This is a cotton leaf with a population of cotton aphids
in which a fungus epizootic is in progress. Live aphids
are yellow, aphids killed by the fungus are gray. Within
days all the aphids on this leaf and within the cotton
field will have been killed by the fungus.
Cotton aphids are difficult to control with insecticides due to resistance and application problems, but it is also difficult to use natural enemies in cotton because of the intensive pesticide usage for a multitude of pests. At present, it is generally impossible for consultants to determine when the numbers of beneficial ladybird beetles, lacewings, and parasitoid wasps are sufficient to control the cotton aphid without resorting to insecticides.,
Aphids Can Be Killed Quickly, Efficiently
In 1988, cotton researchers observed that
a pathogen was rapidly killing off cotton aphids in the mid-South. Entire
fields that had been covered with cotton aphids were clean of aphids within
a week after the disease attacked. In 1990, University of Arkansas entomologist
Don Steinkraus identified the causal agent as Neozygites fresenii, a friendly
fungus that attacks only aphids. Using grants from the Southern Region
IPM program, Cotton Inc., and the U.S. Department of Agriculture, Steinkraus
and his colleagues successfully studied the fungus's biology and epizootiology.
They made two important discoveries:the fungus kills aphids in a very short
period, and it has mechanisms that enable it to spread extremely efficiently
in aphid populations.
From each infected aphid,
3.000 spores are shot into the air. If one spore hits an aphid, the aphid
will die within three days. Further study using aerial spore traps showed
that during an outbreak of the fungus, up to 90,000 spores of N. fresenii
were present in each cubic meter of air in a cotton field. Within an
eight-hour period, these aerial spores could infect 50 percent of the remaining
aphids in a field. Growers and researchers frequently remarked that the
fungus worked better than insecticides, and they wanted to take advantage
of it whenever possible.
Studies on the spatial
and temporal occurrence of the fungus showed it occurred widely-in Arkansas,
Alabama, Georgia, Louisiana, and Mississippi- sometime between June and
August every year.
Growers Now Have Way to Assess Fungus's Activity Level
Laboratory studies resulted in a highly accurate
method of diagnosing fungus-infected aphids, allowing growers to determine
the fungus's activity level in their individual fields. Field studies indicated
when fungus levels reached 15 percent in the aphid population, a major
decline in aphids could be expected in several days.
Based on this research,
Steinkraus and Cooperative Extension agents started an Extension-based
diagnostic service for the aphid fungus in Arkansas in 1993, expanded the
service to Mississippi and Louisiana in 1997, and further expanded the
service to Georgia and Alabama in 1998.
The program now depends
on active involvement of Extension cotton entomologists in all five states.
In each state, a Cooperative Extension coordinator
identifies potential participants for the program among agents consultants,
and growers. In early spring, each participant is mailed a sampling kit
and instructions. During the growing season when aphid populations are
encountered in cotton fields, samples are collected and mailed to Steinkraus's
laboratory at the University of Arkansas via express mail. Using phase
microscopes, trained assistants determine the percentages of infected aphids
in the samples. Results are also supplied to participants within 48 hours.
Results are posted each day on the Web site developed by Don Johnson, Extension
entomologist with the Arkansas Cooperative Extension Service, where growers
in the five-state program can instantly access the latest information on
the progress of epizootics.
If there is no fungus
present-or if the fungus level is less than 15 percent-in a field being
considered for an insecticide spray for aphids, a treatment may be in order,
although at the risk of reducing beneficial insects.
Program a Money Saver, Participants Said
In a survey about satisfaction with the project,
most participants (90 percent) said that the service was very helpful and
saved them money because they had avoided aphid insecticide treatments
when the fungus was present.
There is also a resting spore stage of the
fungus. Resting spores are formed internally in the aphid and are eventually
deposited in the soil, initiating infections in early summer. Aphids are
squashed and different stages of infection diagnosed.
The leg of a cotton aphid showing the adhesive
secondary spores attached to the femur. These spores germinate within 30
minutes and penetrate into the aphid, where the fungus grows vegetatively.
"We had five growers prepared
to spray approximately 2,500 acres until the survey revealed that the fungus
was present. At $7 to $10 an acrethis was a significant savings," said
one participant.
Success of this project
required understanding of the biology of both host and natural enemy. One
participant said that participating helped his understanding of how and
when the fungal disease works so that he can fit reliance of this disease
into a pest management program.
It is estimated that using
the information gained from this research saves cotton growers several
million dollars a year. Funding from the Southern Region IPM program was
crucial in this effort.
A cotton aphid recently killed by the fungus has a granular gray appearance and discharges thousands of spores into the air and onto the leaf near the aphid, infecting other aphids. The bright spots around the aphid are the spores.
The aphid fungus survey for the five states
in the Extension-based diagnostic program is on the Web at:
http://ipm.uaex.edu/Insects/Aphid/StartHre.htm
For more information contact:
| William Yearian Department of Entomology
University of Arkansas Fayetteville, AR72701 (501)575-2451 |
Gus Lorenz III
University of Arkansas CES P.O.Box 391 Little Rock, AR 72203 (501)671-2191 |
