Leafhopper control requires alfalfa regrowth monitoring
UNIVERSITY PARK -
Spraying alfalfa stubble to control
the potato leafhopper the most
serious pest in Pennsylvania
alfalfa may be uneconomical or
ineffective, says Arthur A. Hower,
research entomologist at Penn
State.
Hower said leafhopper activity
on one crop does not provide an
accurate estimate of numers on
subsequent crops. Only 50 to 59
percent of fields harboring
damaging populations before
harvest retained that pressure
after harvest in studies carried out
in Pennsylvania and Virginia.
“The recommended
Lower wheat production
predicted for 1984
HARRISBURG
sylvania’s 1984 production of
winter wheat is expected to be
below 1983 production based on
May 1 conditions, according to the
Pennsylvania Crop Reporting
Service.
Acreage to be harvested is
forecast at 220,000 with a yield of 38
bushels per acre. The resulting
production, at 8,360,000 bushels, is
three percent below the 1983 level.
Total 1983 tobacco production for
Pennsylvania was 22,415,000
pounds. Southern Maryland, "type
32 acres harvested were 4,300 with
an average yield per acre of 1,900
pounds and total production of
8,170,000 pounds. Pennsylvania,
Type 41 acres harvested were 7,700
with an average yield of 1,850 and
production of 14,245,000 pounds
Hay stocks on Pennsylvania
farms as of May 1 totaled 554,000
tons, the lowest level since 1966.
United States winter wheat
production is forecast at 1.98
billion bushels, one percent less
than last year’s production of 1.99
billion bushels, total 1983 U.S.
tobacco production was 1.43 billion
pounds, 28 percent below 1982
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management strategy is to
determine leafhopper numbers
early in regrowth following each
harvest,” Hower affirmed.
Leafhopper buildup should be
monitored every three or four days
after harvest, he said, and in
secticides should be applied only
when the number of insects per
sweep with a net across the field is
large enough to cause economic
loss. |
Use spray table
Hower indicated alfalfa growers
can best determine whether or not
to spray leafrollers by using Table 2
in Penn State’s Special Circular
production. All hay stocks on
farms as of May 1 totaled 20.56
million tons, 29 percent below that
stored on May 1,1983.
Penn-
#284, “A Pest Management
Program for Alfalfa in Penn
sylvania.” This circular is
available from county agents or
from the Mailing Room, 112
Agricultural Administration
Building, University Park, PA
16802.
He gave an example of how the
table works. Plants in a field are 1
to 3 inches high, hay is valued at
$lOO per ton, cost of spray is $lO per
acre, and the grower has 35
leafhoppers in 100 sweeps, an
average of .35 per sweep. The
chart in the table shows that the
leafhopper population in the field is
slightly above the injury threshold
(0.3) so a spray will be profitable.
The Penn Stater cautioned that
sweeps with an insect net may
overestimate survival of nymphs
right after harvest. In a 20-acre
field over two years, the per
centage of nymphs surviving
harvest was high the day hay was
removed. Four to seven days later,
the number of nymphs was down to
10 percent.
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completed a five-year study on tne
effectiveness of alfalfa stubble
sprays and the biology of the
potato leafhopper. Stubble sprays
were evaluated on third cuttings in
a four-cutting system. The sprays
were assessed on alfalfa yield,
quality, and insect population
change.
“Residual nature of an in
secticide should be a major con
sideration when deciding whether
or not to treat stubble,” he stated.
“Nonsystemic insecticides, those
lacking the ability to distribute into
new growth, generally will not
persist long enough for adequate
control,” he added.
The experiments tested systemic
insecticides such as Advantage in
1981 when potato leafhopper
pressure was very heavy. Yields
were significantly higher than the
untreated and regrowth treat
ments. Advantage prevented
damage to regrowth and con
tinually protected the crop during
its first weeks of development.
This was the only time a stubble
spray improved yield.
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On the other hand, non-systemic
insecticides such as Trithion lost
their protection early in the crop’s
development. Insecticides applied
as preventive sprays on stubble did
not control potato leafhoppers
sufficiently. Insect numers
generally reached damaging
levels at least two weeks before
harvest.
Plots treated at 4-to-8 inches
regrowth always harbored less
than damaging numbers at har
vest. Leafhopper numbers in
untreated plots, however, always
exceeded damaging levels at
harvest. The only effective stubble
sprays were Advantage and
Furadan, both having systemic
activity, and Lorsban. These
reduced pest numbers
significantly throughout crop
development.
Over a two-year period, Ad
vantage as a stubble spray was the
only compound giving control
equal to regrowth sprays. Lorsban
was comparable to Advantage the
one year it was tested and Furadan
was effective one of two years
tested.
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