Minute Creatures Raise Mighty Concerns.
Ronald Ochoa, the U.S. Department of Agriculture's expert on mites, is about to transform the 200-year-old study of mites--a science called acarology. Ochoa is a research associate at the Systematic Entomology Laboratory (SEL) of the Agricultural Research Service (ARS) in Beltsville, Maryland. He specializes in the systematics of mites--that is, the discovery, scientific description, classification, and naming of agriculturally important mite species.
"Mites have attacked the world's vertebrates, invertebrates, and plants for millions of years," he says. "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."
Ochoa is also curator of mites for the National Collection of Insects and Mites, housed at SEL and belonging to the Smithsonian Institution's National Museum of Natural History in Washington, D.C. Recently, he teamed up with cytologist William Wergin and botanist Eric Erbe, both of ARS' Beltsville Agricultural Research Center Nematology Laboratory, to study mites with a sophisticated new technology called low- temperature scanning electron microscopy (LT-SEM).
Mites come in a variety of body shapes and normally have four pairs of legs. "But because of their small size--some no bigger than the point of a needle (80 micrometers in diameter)--mites are difficult to study biologically," Ochoa explains. "Their intricate structures and the distribution of their hairs, called setae, are important for identifying them, but their minute size can be a problem."
Lack of detailed, accurate information about the identity, biology, and ecology of mites often has serious consequences for U.S. agriculture. "More than 6,000 mite species infest nearly every plant important to agriculture," notes Ochoa. "In the United States, they cause annual economic losses estimated in the billions of dollars from decreased food, fiber, and ornamental [plant] production. Invasive mite species are among 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 observes. "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 success."
Magnifying the minuscule
Wergin, who developed LT-SEM technology, explains how it works: "Unlike conventional microscopes, a 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."
According to Ochoa, "LT-SEM was used to obtain--for the first time ever--highly magnified, clear images that show the details of intact mites and how they interact with and attack plant and insect hosts." He adds that LT-SEM technology provides a powerful new tool that can benefit not only the systematics of mites but research on other microorganisms as well.
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."
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," explains 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, mouthparts, and other body parts that are so complex that systematists have difficulty comparing or contrasting those of closely related species. Many external characteristics used for identifying flat, rust, gall, and bud mites are related to their body ornamentation."
LT-SEM technology also reveals previously unseen ultrastructural features--such as pores--on the surface of mites' bodies. Observation of these features provides vital information on how mites move and hold on to their hosts. Because 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," notes 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, Ochoa--in cooperation with entomologist Carl Childers at the University of Florida--Lake Alfred and acarologist Barry O'Connor at the University of Michigan--Ann Arbor--has used the new technology to study various mites on plants. He has also used it to compare different traditional slide-mounting methods, examining several plant-feeding species of mites.
"Today, there are 3,400 described species of gall, rust, and bud mites," says Ochoa. "This is just a small fraction--perhaps 5 percent-- of the total number estimated to exist." Many of these species can spread destructive viral, microbial, and fungal diseases. For example, a flat mite can carry the citrus leprosis virus--a very important and costly plant disease in South America.
According to Wergin and Ochoa, the new technique has already led to one important scientific finding--namely, that some setae appear to function as capacitors, possibly helping the mites sense electrical or magnetic fields in their environment. Ochoa believes that these sensing devices may help them find food or avoid predators. If the theory is right, 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.
Curbing weeds and helping bees
Ochoa is also working with ARS plant physiologist John Lydon at the Weed Science Laboratory in Beltsville. They are studying the potential use of a mite species as a 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 Canada thistle on Maryland's Eastern Shore.
"Preliminary results from a survey of Maryland and surrounding states indicate that the mite is abundant here," says Lydon. "Under growth- chamber conditions, mite populations on a Canada thistle can reach very high levels, causing severe damage to the plant," he notes.
The mites' presence leads to a reddish-brown discoloration of thistle leaves, leaf curling, and spindly growth. Says Ochoa, "These mites could also transmit plant diseases--particularly viral diseases--to 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 virus to the weed. A search is under way for viral-infected Canada thistle plants in regions where they were once reported: Denmark, England, and North Dakota. Lydon also plans to determine the specificity of A. anthocoptes--whether it attacks other Cirsium species.
An additional collaboration is ongoing between Ochoa and ARS entomologist Jeffery Pettis at the Beltsville Bee Research Laboratory, to see if the LT-SEM technique can help bee researchers understand how parasitic mites like Varroa interact with their bee hosts. "Varroa mites feed on the blood of adult and developing young bees," observes Pettis. "Parasitized bees may have deformed wings and abdomens and a shorter life span than unparasitized hive mates."
The LT-SEM technique freezes and captures Varroa mites on bees at the moment of parasitization. Consequently, 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.
In this manner, LT-SEM technology is an exciting tool that helps reveal the structural and behavioral features of mites. It is already providing valuable new information that could be used to control some species that are agricultural pests and make others more effective as biological control agents.n
Article adapted from Agricultural Research, October 2000.
Hank Becker is the national media liaison for the Agricultural Research Service information staff in Beltsville, Maryland. He has written about agricultural research for over 21 years. The research described here is part of Crop Protection and Quarantine, an ARS national program (No. 304) accessible through the World Wide Web at www.nps.ars.usda.gov
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|Publication:||World and I|
|Date:||Feb 1, 2001|
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