NEWARK, Del. Before 1975,
more than half of Delaware’s
100,000 soybean acres were
routinely treated with insecticides
to control the Mexican bean beetle,
a voracious pest capable of
reducing yields by as much as 26
percent at high infestation levels.
Last summer, less than 1 percent
of this acreage was treated, thanks
to the hot weather and a busy httle
wasp with a very long name -
Pediobius foveolatus - which lays
its eggs in bean bettle larvae.
From 1976 to 1978 large numbers
of this parasite were released as
part of the University of Delaware
extension service’s integrated pest
management-trap crop program.
In 1980, the Delaware Department
of Agriculture began releasing the
beneficial insect in snap bean
nurse plots strategically planted at
the edges of fields of cooperating
soybean farmers, as part of a
cooperative project funded by the
U.S. Department of Agriculture.
Purpose of the project was to
determine the economic feasibility
of using Pediobius instead of
chemicals to control the MBB on
Delmarva.
“Since initiating the parasite
releases, insecticide use for MBB
control in Delaware has gradually
declined,” reports extension pest
management specialist Joanne
Whalen. “During the first three
seasons of the release program
(1980-1982), only about 20 to 25
percent of the state’s soybean
acreage was treated with in
secticides, as compared to 50 to 60
percent before 1975. Last summer
less than 5 percent of Delaware
soybeans required treatment for
MBB control.”
Whalen and extension en-
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MBB parasite success reported in Del.
tomologist Mark Graustein feel
that declines in both in-season and
overwintering MBB populations
since 1979 have been largely
responsible for this dramatic
reduction in insecticide use.
“Though weather conditions also
have contributed, we feel that
Pediobius deserves a lot of the
credit,” Graustein says. “In parts
of the state where parasites have
never been released, farmers have
still experienced economic levels
of MBB.” As a result, insecticide
use has remained high in these
areas. For example, around
Roxana, 20 to 25 percent of the
soybean acreage had to be treated
for MBB last suihmer.
“Over the past four years we’ve
literally chased the bean bettle up
and down the state with our
parasite release program,”
Graustein says. Whalen agrees;
“It appears that the parasite has
helped signifcantly in holding MBB
populations below an economic
level where it has been used.”
Data collected over the past four
years show that Pediobius is at
least as effective as insecticides in
controlling MBB.
Potential yield loss
Without control- 26% of soybean
r ■■■■
yield lost.
With parasites- 4% loss, 22%
savings with the parasites.
With chemicals- 4% loss, for
same savings.
Translated into dollars using
1981 figures, potential MBB losses
in Delaware could have cost far
mers as much as $11,407,500,
assuming a state yield of 7,020,000
bushels (260,000 acres x 27 bushels
x $6.25 per bushel) with a 26 per
cent loss to the beetle. This com
pares with an estimated loss of
only $1,755,000, using either
chemicals or the parasite to hold
MBB losses to 4 percent.
Is use of Pediobms cost effective,
compared to chemical controls?
Decidedly yes, Whalen says
Compared to an average yearly
chemical cost (excluding ap
plication) of $7 to $9 per acre for a
systemic insecticide, or $3 to $6 an
acre for a foliar spray, the
parasites themselves cost between
10 cents and 12 cents an acre per
year. Beyond that, it costs about
$lOO for the labor, machinery and
seed to establish small snap bean
nurse plots at the edge of a soybean
field. (These plots consist of a
mixture of two-thirds snap beans
to one-third soybeans. The MBB
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greatly prefers snap beans to
soybeans; therefore, over
wintering beetles lay their eggs in
these plots rather than in the
soybean fields. This provides a
concentrated population of larvae
in which the parasitic wasp can lay
its eggs.)
Even with statewide release of
the tiny beneficial insects,
localized MBB outbreaks still
occur, and chemical controls are
required as well, Whalen cautions.
For this reason it is important to
scout fields routinely- especially
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those with early planted, full
season soybeans, where economic
infestations are most likely to
develop.
In spite of occasional rescue
treatments, she and Graustem feel
that in the long run the cost of
controlling the bean beetle with the
parasite will be much lower than
with chemicals.
Unfortunately, since Pediobius
can’t overwinter in Delaware’s
climate, this biological control
against the Mexican bean beetle
requires annual releases.
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