Beet Armyworm Eggs: Not Just a Random Scramble
Under the direction of
entomologist Tom Sappington
(right), University of Texas-
Pan American graduate student
Peter Carreon attaches a beet
armyworm moth to a flight mill.
Just what is it that beet
armyworms want when it comes to choosing a place to raise offspring? After all,
these insects live on a wide variety of plants, including cottona crop to
which they can lay waste.
There is a lot of anecdotal evidence about what attracts the beet armyworm
to particular plants and fields. But what is "known" on that basis
may not be exactly so.
Now, data from ARS scientists at the
Kika de la Garza Subtropical Agricultural Research Center in Weslaco, Texas,
are shedding light on what the pest really prefers when it comes to picking a
host plant for offspring.
ARS entomologists Shoil Greenberg, Allan Showler, and Thomas Sappington, with the Integrated Farming and Natural Resources Research Unit, have spent the past several years separating fact from folklore.
A beet armyworm moth is
attached to a flight mill,
which allows it to fly in
a circular pattern. Each
revolution of the low-
friction flight arm represents
1 meter of flight distance.
"It's well documented in the
literature that pigweed, cotton, peppers, sunflowers, and cabbage, among many
other species, are used as host plants by beet armyworms. But whether all are
preferred equally was a big question because of implications for controlling
this pest," explains Sappington. "Now we've found that beet armyworm
moths display decided preferences when it comes to laying eggs."
Given free choice in laboratory experiments on individual leaves and in
greenhouse studies using potted plants, females laid eggs four to five times
more often on pigweed than on sunflower or cabbage, according to Sappington.
Cotton and peppers were an intermediate choice, receiving only half as many
eggs as pigweed.
In parallel studies, Greenberg and Sappington also found that beet armyworm
larvae thrived better on pigweed and ate less of its leaves. The insects grew
faster and larger than larvae from eggs laid on cotton or peppers, Sappington
To verify which plants
beet armyworms prefer to
eat and lay eggs on, technician
Chuy Caballero (left) and
entomologist Shoil Greenberg
examine leaves of cotton,
cabbage, and pigweed.
There were also differences in
where on the plant eggs were laid, depending on the plant chosen. On cotton,
most eggs were deposited on the undersides of leaves within the top part of the
canopy and horizontally within the central part of the plant. On pigweed, eggs
were also commonly laid within the central part of the canopy, but they were
spread vertically throughout the plants. This information will be useful in
devising efficient procedures for scouting for armyworms in crop fields and
Greenberg is currently studying whether the type of plant the beet armyworm
is hatched on makes a difference in which plant the females choose for the next
generation. He raised three generations each on pigweed, cotton, and cabbage
and then offered the moths a choice of host plants for egg laying. Sappington
also attached fishing line to the female moths and fastened them to rotatable
arms on flight mills to monitor differences in migratory flight behavior
associated with host plant choices.
Preliminary data indicate that the plant these pests are raised on doesn't
influence which plants the adult females seek out.
Leaf damage shows that
beet armyworm larvae prefer
to feed on pigweed (top)
rather than cabbage (left)
or cotton (right).
Showler, meanwhile, is studying
what makes one type of plant more attractive than another. He looked at
egg-laying choices when the insects were limited to their sense of
smellchemical cuesto identify a preferred host plant.
Working by smell alone, not only did the females lay 3.3 times more eggs on
pigweed than on cotton plants, they also laid 4.5 times more egg clusters.
"When I saw these differences, I wanted to understand what was so
attractive about pigweed," Showler says. "Most other host plant
studies have focused on what deters beet armyworms from choosing a plant."
He also knew from other studies that pigweed provided a nutritional
advantage, since the larvae developed faster and larger on it.
One major factor he found was that pigweed has much higher free amino acid
levels than other host plants and a more diverse array of them. Unlike
proteins, free amino acids don't have to be broken down before they can be used
by the insect.
"In pigweed I've identified the presence of 9 of the 10 free amino acids that can provide a nutritional advantage to insects, and the 10th one may be there too," Showler says.
Sometimes it is not the type of
plant but its condition that affects the pest's choice of where to lay eggs.
One commonly held conviction, especially by cotton farmers, is that beet
armyworms are more attracted to drought-stressed cotton plants.
To scientifically validate whether this preference actually exists, Showler
offered egg-laying females the choice of water-saturated cotton plants or
plants that received 1,500, 1,000 or 750 milliliters of water per week.
Indeed, all the water-stressed plants received more eggs and more egg
clusters than did the water-saturated plants. But egg numbers among the three
different levels of water-stressed plants were not statistically different.
Showler also found a significant increase in free amino acid levels in all of
the water-stressed plants, and these levels matched the egg-laying choices.
But what didn't follow the expected course was survivability. Even though
more eggs were laid on the water-stressed plants, the newly hatched larvae
fared very poorly. So it does not follow that drought-stressed cotton is more
likely to suffer damage from beet armyworms.
"There are anecdotal reports that if you are scouting a field for a
beet armyworm outbreak, go first to the drought-stressed plants," Showler
recounts. "That may be true for the presence of eggs, but it doesn't hold
up for larvae because they don't survive well. So the presence of eggs alone is
not necessarily a way to tell whether your field is going to become
Collecting this type of basic biological data is critical because once the
beet armyworm's behavior and development can be accurately predicted, it may be
possible to exploit the information to devise better monitoring and control
Kim Kaplan, 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
Thomas Sappington, and
Allan Showler are in the
USDA-ARS Integrated Farming and Natural Resources Research Unit,
Kika de la Garza Subtropical
Agricultural Research Center, 2413 E. Hwy. 83, Weslaco, TX 78596; phone
(956) 969-4812, fax (956) 969-4800.
"Beet Armyworm Eggs: Not Just a Random Scramble" was published in the June 2002 issue of Agricultural Research magazine.