The aphid war is going on - ABC NEWS
Bug Wars
Aphids Are a Tough Bug to Fight Since Many Come With Bodyguards
A ladybug released at the Midland Trail Golf Course in Louisville, Ky.
earlier this year to promote Aphid control, crawls on a Blackeyed Susan.
Ladybugs eat aphids but sometimes watchful ants will fight them off.
By Amanda Onion
Aug. 28 - Just when biologists announced they had established a natural
predator for one of farmers' most persistent aphid pests, a new breed has
turned up in its place.
The soybean aphid was detected this July in soybean fields in Iowa,
Wisconsin, Minnesota, Michigan and Illinois. So far the tiny, sap-sucking
insect hasn't caused great damage but scientists predict it soon will as it
spreads throughout crop fields in coming seasons.
To take care of a previous infestation by the Russian wheat aphid,
biologists imported parasitic wasps from countries in Europe and the Middle
East. These wasps lay eggs inside the aphids. The eggs develop into wasp
larvae that eventually devour their hosts from the inside out. Now
scientists are searching for insect predators and chemicals to combat this
latest intruder which most likely arrived from China.
But it's never easy taking on an aphid.
Aphid Henchmen
One big reason is about half of the 4,500 species of aphids come equipped
with their own bodyguards in the formidable form of ants (that's formidable
in the insect world anyway). As biologist John Addicot from the University
of Alberta in Canada explains, the ants "basically run a protection racket"
for the aphids.
While henchmen demand payment from their subjects, ants demand
something a little easier to come by - the aphids' excrement. Addicot
explains that aphids feed by inserting their beaks into stems, leaves, or
roots, and sucking the plant's juices. The insects then extract nitrogen
from the sap and excrete copious amounts of a sugary substance, known as
honeydew, from their anuses. This sticky, sweet excrement offers high
calorie fuel for ants.
"Any given aphid is probably going to produce a droplet of honeydew
every hour or so," says Addicot. "Now imagine you have a colony of aphids
that has 1,000-10,000 individuals, then you have a large supply of food for
an ant colony."
Just as farmers protect their cattle from predators, ants are known to
fight off lady bugs and wasps only when they're tending to their aphids.
They also "herd" the aphids to different regions of the plant that will
provide more sap.
The ants even know how to "milk" their aphids. When a foraging ant such
as the common black ant approaches aphids, they stroke or pat the abdomen of
several aphids with the tips of their antennae. The aphids respond by gently
releasing drops of honeydew. Normally the aphids would use their hind legs
to flick their excrement far away.
"Catch an aphid colony in the right light and you can see arcs of
honeydew sailing away from the community," says Addicot.
Finally, if ants find some aphids are no longer producing adequate
amounts of honeydew, they will carry the aphids away to their nest for
consumption by the colony.
A Two-Way Relationship
In return, the aphids don't only win protection from the ants, often they
also gain a warm, safe place to spend the winter. When crops have been
harvested and temperatures drop, some ant species take in aphid eggs and
larvae and host them in leafy nests. In Australia and Africa, certain ant
species even build their aphids shelter during the warm seasons to protect
them from torrential rainfalls.
The ant-aphid relationship is known as a mutualistic relationship,
meaning each species benefits, but is not dependent upon the other.
"There are some aphids that absolutely require this mutualism with
ants," says Manya Stoetzel, an aphid expert with the Entomology Laboratory
of the United States Department of Agriculture. "You just never find certain
colonies of aphids without the ants."
But several aphids do live without the protection of ants. To ensure
their survival, these aphids rely on one of nature's most effective
defenses - rapid reproduction. In warm temperatures, many aphid species can
turn out a new generation in as little time as a two to three weeks. Their
reproductive cycles are also difficult to trace since it involves many
different stages that occur in many different hosts.
Confusing the Enemy
In a typical life cycle, several generations of wingless females reproduce
by making cloned copies of themselves. These winged, female clones then give
birth to generations of winged females, which bear sexually reproducing,
egg-laying generations of males and females. The various generations make
their homes in different host plants and in different parts of each plant.
So while farmers may manage to kill off aphids from their crop plants,
another generation may be waiting in the wings in nearby grasses and trees.
Right now scientists are still trying to get a firm understanding of
the newly emerged soybean aphid, its life cycles and feeding habits.
"It's so new we haven't yet gathered all the literature on it," says
Kevin Staffey, an entomologist at the University of Illinois.
What is known is that in China the aphids reduced seed yields by nearly
30 percent. That could translate into thousands, maybe millions of dollars
of lost crop sales for U.S. farmers. Past experience shows aphid
infestations can be costly. Despite efforts in the past decade to fight off
the Russian wheat aphid, the insect cost farmers about $1 million in lost
crops, pesticides and other management measures. And healthy populations of
the aphid still reside in Colorado.
Staffey and others are busy looking for a local natural predator of the
soybean aphid which they might enhance to ward off the insect. It appears
that the soybean aphid may be more difficult to thwart since it seems to be
able to adapt to both dry and wet soil environments.
"The Russian aphid was bad, but this guy is probably going to be an
even bigger problem," says Stoetzel.
Still, preliminary studies suggest at least one hopeful fact. It
appears the soybean aphid is not normally protected by ants. That means
farmers might be contending with the aphid - on its own.
