Mites Get Frozen, Photographed, and Identified
Here a yellow mite, Lorryia formosa, commonly found on citrus plants,
shown among some fungi. Magnified
Ronald Ochoa, the U.S. Department
of Agriculture's chief expert on mites, is about to transform the 200-year-old
study of mitesthe science called acarology. Ochoa is a research associate
at the Agricultural Research Service's
Systematic Entomology Laboratory (SEL) in Beltsville, Maryland. He specializes
in the systematics of mitesthat is, the discovery, scientific
description, classification, and naming of agriculturally important mite
species. He is also curator of mites for the National Collection of Insects and
Mites, housed at the SEL. The collection belongs to the Smithsonian
Institution's National Museum of Natural History in Washington, D.C.
Recently, Ochoa teamed up with cytologist William P. Wergin and botanist
Eric F. Erbe, both of ARS' Beltsville Agricultural Research Center Nematology
Laboratory, to study mites by using newly developed technology called
low-temperature-scanning electron microscopy (LT-SEM).
A low-temperature scanning electron microscope (at left) enables acarologist
Ron Ochoa (background) and botanist Eric Erbe to observe tiny mites in detail
impossible with conventional microscopes and slide-mounting techniques. Liquid
nitrogen is first used to flash-freeze mites on their hosts in their natural
"Mites have attacked the
world's vertebrates, invertebrates, and plants for millions of years,"
Ochoa says. "And although they remain a constant threat to economically
important crops, stored grains, livestock, wildlife, and humans, only about 10
percent have been described or named."
Mites come in a variety of body shapes and normally have four pairs of legs.
"But because of their small sizesome no bigger than the point of a
needle (80 micrometers in diameter)mites are difficult to study
biologically," Ochoa says. "Their intricate structures and the
distribution of their hairs, called setae, are important for identifying them,
but their minute size can be a problem."
Scanning electron microscopy allows mites to be viewed from different angles.
This is an overhead view of the Lorryia formosa mite shown on the cover.
Magnified about 200x.
Lack of detailed information about
mites' correct identity, biology, and ecology often causes serious consequences
to U.S. agriculture. "More than 6,000 mite species infest nearly every
plant important to agriculture," Ochoa says. "In the United States,
they cause annual economic losses estimated in the billions of dollars from
decreased food, fiber, and ornamental production. Invasive mite species are one
of the primary culprits."
Increased world trade will continue to distribute mite infestations widely.
"Once they've become established in a new area, certain biological
characteristics allow mite populations to escalate rapidly to pest
status," Ochoa says. "High egg production, various modes of
reproduction, short life cycles, many dispersal techniques, and adaptability to
diverse environmental and ecological conditions all contribute to their
A straw itch mite (Pyemotes tritici)
on the back of a caterpillar. A
major problem in stored grain, this
mite is also a potential biocontrol
agent. Magnified about 375x
Magnifying the Minuscule
Ochoa says that LT-SEM technologydeveloped by Werginprovides a
powerful new tool that can benefit not only the systematics of mites, but also
research on other microorganisms.
"Unlike conventional microscopes," Wergin explains, "an SEM
apparatus does not use light passed through a glass lens to magnify images of a
specimen. Rather, the images are formed and magnified by electrons passing
through a magnetic field that functions as a lens. The images can be displayed
and recorded on a cathode ray tube similar to one in a television."
Says Ochoa, "LT-SEM was used to obtainfor the first time
everhighly magnified, clear images that show the details of intact mites
and how they interact with and attack plant and insect hosts."
The dust mite (Tyrophagus putrescentiae) is common on plant leaves and in
stored grain and animal feed. Magnified about 100x.
To prepare, or fix, a mite
specimen for viewing, Ochoa and Erbe use liquid nitrogen, which is at
320°F. This cryofixation instantly freezes the mite in its natural
state on the host and prevents it from moving or becoming distorted.
"Although conventional SEMs have been used by acarologists for the past
30 years, Wergin's technique has many advantages," says Ochoa. "We
can magnify mites more than 50,000 times. For the first time, we can observe
very delicate structural details at different angles and can see mites in three
dimensions, fully hydrated, and on their hosts in natural positions. Using
LT-SEM will help us determine how these arthropods perceive and react to their
environment and how they're affected by factors like light, temperature,
moisture, and pressure."
The peacock mite (Tuckerella sp.),
a beautiful but important pest on
citrus is the Tropics, is shown on
tea stem. Magnified about 65x.
According to Ochoa, one major problem in understanding and managing mite
pests has been scientists' inability to observe the relative position and
function of organs in a slide-mounted mite.
"Traditional slide-mounting procedures flatten or distort specimens,
which seriously limited our ability to accurately see the relationships between
mites and their hosts," says Ochoa. "Recent studies show that the
chemicals used to fix specimens may also cause physical changes that obscure
important distinguishing characters, like setae, used to identify species.
"Mites have many setae, sensory organs, mouth parts, and other body
parts that are so complex that systematists have difficulty comparing or
contrasting those of closely related species," says Ochoa. "Many
external characteristics used for identifying flat, rust, gall, and bud mites
are related to their body ornamentation."
The rust mite (Aceria anthocoptes), here on Canada thistle, may have potential
as a biological control agent of this weed. Magnified about 700x.
LT-SEM reveals ultrastructural
featureslike poreson the surface of mites' bodies that have never
been seen before and that provide vital information on how they move and hold
onto their hosts. Since many plant-feeding mites are host specific, such
information could be very valuable.
"All these new data will increase our knowledge of the plant-feeding
mites already in the United States and of those being introduced," says
Ochoa. "It will help determine the potential destructive nature of new
invasive species and help scientists and quarantine officials develop better
management, control, and quarantine methods."
So far, Ochoain cooperation with entomologist Carl C. Childers at the
University of Florida-Lake Alfred and acarologist Barry M. OConnor at the
University of Michigan-Ann Arborhas used the new technology to study
several mites on plants. He's also used the LT-SEM technique to compare
different traditional slide-mounting methods, examining several plant-feeding
species of eriophyid and flat mites.
The flat mite (Brevipalpus phoenicis) carries the leprosis virus in citrus, a
disease currently in South America but moving north. Magnified about
"Today, there are 3,400
described species of gall, rust, and bud mites," Ochoa says. "This is
just a small fractionperhaps 5 percentof the total number estimated
Many of these species can spread destructive viral, microbial, and fungal
diseases. For example, a flat mite can carry the citrus leprosis virusa
very important and costly plant disease in South America.
According to Wergin and Ochoa, one important scientific observation and
theory of mites has already come from using this new technique: the discovery
that some mites' setae appear to function as capacitorspossibly helping
the mites sense electrical or magnetic fields in their environment.
"These sensing devices may help them find food or avoid
predators," says Ochoa. If the theory is true, perhaps artificial fields
could be generated to confuse or disorient the mites and reduce the damage they
cause to people and agriculture. This discovery could help scientists develop
safer and more effective methods of controlling these pests," he says.
The bee-infesting Varroa mite
causes considerable economic
losses for beekeepers and
agriculture. Cytologist Bill Wergin
(left) and entomologist Jeff Pettis
examine a highly magnified photo
of a honey bee infested with Varroa.
To Curb a Weed
Ochoa is also working with ARS plant physiologist John Lydon at the Weed
Science Laboratory in Beltsville. They are studying a mite as a possible
biological agent to control Canada thistle (Cirsium arvense), a major
invasive weed pest in U.S. pastures. First identified in Europe over 100 years
ago, this mite (Aceria anthocoptes) was discovered in the United States
in 1998, when Lydon collected some Canada thistle on Maryland's Eastern Shore.
"Preliminary results from a survey of Maryland and surrounding states
indicate that the mite is abundant here," Lydon says. "Under
growth-chamber conditions, mite populations on a Canada thistle plant can reach
very high levels, causing severe damage to the plant."
Their presence leads to a reddish-brown discoloration of thistle leaves,
leaf curling, and spindly growth. Says Ochoa, "These mites could also
transmit plant diseasesparticularly viral diseasesto the weed. The
LT-SEM will allow us to measure the number of mites per leaf and see their
damage to leaves."
Lydon wants to determine the size of the mite population needed to
significantly impede growth of Canada thistle and whether this mite can
transmit a disease or a virus to the weed. A search is under way for
viral-infected Canada thistle plants in the areas where they were once
reportedDenmark, England, and North Dakota. Lydon also plans to determine
the specificity of A. anthocopteswhether it attacks other
The mite species Aceria anthocoptes may prove useful as a biological control
agent for weeds. From left to right, Chris Frye, botanist with the Maryland
Department of Natural Resources, ARS acarologist Ron Ochoa, and ARS plant
physiologist John Lydon identify a thistle species as Cirsium horridulum and
note where the highest mite population is most likely to be.
Helping Bees With
Ochoa is working with ARS entomologist Jeffery S. Pettis at the Beltsville
Bee Research Laboratory to see if the LT-SEM technique may help bee researchers
better understand how parasitic mites like Varroa interact with their
"Varroa mites feed on the blood of adult and developing
bees," says Pettis. "Parasitized bees may have deformed wings and
abdomens and a shorter life span than unparasitized hivemates."
Because the LT-SEM freezes and captures Varroa mites on bees at the
moment of parasitization, the scientists have discovered some intriguing
behavioral and structural patterns that allow the mite to hide on the bee.
"Varroa mites may camouflage themselves by aligning their setae
with the hairs on the bee's body. In doing so, they could escape detection by
the bee when it grooms itself or when it's being groomed by another bee,"
says Pettis. If this hypothesis is correct, it may be possible to breed bees
that are better able to defend themselves from the mites, he says.
LT-SEM technology is an exciting new tool to help reveal the systematics and
behavior of mites. It is already providing valuable new information that could
be used to control them as agricultural pests or to make them more effective as
biological control agents.By Hank
Becker, Agricultural Research Service Information Staff.
This research is part of Crop Protection and Quarantine, an ARS National
Program (#304) described on the World Wide Web at
Ronald Ochoa is with the
USDA-ARS Systematic Entomology
Laboratory, 10300 Baltimore Blvd., Bldg. 005, Room 137, Beltsville, MD
20705; phone (301) 504-7890, fax (301) 504-6482.
"Mites Get Frozen, Photographed, and
Identified" was published in the
issue of Agricultural Research magazine.