide prior to leaf drop of the plant. Though this process will kill beneficial mites, it will reduce the possibility of mite-induced galls in the next year’s new foliage. You must have precise timing to be effective with late-season application, so it can be hard.
The good thing: you don’t have to worry about the mites affecting the other plants in your landscape because Scotch broom gall mites have specialized feeding habits. It’s up to you: if your Scotch broom plants have gall mites, you can fight them, or pull out your Scotch brooms and plant something else.
Oneto, S. (2015). Scotch Broom Gall Mite: A new natural enemy to California. Retrieved September 27, 2017, from http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=17357
Broom Gall Mite. (2014). The Biological Control of Weeds Book. Retrieved September 27, 2017, from https://www.landcareresearch.co.nz/__data/assets/pdf_file/0018/20565/Broom_Gall_Mite_Dec_14.pdf
Wawrzynski, R. P., Hahn, J. D., & Ascerno, M. E. (2015). Insect and Mite Galls. Retrieved September 27, 2017, from https://www.extension.umn.edu/garden/insects/find/insect-and-mite-galls/
Graham, J., & Johnson, W. S. (2004). Ornamental Plant Damage by Eriophyid Mites. Retrieved September 27, 2017, from https://www.unce.unr.edu/publications/files/nr/2004/FS0447.pdf