Fayetteville, Arkansas
February 13, 2008
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USDA image shows
an adult male panicle mite. |
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A researcher at the
University of Arkansas'
statewide Division of Agriculture is investigating an imported
pest less than one-one hundredth of an inch in length that may
pose a new threat to Arkansas rice crops.
The panicle mite has been found in farm fields in Texas and
Louisiana, but not in Arkansas, said entomologist Ashley
Dowling.
Dowling said the panicle mite is present throughout rice-growing
areas of the Caribbean and Central America, including Mexico,
less than 10 years after its accidental introduction. It may
have entered the U.S. in rice seedlings shipped from winter
nurseries in these countries.
The mite's origins are not known for certain, Dowling said, but
it is an important pest throughout tropical regions of Asia,
specifically China and Taiwan where it has been destructive. The
mite's role in temperate regions of these countries has not been
well studied, but reports do not indicate severe losses like
those in tropical regions.
Dowling said it's not known why the panicle mite has less impact
in temperate Asia, but scientists suspect either that the mite
cannot survive winters in four-season climates or that it has
natural enemies that keep its population in check in those
areas.
Whether the panicle mite will be a problem in Arkansas is not
known, Dowling said. It may not be able to survive the state's
winters, or it may be able to overwinter in host plants other
than rice. Louisiana researchers are finding the mites on many
plants that commonly grow near rice fields, but it is not known
whether the mites can use them as host plants between planting
cycles, he said.
"In the meantime, it's best to be aware of the potential threat
now, before it becomes a problem," Dowling said.
The panicle mite feeds on the rice plant as well as inside the
endosperm of the seed. "It will completely hollow out the rice
hull," Dowling said. In some cases, after milling, as much as 20
percent of a crop has simply disappeared, he said, because the
mites left nothing but the hulls.
Feeding inside the seed also raises the danger of spreading the
mite by transporting the grain, Dowling said.
How the mite moves from field to field is virtually unknown and
is another area of the pest's biology that requires further
study, Dowling said.
"There are also several diseases associated with the panicle
mite," Dowling said. "Fungal sheath rot and bacterial panicle
blight have been found associated with mite feeding." He said
it's the combination of the mite and the diseases that lead to
crop losses of 80 percent or more.
Dowling said pesticides don't seem to control the panicle mite
very effectively. John Bernhardt, entomologist at the division's
Rice Research and Extension Center near Stuttgart said the mite
attacks the rice plants behind the leaf sheath, where pesticides
don't reach very effectively. Because of this, cultural
practices and natural enemies have the best potential for
controlling the pest.
Dowling is beginning a population genetics study of the panicle
mite to backtrack to its place of origin. "If we know where it
comes from, we'll know where to look for natural enemies that we
may be able to use for biological control," he said.
The genetic data will also be used to develop molecular tools
for identification and detection of the panicle mite, Dowling
said.
Bernhardt said the USDA's Animal and Plant Health Inspection
Service (APHIS) has been working closely with the State Plant
Board and monitoring Arkansas rice for the panicle mite.
"APHIS has been involved because the panicle mite is a pest
that's new here and that we don't want around here," Bernhardt
said.
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