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Note: this information is considered unpublished work and should not be used as final or finished results.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.
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                 Mean number of MRB larvae per ft² on
Field        7/15          7/30      8/13         post-harvest¹
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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
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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.