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Control of Mint Root Borer Research Progress Report - 1992
Prepared by Joyce Takeyasu
Note: this information is considered unpublished work
and should not be used as final or finished results. It has been included in IPMP 3.0
because it may not be available from other sources, and in some cases may include
information that may not reach final publication.
The mint root borer continues to be a serious pest in
peppermint; however, excellent control can be achieved with the entomopathogenic nematode,
Steinernema carpocapsae. Experiments conducted in 1991 justified lowering the
recommended application rate from 3 billion to 2 billion infective juveniles per acre. In
1992, we continued to investigate whether the rate could be reduced further. In addition,
studies were conducted to determine the optimum timing of nematode application to achieve
maximum control.
Pre harvest studies: Two experiments were conducted. The first experiment compared five
nematode rates (O, 0.5, 1.0, 1.5 and 2.0 billion infective juveniles (IJs / per acre)
using 8'x30' plots in a field located north of Eugene. A randomized complete block design
was used with treatments replicated five times. On July 14, the plots were irrigated for
approximately 30 minutes to wet the ground prior to nematode application with a carbon
dioxide driven backpack sprayer (25 GPA at 30 psi). The nematodes were applied with the
irrigation running, followed by approximately one inch per acre of water. Samples were
taken on August 8. Six 1/2 sq. ft. soil samples were taken per plot to a depth of two
inches. Rhizomes were separated from soil and placed in Berlese funnels to extract live
MRB larvae.
The second experiment compared 3 nematode rates (O, 1 and 2 billion IJs/acre) using the
length of an irrigation line as the experimental unit. Again, a randomized complete block
design was used, but this time treatments were replicated 3 times. The experiment was
located in 2 fields: one replicate was located in the same field as the small plot
experiment while the other two replicates were located in a field in Jefferson. Nematodes
were applied in the Eugene field on July 21 and 22, and in the Jefferson field on July 27
and 28. The ground was pre-irrigated prior to injecting the nematodes into the irrigation
line, followed by approximately one inch per acre of water. The first set of samples were
taken on August 8 and 15 in the Eugene and Jefferson fields, respectively. Another set of
samples were taken on August 25. Sixteen 1/2 sq. ft. soil samples two inches deep were
taken alone each irrigation line. MRB larvae were extracted with Berlese funnels.
A nematode persistence study was conducted utilizing the irrigation lines receiving the 2
billion IJs/acre rate. On three sampling occasions (immediately after application, 2 weeks
after application, and 4 weeks after application), soil samples were taken at ten random
sites alone each irrigation line. Two 150 cubic cm samples to a depth of 6 cm were taken
at each site and combined. The soil was then sieved to remove large particles and placed
in an 8 oz. plastic cup. The soil was moistened, if dry, and five late instar Galleria
mellonella were placed on the soil surface and incubated for six days at 20 degrees
F. Galleria larvae are highly susceptible to the nematodes making them an ideal bioassay
organism. After six days, the Galleria were removed from the soil and replaced
with another five larvae. This second set of larvae were removed after six days. Dead
larvae were dissected to determine the presence of nematodes. If nematodes were found in
the cadaver, their numbers were recorded.
In both the small plot and the large scale experiment, all nematode treatments
significantly reduced MRB larvae compared to the untreated check. Differences between
nematode rates were not detected, but the 2 billion IJs/acre rate always resulted in the
highest % reduction (Table 1). Although MRB larvae were reduced below the treatment
threshold, the % reduction for the small plot experiment ranged from 64.4% to 69.5% which
is somewhat low. Two possible explanations are: 1 ) the nematodes are less effective
against early instar MRB, or 2) the nematodes were applied too early and did not persist
long enough to provide continuing control of later hatching larvae. The second explanation
seems more likely since the nematodes have been observed successfully attacking early
instar MRB in the laboratory. Furthermore, the nematode numbers drop dramatically two
weeks after application. This, in combination with pheromone trap catches, indicates that
the nematodes may have been applied too early. The application date, July 14, occurred
prior to a peak in trap catch. Since only male moths are caught in the traps, and female
moths emerge slightly later than the males, we would expect the larvae resulting from this
peak to enter the soil when nematode numbers have declined.
Table 1. Results of pre-harvest nematode application against MRB larvae, 1992.
____________________________________________________________________
SMALL PLOT EXPERIMENT
Mean No. MRB 1/ Mean No. MRB 2/
in 3 sq. ft.
per
sq. ft.
Treatment
(mean + SEM) (mean + SEM) %
Reduction
Control
11.8 + 3.1a
3.9+1.0
--
0.5 bil IJs/A
4.0 + 0.9b
1.3+0.3
66.1
1.0 bil IJs/A
4.0 + 1.6b
1.3+0.5 66.1
1.5 bil IJs/A
4.2 + 3.8b
1.4+0.6 64.4
2.0 bil IJs/A
3.6 + 1.1b
1.2+0.4 69.5
LARGE SCALE EXPERIMENT
Mean No. MRB Mean No. MRB
in 8 sq. ft.
per sq. ft.
Treatment
(mean SEM) (mean SEM)
% Reduction
____________________________________________________________________
Sampled on August 8 and 15, 1992
Control
40.3 + 1.2 a 5.0 + 0.2
--
1.0 bil IJs/A
15.3 + 6.5 b 1.9 +
0.8
62.0
2.0 bi IJs/A
7.0+ 1.0 b 0.9 +
0.1
82.6
Sampled on August 25, 1992
Control
53.7 + 14.8 a 6.7 + 1.8
1.0 bil IJs/A
21.3 + 3.7 b 2.7 + 0.5
60.3
2.0 bil IJs/A
17.7 + 8.7 b 2.2 + 1.1
67.1
____________________________________________________________________
1/ Means followed by the same letter are not significantly different. Separation of means
by FPLSD, p = 0.05.
2/ Estimated treatment threshold is 2 to 3 MRB Per sq. ft.
The results of the large scale experiment also suggest the nematodes were applied too
early. Two and a half weeks after application, results of the large scale experiment show
82.6% reduction with the 2 billion IJs/acre rate, but this decreased to 67.1% on August
25. This decrease in % reduction reflects an increase in MRB numbers in the Jefferson
field where two of the three replicates were located. MRB development in the Jefferson
field lagged behind the Eugene field. Moths were observed flying in the field as late as
August 15 whereas moths were not seen in the Eugene field after July 23. An increase in
MRB numbers from the first sampling date to the second did not occur in the Eugene field,
indicating the application on July 21 and 22 was timed properly.
Post-harvest studies: As with the pre-harvest experiments, the post-harvest experiments
also consisted of a small plot experiment and a large scale experiment. Both the small
plot and large scale experiments compared 4 treatments:
1) Untreated check
2) 1.0 billion IJs/acre 3) 2.0 billion IJs/ acre 4) Lorsban (2 Ib ai /acre)
3) 2.0 billion IJ's/ acre
4. Lorsban (2 lb ai / acre)
A randomized complete block design was used with each treatment replicated four times. The
small plot experiment was located in a field south of Corvallis and the treatments applied
August 28. The large scale experiment was again located in two fields. Three replicates
were located in the Corvallis field and the treatments were applied between September 1-3.
The remaining replicate was located in a field in Jefferson and the treatments were
applied between September 4-7. Nematodes were applied using the same methods as in the
pre-harvest experiments; Lorsban was applied in the same way as the nematodes. Two weeks
after application, six 1/2 sq. ft. soil samples per plot were taken from the small plot
experiment and twenty 1/2 sq. ft. soil samples per irrigation line were taken from the
large scale experiment. MRB larvae were extracted using Berlese funnels. In addition, the
soil was carefully checked for the presence of MRB hibernacula.
In both the small plot experiment and the large scale experiment, nematode treatments
significantly reduced the mint root borer compared to the untreated check (Table 2). No
differences were detected between the two nematode rates although better control appears
to be achieved with the higher rate. Comparison of the nematodes with Lorsban showed no
detectable differences in the large scale experiment. However, in the small plot
experiment, there was a significant difference between Lorsban and the higher nematode
rate. Application methods differed between the small plot and large scale experiments,
suggesting that Lorsban may provide better control when chemigated. It should be noted
that there was again a difference in MRB development between the two fields. On September
11, the Corvallis field had an average of 44.9% hibernacula formation, whereas the
Jefferson field had only 3.5% hibernacula formation on September 15. The MRB hibernacula
is resistant to both nematodes and Lorsban, so it is important to treat the field before
hibernacula form. Since three out of the four replicates were located in the Corvallis
field, the observed control may be an under estimation.
Table 2. Results of post-harvest nematode application against MRB larvae, 1992.
____________________________________________________________________
SMALL PLOT EXPERIMENT
Mean No. MRB1/ Mean No. MRB 2/
in 3 sq. ft.
per
sq. ft.
Treatment (mean + SEM)
(mean + SEM)
% Reduction
___________________________________________________________________
Control
8.0 + 1.8 a
2.7 + 0.6
--
Lorsban
6.5 + 1.8 ab 2.2 + 0.6
18.8
1.0 bil IJs/A 3.0
+ 0.8 bc 1.0 + 0.3
62.5
2.0 bil IJs/A 1.5
+ 0.5 c 0.5
+ 0.2
81.2
___________________________________________________________________
LARGE SCALE EXPERIMENT
___________________________________________________________________
Mean No. MRB Mean No. MRB
in 10 sq. ft.
per sq. ft.
Treatment (mean + SEM)
(mean + SEM)
% Reduction
___________________________________________________________________
Control
29.2 + 10.4 a
2.9 +
1.0
--
Lorsban
7.8 + 2.8 b
0.8 + 0.3
73.5
1.0 bilIJs/A
7.2 + 2.0 b
0.7 + 0.2
75.2
2.0 bil IJs/A
4.2 +
1.6 b
0.4 + 0.2
85.5
___________________________________________________________________
1/ Means followed by the same letter are not significantly different. Separation of means
by FPLSD, p = 0.05.
2/ Estimated treatment threshold is 2 to 3 MRB per sq. ft.
Application of S. carpocapsae can effectively control the mint root borer, but
timing is critical to achieve good control. It is extremely important that the nematodes
are applied early, before hibernacula form. If the crop is to be harvested late, a
pre-harvest nematode application should be considered. However, a pre-harvest nematode
application can be applied too early. Nematode numbers in the soil decreased noticeably
two weeks after application so the nematodes cannot be relied upon to persist in the
field. Since MRB adults emerge over a two month period, care must be exercised not to
apply the nematodes prematurely. The variation in development between fields is another
concern in the proper timing of a treatment. From observations this field season,
generalizing MRB development over the entire Willamette Valley can no longer be done.
Further study is needed to determine if the differences in development are regional or
random. Although no differences were detected between any of the nematode rates in either
the pre-harvest or post-harvest experiments, the data suggest some control is sacrificed
with a lower rate. However, all nematode rates, including the 0.5 billion IJs/acre rate,
resulted in significant MRB control. |
