Growing Knowledge
Small and mite-y
OSU researchers investigate after a new microscopic
pest causes damage to linden trees
mike reitmajer
mites, they were suspected of causing
problems in linden seedlings — specifically, tip damage, reduced growth and
compressed internodes.
OSU faculty and graduate students
took the lead to verify the association
between the mites and damage. They
also worked to determine the source of
the infestation. This led to a four-year
study of eriophyid mites in linden trees,
and the discovery of a new U.S. record
of a European mite immigrant.
Eriophyids are four-legged mites, less than .01 inches long, which drag their elongated body along
behind them as they travel across a plant surface.
By Betsey Miller, Robin Rosetta and
Vaughn Walton
DECEMBER 2011
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Mites and insects are regular residents in agricultural landscapes. Many
cause no problems, but every grower
knows the challenge of managing a
“bug” that has become a pest.
In 2007, a Willamette Valley nursery manager noticed that some shade
trees, linden (Tilia sp.) in particular,
were exhibiting atypical symptoms of
disease. Upon closer investigation of
leaf samples under a dissecting microscope, he found that they were infested
with tiny mites. Identified as eriophyid
Identification and detection
In pest management, the first step
is identification. Eriophyid mites present a real challenge in this regard.
Identification of these mites is nearly
impossible without the use of a compound scope. Mites must be mounted
carefully to keep organs and other distinct morphological features intact.
Dr. James Amrine, an eriophyid
specialist at West Virginia University,
identified the mite found in Oregon
nurseries as the rust mite Aculus ballei.
Previously only known in Poland, this
is the first record of the European immi-
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Small and mite-y
grant in the United States. Undoubtedly,
this cryptic species has existed here for a
long time and has only escaped observation due to the difficulty in detecting and
identifying eriophyid mites.
Orange in color and shaped like a
carrot, eriophyids are so small (less than
1/100 of an inch long) that they cannot be
seen without a microscope or hand lens.
Unlike other mites, which have
eight legs, these mites have just four
and they drag their elongated body
along behind them as they travel across
a plant surface. They usually cause distinct symptoms in their host, which can
aid in detection and identification.
Eriophyid mites are classified by
the damage they cause and are divided
into four major groups. Gall mites are
the easiest to identify, as they cause
morphological changes in the plant tissues they are feeding on, resulting in
the formation of galls that serve as a
refuge for the mites.
Bud mites feed on buds and fruit,
causing distortion or abortion. Blister
mites cause discoloration and deformation
of leaf tissue. Rust mites can also cause
discoloration of leaves, but may cause
deformation of young growing tissues.
Rust mites do not inhabit buds or
galls and are called vagrants, as they
are free living on plant surfaces. Vagrant
mites feed on young, rapidly growing plant tissues and actively move
around on a host plant in search of
new growth. Their feeding can distort
foliage and growing tips or may lead to
more subtle symptoms, such as reduced
growth or discoloration.
One problem with free-living mites
is that growers seldom realize that these
subtle symptoms are related to mite
infestation, often contributing the dis-
coloration or delayed growth to nutritional deficiencies. In addition, because
the damage is the only way to detect
eriophyid mite infestation, timely management is a challenge.
Although not generally harmful to
the plant itself, damage from feeding
mites can lead to a reduction in aesthetic quality, and ultimately devalue the
crop. For this reason, many growers do
preventative sprays on a routine basis,
which may unnecessarily increase production costs and upset the predatorprey balance in the nursery.
Distribution and behavior
Because of their small size and
awkward locomotion, eriophyids are
generally dispersed via wind currents.
Propagation practices are also a likely
suspect in dispersal.
Like many shade trees, lindens are
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propagated by budding, typically with
chip buds from on-site scion wood.
Linden mites overwinter in cracks and
crevices on trees and prefer older
wood, most likely because it provides
more suitable hiding places.
Research of propagation tissues
showed that nearly half of the overwintering mites can be found in old
pruning wounds, where the bark has
separated from the wood. The rest can
be found in other wounds on the bark
surface, under bud scales, in bark folds
at branching points, and in lenticels.
In these protected sites, they spend
the winter in a resting state that looks a
lot like a macaroni noodle.
A field survey of twelve Willamette
Valley shade tree nurseries indicated
that the rust mites were present on 100
percent of linden scion tree samples.
The next step was to figure out
when the mites emerge from their “caves”
to begin feeding in the spring and when
they return at the end of the growing
season. To track mite movement, double
sided sticky tape was placed on branches
near overwintering sites to trap mites
moving toward the branch tips and new
plant tissue.
The first detectable movement of
mites occurs during May, when buds
are breaking and plants are leafing
out. Eriophyids are well-tuned to their
plant hosts, so this emergence is likely
to be timed with the plant’s phenology
as well as photoperiod and average
daily temperatures.
The linden mites feed and proliferate throughout the summer, reaching a
peak in late June or July. In August, the
feeding population spikes again before
the autumn migration to overwintering
sites. Typically, by the end of September,
the mites have returned to their crevices
to hibernate. The overwintering females
(called deutogynes) can survive adverse
winter conditions. These females have
mated, but will not lay eggs until they
emerge in spring.
DECEMBER 2011
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SMAll AND MITE-y
oreGon state uniVersitY
Researchers trapped eriophyid mites by placing double-sided sticky tape on branches. The mites were
captured as they moved toward the branch tips and new plant tissue.
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Damage and control strategies
Symptoms of eriophyid damage on
linden occur in late spring and summer. Because they are free-living rust
mites, the damage is subtle. Symptoms
of rust mite feeding such as chlorosis,
leaf bronzing and leaf distortion can be
caused by other pests, diseases, or even
nutrient deficiencies.
The most indicative symptom in linden seedlings, die-back and distortion
of growing tips, appears in July. The
damage can slow or halt plant growth
and shorten internodes. This may lead
to a change in the plant’s architecture,
which does not become evident until
the following season.
In an attempt to understand the association between eriophyids and internodal
shortening, researchers compared the
weekly growth of a subset of branches
to mite counts from leaf samples. In both
seedlings and mature trees, a decrease in
growth followed an increase in the mite
population, and vice versa.
It’s yet to be determined whether
the mites cause the decrease in growth,
or the decrease in growth reduces the
mite population. However, an association between the two is apparent.
With this new information, the
final step was to investigate appropriate control tactics. Because of the mites’
overwintering habits, applications of
dormant oil can be largely ineffective.
The mites are well-protected within
their crevices, and they may not be as
susceptible to pesticides. This leaves
a window during the growing season
when the mites are active.
From an integrated pest management (IPM) perspective, spring emergence would be the ideal time to control
the population. During this period, mites
are out of their protected overwintering
sites, and the small amount of foliage
limits the mites to a smaller area, making
them easier to target.
Another good strategy is to target
pre-overwintering mites before migration to hibernation sites. Timing of fall
applications was tested with a foliar sulfur spray. One application was made in
Acknowledgements
This research was made possible by
the collaboration and assistance of Sam
Doane, Lance Lyon, Jim Owen, Rich
Regan, Marc Anderson, Mike Reitmajer,
Tyler Kilkenny, Drew Mahedy, Sam
Tochen and James Amrine. Funding
was provided in part by the Oregon
Association of Nurseries, The J. Frank
Schmidt Charitable Foundation and the
Oregon Department of Agriculture.
Betsey Miller is a recent graduate now
working for Oregon State University as a
research assistant in integrated pest management. Dr. Vaughn Walton is an assistant professor of horticultural entomology
for the Department of Horticulture at
OSU. Robin Rosetta is a regional extension educator for OSU, stationed at the
North Willamette Research and Extension
Center. For more information, contact
Betsey Miller at 541-737-3913 or
[email protected].
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August, before migration, and one was
made in late September, after migration.
In the first year of the trial, the
early spray was significantly more
effective, reducing the overwintering
population to less than 1 percent of the
late-summer feeding population (in control plots, the natural population was
reduced to 8 percent of the summer
population). The late spray reduced the
population to 5 percent of its original
size. Second year results were less clear,
with both early and late sprays reducing
populations to less than 2 percent of
their summer numbers.
Typically, preventative control measures can save cost and labor down
the road. Attention to cleaning up eriophyid mite infestations in scion material will help to ensure clean stock. It
may reduce eriophyid mite damage and
lessen the need for control measures in
nursery production blocks.
Observation is our greatest tool in
managing pests and disease. Thanks to
the keen eye of a nursery manager, we
now have new knowledge of a previously unknown pest.
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