August 25, 2025
Post-harvest scouting is the best way to measure mint root borer populations in your fields
Mint root borers damage mint stands by feeding on the roots and rhizomes in the late summer and fall. After hatching, larvae feed on mint foliage for a few days before dropping to the ground and searching for rhizomes to feed on. Correct timing is the key to successful larval scouting. If you scout too early (July and early August), the larvae are too small to be found reliably. If you scout too late (after mid-September), much of the damage has already been done, and larvae have started to form hibernacula, which protects them from insecticides. The best time to scout is late August to early September. At this time larvae are large enough to see, and control efforts will be effective at providing some crop protection.
To scout for larvae, collect soil/rhizome samples from 1 ft x 1 ft areas in the mint field. Samples should be 2-3 inches deep and include the mint rhizomes in that area. You should collect at least 25 samples, or two samples for every 2-3 acres. You can screen the samples in the field, or extract larvae using the Berlese funnel method. The treatment threshold is 2-4 larvae per square foot, depending on age and vigor of the stand, other pests or stresses, and oil price.
Based on the Growing degree model, 5% of larvae are expected to have formed hibernacula by August 27 in the Grande Ronde Valley, and September 12 in the Baker Valley.
July 31, 2025
The adult mint root borer capture rate declined significantly to 0.6 moths/trap, compared to 2.8 moths per trap last week. Since mint has matured early due to weather conditions this growing season, mint harvest in Baker Valley is taking place approx. two weeks earlier than normal. Two fields were swathed and harvested this week, and the remaining fields will be harvested next week. This is the last week of trapping for this season.
Based on the growing degree model for mint root borers, peak egg-laying was on July 21 near Imbler and July 30 in Baker Valley, with peak hatch on July 29 and August 7 respectively. This means that harvest operations may capture larvae before they begin feeding on the rhizomes. Growing degree accumulation for mint root borers is about 10 days ahead of the 30-year average.
Mint root borer larvae damage mint stands by feeding on roots and rhizomes, so it is always a good idea to sample for larvae in the fall and treat when needed. A good time to sample is ahead of 5% hibernaculum formation, which is forecast for August 26 and September 9 for Imbler and Baker Valley, respectively.
July 25, 2025
Time to wrap up in-season Vantacor applications
Mint root borer trap counts averaged 2.8 moths per trap this week (July 16 – 23) and is up slightly from the previous monitoring week (July 10 – 16). Overall, mint root borer moth activity is considered low in Baker Valley, but individual traps had between 0 and 7 moths. Peak moth activity was predicted for July 16 and, similar to prior years, this “peak” activity has stretched out over a two-week period starting before and diminishing after the peak date. Peak egg hatch is predicted for August 5 in Baker Valley, and July 29 near Imbler, so I anticipate harvest activities to disrupt successful recruitment of MRB larvae to some degree as it can take 5 – 10 days for the eggs to hatch. Sweep net and foliage/soil surface inspections did not yield any cutworm larvae and only 1 looper. We will continue monitoring the sites through harvest and will collect post-harvest root samples in early September. 5% hibernaculum formation is expected around August 25 near Imbler and September 9 in Baker Valley.
This week’s pest alert was provided by Darrin Walenta. Darrin runs the mint pest alert field monitoring program in northeastern Oregon.
July 17, 2025
Time for in-season Vantacor applications
We are currently in the optimal window for in-season Vantacor applications for mint root borer control. Vantacor applications between peak mint root borer moth catch and peak egg laying can control mint root borers as well as variegated cutworms and loopers with a single spray.
In the Baker Valley, peak catch was yesterday, July 16 and peak egg laying is forecast on July 28. In Imbler, peak catch was on June 10, and peak egg laying is expected on July 21. Later applications have also been shown to be effective through peak egg hatch. This is expected on July 28 for Imbler, and August 4 for Baker Valley.
In field monitoring has detected low mint root borer numbers so far this season. Last week there was an average of one moth per trap. This week, one trap had six moths, up from two moths last week. The remaining four traps had one or two moths each, resulting in an average of 2.2 moths per trap. In addition to MRB pheromone traps, sweep net and foliage/soil surface inspections have not detected any cutworm larvae so far this season at any of the monitoring sites. However, an occasional looper does show up.
July 10, 2025
The growing degree model for variegated cutworms predicts that third-instar larvae peaked on July 14 near Imbler and will peak on July 19 for Baker Valley. Expect fifth-instar larvae to peak on July 30 and August 4 for Imbler and Baker Valley, respectively.
The growing degree model predicts that peak moth catch will be on July 10 near Imbler, and July 15 near Baker Valley. Try to complete your sprays before peak egg laying, which is predicted to occur on July 20 or July 26 in Imbler and Baker Valley, respectively. Check the app to stay up to date with any changes in these predictions.
Previous OSU studies suggest that the best time to apply Vantacor to get the best overall control of mint root borers, loopers and variegated cutworms is between peak moth catch and peak egg laying based on the mint root borer growing degree model. Vantacor is a systemic insecticide that is absorbed by the plant, and controls insects over an extended period. It works best on insects at the egg and early larval stages. Since insects lay eggs over a couple week period, you want to spay early enough that the earliest laid eggs are still vulnerable, but late enough that the product is still effective when the later eggs are laid. The model for variegated cutworms predicts that third instar larvae will peak on July 14 near Imbler and should peak on July 18 near Baker Valley.
July 3, 2025
Hello and welcome to the Mint Pest Alert for 2025
This email newsletter is designed to help you control mint root borers, variegated cutworms, and loopers in mint fields. Over the next couple weeks, I will share the results from a trapping program in your region. I will also include information from growing degree models for mint root borers and variegated cutworms. These models predict when the insects will reach various growth stages, which can help you figure out the most effective time to spray.
A new addition to the Mint Pest Alert this year is a growing degree web app. This means you can get updated model predictions any time. You can see the app by visiting the Mint pest alert blog site at https://blogs.oregonstate.edu/mintpestalert/production-regions/eastern-oregon/. Select the weather station closest to you using the options on the left.
The growing degree model is predicting peak moth catch for mint root borer moths on July 9 near Imbler, and July 14 in Baker Valley. For variegated cutworms, first instar larvae are expected to peak on July 3 and 7 in Imbler and Baker Valley, respectively.
Previous research has shown that the optimal time to use Vantacor to control mint root borers, variegated cutworms and loopers is between peak moth catch for mint root borers, and peak egg laying. This timing can give you control of these pests with one spray.
Archived Reports – 2024
August 28, 2024
Post harvest larval sampling is the best way to measure mint root borer populations in your fields
Mint root borers damage mint stands by feeding on the roots and rhizomes in the late summer and fall. After hatching, larvae feed on mint foliage for a few days before dropping to the ground and searching for rhizomes to feed on. Correct timing is the key to successful larval scouting. If you scout too early (July and early August), the larvae are too small to be found reliably. If you scout too late (after mid-September), much of the damage has already been done, and larvae have started to form hibernacula, which protects them from insecticides. The best time to scout is late August to early September. At this time larvae are large enough to see, and control efforts will be effective at providing some crop protection.
To scout for larvae, collect soil/rhizome samples from 1 ft x 1 ft areas in the mint field. Samples should be 2-3 inches deep and include the mint rhizomes in that area. You should collect at least 25 samples, or two samples for every 2-3 acres. You can screen the samples in the field, or extract larvae using the Berlese funnel method. The treatment threshold is 2-4 larvae per square foot, depending on age and vigor of the stand, other pests or stresses, and oil price.
Based on the Growing degree model, 5% of larvae are expected to have formed hibernacula by September 2 in the Grande Ronde Valley, and September 16 in the Baker Valley.
August 15, 2024
Pheromone traps were removed from fields on August 6 in preparation for harvest. Last week, numbers declined to an average of 0.5 moths per trap. Based on the growing degree-day model, 90% moth catch was August 4 in the Grande Ronde Valley, and August 11 in the Baker Valley.
As the season wraps up, plan to scout for mint root borer larvae in soil and rhizomes in late August or early September. Fall sampling is the best way to determine if your in-season treatments were effective, and decide if fall control is needed. 5% hibernaculum formation is expected on August 31 and September 12 in the Grand Ronde and Baker Valleys, respectively.
August 1, 2024
Mint root borer trap captures decreased this week. A total of 7 moths were caught at two of the 6 monitoring sites for an average of 1.17 moths per trap. Based on the growing degree model, peak hatch will be on August 2 in the Grande Ronde Valley, and August 6 in the Baker Valley. 5% hibernaculum formation is expected on August 29 and September 8, respectively.
No variegated cutworms or loopers were caught in sweep net or foliage shaking samples. The variegated cutworm model predicts that 5th instar larvae will peak on August 3 and 7 in the Grande Ronde and Baker Valleys, respectively.
July 25, 2024
The optimal spray window for Vantacor is wrapping up. The model-predicted peak egg-laying date, which marks the end of the best time to apply Vantacor in-season, was on July 24 near Imbler, and will be on July 31 in Baker Valley. Peak egg hatch for mint root borers is expected on August 2, and August 7 for Imbler and Baker Valley respectively. The growing degree model for variegated cutworms predicts peak fifth instar larvae on August 4 and August 7 for Imbler and Baker Valley, respectively.
This week we caught an average of 1.8 mint root borer moths in the traps, a slight increase from 1.5 last week. There was some variation in capture numbers, with a max of 5 moths in one field, while no moths were caught at two other sites. Sweeps and foliage shaking did not reveal any variegated cutworms or loopers.
July 17, 2024
It is time for Vantacor applications. Peak catch of mint root borers, which indicates the start of the optimal spray window, was yesterday (July 16) near Imbler, and is expected on July 19 in Baker Valley. For the best efficacy, we recommend that sprays are completed by peak egg-laying, which is expected on July 24 and 28 in Imbler and Baker Valley, respectively. Overall mint root borer moth captures have been consistent across the last few weeks. There were an average of 1.5 moths per trap this week.
No cutworms or loopers were caught in sweep net, or foliage shaking samples this week. The growing degree model predicts that third instar variegated cutworm larvae will peak on July 19 and 22 in Imbler and Baker Valley, respectively.
July 10, 2024
With the heat wave over the last few days, insect development has been moving quickly, and model predictions are a few days earlier than we expected as of a week ago. If you are planning to use an in-season Vantacor application on your mint, the best time to spray will be starting in the next few days. The growing degree model predicts that peak moth catch will be on July 15 near Imbler, and July 18 near Baker Valley. Try to complete your sprays before peak egg laying, which is predicted to occur on July 26 or July 30 in Imbler and Baker Valley, respectively. Check the app to stay up to date with any changes in these predictions.
Previous OSU studies suggest that the best time to apply Vantacor to get the best overall control of mint root borers, loopers and variegated cutworms is between peak moth catch and peak egg laying based on the mint root borer growing degree model. Vantacor is a systemic insecticide that is absorbed by the plant, and controls insects over an extended period. It works best on insects at the egg and early larval stages. Since insects lay eggs over a couple week period, you want to spay early enough that the earliest laid eggs are still vulnerable, but late enough that the product is still effective when the later eggs are laid.
Mint root borer moths have been captured in two of the six sites with traps this year. Last week, a total of 8 moths were caught between two sites. No moths were found in the other four fields. These moth counts are just slightly higher than last week. No variegated cutworms or loopers were caught in sweep net samples in the fields yet. The model for variegated cutworms predicts that first instar larvae peak on July 10 near Imbler and should peak on July 12 near Baker Valley. Third instar larvae should peak on July 19 and 22, respectively.
July 3, 2024
Hello and welcome to the Mint Pest Alert for 2024. This website is designed to help you control mint root borers, variegated cutworms, and loopers in mint fields. Over the next couple weeks, I will share the results from a trapping program in your region. I will also include information from growing degree models for mint root borers and variegated cutworms. These models predict when the insects will reach various growth stages, which can help you figure out the most effective time to spray.
A new addition to the Mint Pest Alert this year is a growing degree web app found at the top of this page. This means you can get updated model predictions any time. Select the weather station closest to you using the options on the left.
Traps were put out in mint fields last week. The growing degree model is predicting peak moth catch for mint root borer moths on July 17 near Imbler, and July 20 in Baker Valley. For variegated cutworms, first instar larvae are expected to peak on July 9 and 11 in Imbler and Baker Valley, respectively.
Previous research has shown that the optimal time to use Vantacor to control mint root borers, variegated cutworms and loopers is between peak moth catch for mint root borers, and peak egg laying. This timing can give you control of these pests with one spray. Keep in mind that model predictions might change depending on how much warm weather this upcoming heat wave brings.
Archived posts from 2023
Fall Chemical Management
Late Aug/early Sep is the time to scout for Mint Root Borer (MRB) larvae in the soil if fall control is being considered. MRB larvae drop from the foliage to the ground in August and tunnel into rhizomes, where they will feed into the fall.
Larvae are generally too small to see in August, while sampling in late September is too late as damage is done.
Control treatments must be applied before larvae form hibernacula (overwintering, soil-based cocoons). Based on the insect development model, 5% hibernacula formation is expected September 11 in the Baker Valley, and occurred on August 29 near Imbler.
Sampling Recommendations for MRB larvae:
- Sample 1ft2 of soil to a depth of 2-3” (include rhizomes)
- 2 samples taken every 2-3 acres
- Minimum 25 samples per field
- Screen samples in the field or with Berlese funnels in the laboratory
Treatment threshold = average of 2-3 larvae/ft2 sample
Northeastern Oregon – August 25
Northeastern Oregon – August 3
Archived posts from 2022
Northeastern Oregon Mint Pest Alert Newsletter – August 31
Northeastern Oregon Mint Pest Alert Newsletter – August 10
Northeastern Oregon Mint Pest Alert Newsletter – August 4
Northeastern Oregon Mint Pest Alert Newsletter – July 28
Northeastern Oregon Mint Pest Alert Newsletter – July 20
Northeastern Oregon Mint Pest Alert Newsletter – July 15
Northeastern Oregon Mint Pest Alert Newsletter – July 6
Northeastern Oregon Mint Pest Alert Newsletter – June 29
Archived posts from 2021
Northeastern Oregon Mint Pest Alert Newsletter – August 28
Time for fall treatment.
Northeastern Oregon Mint Pest Alert Newsletter – July 30
Plan to sample early for larvae in soil.
Northeastern Oregon Mint Pest Alert Newsletter – July 23
MRB captures continue to decline
Northeastern Oregon Mint Pest Alert Newsletter – July 16
Optimum application window closed/ closing.
Northeastern Oregon Mint Pest Alert Newsletter – July 10
The optimum application window for MRB and VC control with Coragen or Vantacor is ending soon.
Northeastern Oregon Mint Pest Alert Newsletter – July 3
The optimum application window for MRB and VC control with Coragen or Vantacor is now
Eastern Oregon Mint Pest Alert Newsletter – June 18
Archived posts from 2020
Northeastern Oregon Mint Pest Alert Newsletter-September 11
Sampling Recommendations for MRB larvae:
•Sample 1ft2of soil to a depth of 2-3” (include rhizomes)
•2 samples taken every 2-3 acres
•Minimum 25 samples per field
•Screen samples in the field or recover larvae with Berlesefunnel in the laboratory
•Treatment threshold = average of 2-3 larvae/ft2sample
Northeastern Oregon Mint Pest Alert Newsletter- September 3
Late Aug/early Sep is the time to scout for Mint Root Borer (MRB) larvae in the soil if fall control is being considered (i.e. Lorsban, Mocap, nematodes). MRB larvae drop from the foliage to the ground in August and tunnel into rhizomes, where they will feed into the fall.
Larvae are generally too small to see in August, while sampling in late September is too late as the damage is done.
Control treatments must be applied before larvae form hibernacula (overwintering, soil-based cocoons). Based on the insect development model, hibernacula formation in Grande Ronde Valley should be 5% by September 11th and will reach 50% by October 14th. Baker Valley should expect 5% hibernacula by Sept 26th.
Northeastern Oregon Mint Pest Alert Newsletter- August 5
Recently, a new invasive leafhopper species, known as Ligurian leafhopper (Eupteryx decemnotata Rey, Hemiptera: Cicadellidae) was reported by Bill Gerth, OSU Plant Clinic and this prompted us to look out for this insect while sampling the commercial mint field in Western Oregon. This insect species is of Mediterranean origin and is a pest of many plants in the family Lamiaceae which include peppermint, sage, basil, oregano, rosemary, lemon balm, and other culinary herbs.
Ligurian leafhoppers are tiny, less than 3mm long or 0.12 inches, yellowish-green leafhopper (see image) with a characteristic pattern of spots on the head and the wings.
The Ligurian leafhopper is an important pest in cultivations of plants from the family Lamiaceae. Just like other leafhoppers, they are sap-feeding insects causing damage by puncturing cells and removing their contents. The characteristic stippling is produced (see image) when pest density is high. The damage can be mistaken for thrips or mite injury. The occurrence of this insect pest in Northeastern mint production systems and damage potential in commercial fields is still unknown.
Northeastern Oregon Mint Pest Alert Newsletter- July 31st
We are now at the optimal in-season application window at both Agrimet sites (the optimal window for Grande Ronde Valley will be closing soon).
Northeastern Oregon Mint Pest Alert Newsletter -July 24th
Peak moth flight of Mint root borer adults was reached July 21st in the Grande Ronde Valley, optimal control extends from this time through peak egg-laying (Aug 1st). The NMME forecast still puts peak moth flight for Baker Valley on July 26th. Good control can be achieved with Coragen® from peak moth catch through peak egg-laying.
Variegated Cutworm larvae should be controlled before 5th instar when they drop to ground and become difficult to control. Treatment threshold is 0.6 larva/sweep or 1.3 larvae/sq ft.
If applying Coragen, follow the label: http://www.cdms.net/ldat/ld8KF045.pdf
Northeastern Oregon Mint Pest Alert Newsletter- July 17th
Mint root borer capture rates in the pheromone traps were low and have yet to reach peak moth flight in Baker Valley. No variegated cutworms were found. Only one mint cutworm (See image below) was discovered during crop inspections and they are not generally considered a pest of economic concern as other cutworm species.
Northeastern Oregon Mint Pest Alert Newsletter- July 10th
Recent cooler weather has pushed back the predicted peak flight activity by a couple of days.
Northeastern Oregon Mint Pest Alert Newsletter- July 2nd
The peak flight activity is approaching in the coming weeks.
Northeastern Oregon Mint Pest Alert Newsletter- June 26
This week’s newsletter presents “PAMS” – Prevention, Avoidance, Monitoring, and Suppression framework for mint IPM ranging from proactive to reactive approaches in bio-intensive to conventional settings.
Northeastern Oregon Mint Pest Alert Newsletter- June 19th
Regular scouting and monitoring determine the presence and potential insect pest damage to mint fields in Oregon. Management practices, such as crop rotation, tillage practices, varieties used, etc., reduce the occurrence of many of these pests. This newsletter is a decision support system that utilizes resources such as Online phenology and degree-day models (http://pnwpest.org/dd/model), and the Pacific Northwest Pest Management Handbooks (https://pnwhandbooks.org/insect/crop-pests?combine=mint ) to facilitate the efficient use of chemical control options.
Archived Reports – 2019
NE Oregon
Northeastern Oregon Mint Pest Alert Newsletter – August 29th
Northeastern Oregon Mint Pest Alert Newsletter – August 7th
Northeastern Oregon Mint Pest Alert Newsletter – August 1st
Northeastern Oregon Mint Pest Alert Newsletter – July 24th
Northeastern Oregon Mint Pest Alert Newsletter – July 17th
Northeastern Oregon Mint Pest Alert Newsletter – July 10th
Northeastern Oregon Mint Pest Alert Newsletter – July 5th
Northeastern Oregon Mint Pest Alert Newsletter – June 27th
We just finished the second week of trapping for MRB adults in Baker County and the numbers are very low so far. A total five (5) MRB moths have been capture over the two week period. We are not doing any area-wide monitoring in the Grande Ronde Valley but we are providing the phenology development updates based on predictive models for the GRV. As expected, GDD accumulation is running behind a few days compared to last year.
Northeastern Oregon Mint Pest Alert Newsletter – June 19th