Time to Spray for Leaf Folders

Last year, some growers saw an explosion of leaf folders that caused considerable damage. By the time growers noted the problem, it was too late to control them. This is because leaf folders, unlike true leafrollers, tend to stay within the leaf roll day and night, protecting them from insecticide sprays. The best time to control leaf folders would be when they first appear and begin to roll up leaves.

On a research team visit to our research garden yesterday, it didn’t take long to find leaf folders on the emerging tops. This is a 4-year garden with most of the tops still arching out of the straw, with all of the leaves rolled up and pointing downward. A few more advanced plants had leaves fully unfurled and pointing upwards, although still less than 15 cm (6 in.) tall. On plants with fully unfurled leaves, it was relatively easy to find the leaf folders rolling up the end of a leaflet (Figure 1). Unrolling the leaf would reveal the small green caterpillar, which is about 5-7 mm long at this stage, often coiled into a circle (Figure 2). Caterpillars could also be found in tops that were still fully rolled up and pointing downwards. In these cases, the caterpillars were hiding between the leaves (Figure 3). These caterpillars were much harder to find and required us to physically open the bunched-up canopy.

Figure 1. A leaflet rolled up by a leaf folder.

Figure 2. A caterpillar of the zigzag Herpetogramma leaf folder, found within a leaf roll.

Figure 3. A caterpillar (and silk) hiding between the leaves of a newly emerging 4-year-old top.

Leaf folder populations are very high in the research garden. We found about two caterpillars per metre of bed. Given the difficulty in finding the caterpillars, populations could be even higher. It is also possible that more caterpillars will emerge over time. This was in a section of the garden with low leaf folder populations in 2024. Some commercial fields had much higher populations than the research garden last year. As a result, there is the potential for another large outbreak of leaf folders in some fields.

Given this finding, NOW is the time to start controlling leaf folders. Not all gardens have problems with leaf folders, so it is best to base an insecticide application on scouting or field history. If there were leaf folder problems last year in a field, that field (or area of the field) will require insecticide application. Areas near forests are more likely to have issues with leaf folders, but other areas can also have problems on occasion. Our research garden, for example, is nowhere near a woodlot. When you are unsure, scout fields thoroughly and regularly over the next few weeks. There are no exact thresholds for what populations warrant spraying, but likely finding one or more caterpillars per metre of row is a good starting point for deciding to spray. Scout multiple areas of the field to determine if the whole field needs to be sprayed or just a section of the field.

While there are no insecticides registered for this species of leaf folder in ginseng, insecticides applied for control of leafrollers should also control leaf folders if applied at the right time. Given the potential for more caterpillars to emerge in the next week or two, it is best to make at least two applications 7 to 10 days apart. The options available for leafroller control are Dipel 2X DF, Success/Entrust, and Delegate. Success and Entrust are different formulations of the same active, and Delegate has a similar mode of action to Success/Entrust. While we don’t know for sure what will provide the best control, try Dipel 2X DF now, since it is most effective on young larvae, and then apply one of the other products 7 to 10 days later. Scout again after each spray to determine whether further controls are necessary. Keep in mind that once a leaf is rolled up, it will remain that way even if the caterpillar is dead. You may need to open up some rolls to confirm whether the caterpillar is still alive. Read the insecticide labels carefully because some require application in the evening for best control.

If you have any questions or observations about leaf folders or their control, contact our research team:

Sean Westerveld, OMAFA: sean.westerveld@ontario.ca

Amy Shi, OGGA: amy.shi@ginsengontario.com

Forecasting Frost Events

This is the time of year when growers keep an eye on the forecast for the potential for frost. There is a risk of some frost over the next two nights. At this time, the current forecasts range from a mild frost to no frost at all. Here are some reminders for determining frost risk:

1. Forecasts are based on temperatures at eye level. Temperatures on the ground can be a few degrees colder on calm and clear nights, depending on the humidity level.
2. The dew point is most important for determining frost risk because it gives an indication of how low temperatures could get on the ground level in calm and clear conditions. Colder air holds less moisture than warmer air, so as an airmass cools at night, eventually the air can no longer hold the moisture and dew or fog forms. The point at which this occurs is called the dew point. At this time of year, the temperatures rarely fall below the dew point even at ground level because as moisture comes out of the air, some heat is released that keeps the temperature relatively steady at the dew point.  If a forecast does not include the dew point, it can be calculated based on the temperature and relative humidity. Here is an example dewpoint calculator: https://www.calculator.net/dew-point-calculator.html
3. Use a mix of forecasts to determine the frost risk. They all have some benefits and some limitations. In my experience, model-based forecasts tend to be more accurate in the long term for predicting extreme weather like frost, although they fluctuate considerably from day to day as the models change. On the other hand, forecasts involving a mix of models and meteorologists, such as Environment Canada and The Weather Network tend to moderate temperatures in the long term (i.e., warmer night temperatures than what the models show) and then become more accurate as the event approaches.  Here are some options with Simcoe programmed in:
   • Meteoblue (forecasts based on models that are specific to a location; includes relative humidity and temperature for calculating the dew point)
   • Weather Underground (forecasts based on models that are specific to a location; includes a dew point forecast)
   • Environment Canada (forecasts are based on one location in a county, and all other locations in the county have the same forecast; no dew point or relative humidity forecasts)
   • The Weather Network (forecasts are specific to location; includes relative humidity and temperature for calculating the dew point).
4. Forecasts change regularly, and none are perfect. Keep looking back at the forecasts as a potential frost event approaches.