IPMP3.0, Oregon State University, Copyright 2000 Mint Root Borer
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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Note: this information is considered unpublished work and should not be used as final or finished results.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.