On the Lookout for Scaley Invaders
Miller (foreground) and Gary
Miller pull slides from the
Collection of the National
Museum of Natural History
to compare with a scale
It's hard enough keeping scale insects out of your garden.
Imagine trying to keep them out of the country!
"They're small and evasive," says entomologist
Douglass Miller, lead scale insect scientist at the ARS
Systematic Entomology Laboratory (SEL) in Beltsville, Maryland. "Many
gardeners hardly notice them until they discover the gooey mess many
of them make of their plants. In cargo, they're extremely hard to detect,
especially when there aren't many of them."
Miller is in a unique position to monitor invasive scale insects. Port managers, ordinary citizens, and officials at USDA's Animal and Plant Health Inspection Service routinely call on him to identify scale insects suspected of being invaders. As "resident expert," he is often the first to tell whether a species is new to this countryor even new to the list of known insects.
These invasive scale insects,
when disturbed, secrete a red
liquid as a means of defense.
Note the two droplets on the
large mealybug in the center.
"Scale insects are among the nation's most destructive
pests," Miller says. "One reason for this is that they often
go undetected until they've become established and caused damage."
He adds that most pest scale insects in the United States today are
Miller says that once a destructive invasive scale enters
the country, "it's important to know its scientific name, where
it comes from, and any other information we can get. That way, if we
need to implement a biocontrol or pest-management program, we have the
He cites studies stating that control efforts against
scales, plus repairing the damage they cause to plants, cost up to $500
million annually in the United States and $5 billion worldwide. He estimates
three-fourths of the U.S. totals are attributable to invaders.
The most common scale insects are identifiable by the scaley, hard cover that's their natural protection. But others are covered by mealy waxes that come in a variety of colors. There are 22 familieswith more than 7,300 speciesthe largest and most common being Diaspididae, or armored scales; Coccidae, or soft scales; and Pseudococcidae, commonly known as mealybugs.
The protective cover of the
Calico scale (Eulecanium
cerasorum), an invasive
species, is readily apparent.
Scale insects' unique characteristics fascinate scientists.
They vary in appearance, from tiny organisms whose covers resemble oyster
shells to shiny, hard, pearl-like bugs that live in the soil. Females
are wingless, with heads that are indistinct from their bodies, and
spend most of their lives in one spot extracting plant fluids through
a long, thin tube. Meanwhile, tiny male scales have wings and live for
just a few days, without ever feeding.
It's their ability to spread quickly, and undetected, to all parts of the world that gets the attention of SEL scientists, especially Miller, who has monitored scales for ARS for 34 years. For about 7 years, he was aided by fellow entomologist Gary Miller, who recently became SEL's main aphid scientist.
Scale insects are most often palaearctic in origin, meaning they come from a region that includes Europe, Asia north of the Himalayas, northern Arabia, and Africa north of the Sahara. But today they are found everywhere, from the tundra to the Tropics.
Entomologist Gary Miller
illustrates details of an adult
male scale insect he is
inspecting with a
At least 1,000 species can be found in the United States, 253 of which
are invasive. Douglass Miller says 20 of these invaders have established
themselves here over the past 2 decades, and 11 of these are now considered
Gary Miller says global trade has been a major factor in their spread.
"And when you consider trade, natural insect-migration patterns,
and intangibles such as people who sneak fruits into the country, you
can see why scale insects slip very easily under the radar."
Scale insects enjoy perennial plants and can devastate nut and fruit
trees, greenhouse plants, forest vegetation, woody ornamentals, and
house plants. They thrive on nearly all parts of host plants, sometimes
settle under bark, and cause a variety of plant deformities.
All that is in addition to their best-known calling carda sticky, sweet substance called honeydew that many secrete while feeding. The honeydew's stickiness and the black sooty mold that grows on it can make a mess of plants and anything under infested trees.
With a careful eye,
entomologist Gary Miller
takes a closer look at a fern
scale (Pinnaspis aspidistrae).
Among the most serious of recent invasive scales is the pink hibiscus mealybug (Maconellicoccus hirsutus), an Asian native that affects more than 200 plant species. Another is the vine mealybug, Planococcus ficus, a Middle Eastern native that since 1994 has damaged grape crops in some parts of California. Also troublesome is the cycad aulacaspis scale, Aulacaspis yasumatsui, which originates in Southeast Asia and attacks Florida's cycads (palmlike trees).
Getting a Handle on Them
In addition to making crucial identifications, Miller and Miller have
developed another tool for combating scales: the first known full inventory
This inventory is part of ScaleNet, an ARS-run Internet database that
allows clients to use a query system to gather information about scales.
Douglass Miller says that ScaleNet, which is about 75 percent complete,
is currently accessible at www.sel.barc.usda.gov/scalenet/scalenet.htm.
He says full listings for 7,300 species should be available by the end
Meanwhile, the watch for destructive invasive scales goes on. Florida alone is home to six of the newest pests:
Gary Miller says the inventorywhich not only lists the invasive
scales, but provides data about their origins and food choiceswill
prove invaluable. "It will give us a look at the past as well as
future and give us an idea of not only where the insects came from,
but where the next invader will come from," he says.By Luis
Pons, 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 www.nps.ars.usda.gov.
"On the Lookout for Scaley Invaders" was published in the December 2003 issue of Agricultural Research magazine.