"Ash Flower Gall" - An Eriophyid Mites
Eriophyid Mites (Aceria guerreronis)
Eriophyid mites are more closely related to arachnids (spiders) than insects. They are microscopic and cannot be seen with the naked eye. They are noticed by the abnormalities created in soft plant tissues when they feed.
Commonly Effected Trees:
A hand lens or microscope can be used to identify these critters, however it is very difficult to differentiate between species using this method.
Eriophyid mites are easily identified by the signature mutations each species causes within its host plant. As the mite feeds, by piercing and sucking the soft cells, it introduces a growth regulator through its saliva into the plant tissue, which then induces a desired reaction from the plant. This reaction or mutation then acts as food and protection for the parasite. They are host specific, which means that each species of eriophyid mite has one type of plant that it lives its life in and cannot reproduce within another plant system.
There are basically five types:
Eriophyid mites overwinter as females in a hardened state either underneath the bark or in bud scales. In the spring they lay eggs which hatch after fertilization and then go through three more stages: nymph 1, nymph 2 and adult. They have multiple generations per year, usually 3 or 4, with some generations being all female which is then followed by a generation with males.
Their physical structure is elongated with 2 leg pairs which are near the head. They have a very difficult time moving around by themselves and depend on wind, insects or birds to carry them to a new host.
Damage is typically cosmetic and rarely harmful to plant health. Severe infestations can cause twig dieback on some plants and can be fatal in some instances.
Damage is always to soft plant parts like leaves, flowers and new growth.
Control of this pest is fairly easy. Removal of infected plant parts is essential, but not always practical for control. If the problem is aesthetically displeasing there a few effective sprays that can be used:
Conventional: A pesticide called Carbaryl applied in mid spring as the buds break should give season long control. It may be necessary for 2-3 seasons and then monitoring for new outbreaks thereafter should suffice.
Non-Toxic (almost organic): A dormant oil in the fall just before leaf drop will give a fair control over the next years 1st generation of this pest. This should be followed up the following season with several horticultural oil sprays to continue smothering adults that will reproduce. Probably 3-4 times during the growing season.