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Mint Root Borer Development
Research Progress Report - 1993
Joyce Takeyasu, Ralph Berry, and Marvin ButlerNote: this information is considered unpublished work and should not
be used as final or finished results. It has been included in IPMP 2.0 because it may not
be available from other sources, and in some cases may include information that may not
reach final publication.
Introduction
Variability in mint root borer development was observed during the 1992 season. This has
consequences in timing treatments for mint root borer control since hibernacula are
resistant to both chlorpyrifos (Lorsban) and the entomopathogenic nematodes. For optimum
control, treatments must be applied before hibernacula form. In other words, fields must
be sampled and assessed as early as possible. This is particularly important for heavily
infested fields which may require a pre-harvest application of entomopathogenic nematodes.
Pheromone traps have been used to monitor adult emergence and soil sampling, coupled with
the use of Berlese funnels, to assess larval populations. Soil sampling and Berlese funnel
extraction of larvae is a time-consuming process but is a direct measure of larval
numbers. However, if there is a strong correlation between pheromone trap catches and the
level of larval infestation at the end of the season, trap catches could be used as a
diagnostic tool. To correlate pheromone trap counts with larval infestation and to
determine how early fields can be sampled, fields throughout the Willamette Valley were
monitored during the 1993 field season.
Materials and Methods
Western Oregon
Pherocon lC sticky traps containing a mint root borer pheromone cap (Trece, Inc., Salinas,
CA) were placed in 16 peppermint fields throughout the Willamette Valley and checked on a
weekly basis. Starting in mid-July, soil samples were taken every two weeks from eight
fields. The other eight fields were sampled just one time post-harvest to determine the
level of mint root borer infestation. Rhizomes were separated from the soil and placed in
Berlese funnels to extract mint root borer larvae. The soil was sifted with a 1/4 mesh
screen and carefully examined for larvae. The samples taken in September were also
examined for hibernacula.
Central Oregon
Seventeen mint root borer sex pheromone traps (Pherocon lC) were placed
in peppermint fields in the different growing areas in central Oregon to determine the
distribution of mint root borer in central Oregon and peak adult emergence. Traps were
located in the following growing areas: Prineville/Lone Pine, Culver, Lower Bridge, and
Agency/Little Agency Plains. Traps were placed in the fields on June 14 and monitored
weekly until early September. All moths were sent to OSU for identification.
Results and Discussion
Western Oregon
Of the eight fields that were sampled extensively, four had mint root borer levels above
the treatment threshold of 2 larvae per ft² while the remaining four had infestation
levels below the threshold. The category to which a field belonged was correctly diagnosed
from soil samples taken on August 13, 1993. More heavily infested fields were diagnosed as
early as July 15, 1993. This demonstrates that sampling for mint root borer can begin
sooner.
The first hibernaculum was found during the first week of September in a field near
Monroe. This suggests mint root borer sampling should be completed by the end of August to
allow for prompt treatment of fields in the first half of September. Correlation between
pheromone trap counts and level of larval infestation is very weak (R² = 0.02).
Therefore, trap catches should not be relied upon to predict larval infestation later in
the season.
Table 1: Mean number of mint root borer larvae per ft² on different sampling dates, 1993.
_____________________________________________
Mean number of MRB larvae per ft² on
Field 7/15
7/30 8/13
post-harvest¹
_____________________________________________
1
0.0 0.0 0.0
0.5
2
0.8 0.0 1.0
0.8
3
0.2 0.0 0.2
0.9
4
0.0 0.1 0.4
1.0
5
1.0 0.5 3.0
---²
6
1.6 1.2 4.9
6.5
7
1.0 3.9 6.1
---²
8 10.4
15.2 15.6
---³
______________________________________________
¹ sample dates varied from 9/1 to 9/16
² data unavailable due to rotation to another crop
³ soil samples not taken on this sample date
Central Oregon
As in 1992, the pheromone traps demonstrated that
mint root borer is widespread in central Oregon. The largest number of adult males were
caught in the Agency Plains area, particularly at one location. Adult males also were
abundant in fields in the Lower Bridge area. In 1992, peak adult emergence of males
occurred on July 23, but peak emergence was delayed about two weeks in 1993. Peak
emergence occurred on August 2 in the Agency Plains area and on August 9 in the Lower
Bridge area which shows differences in emergence patterns in central Oregon. However, this
phenomenon is not surprising since growers use different production practices in different
areas and weather conditions vary from one growing area to another, even in central
Oregon. |