Other than localized storm damage to shade structures over the past few weeks, there are currently no major unusual issues for ginseng. However, prolonged wet, humid and cooler weather over the past week and more wet weather to come this weekend could greatly increase foliar diseases, phytophthora, and slugs. Make sure you are on top of your spray schedule.
As we approach mid-season, it is also a good time to thoroughly re-calibrate your sprayers, replace worn nozzles and ensure even spray volume and patterns are coming from each nozzle. For an overview of sprayer calibration for ginseng and how to obtain optimal coverage consult the article “How to Spray Ginseng” on Sprayers101 (https://sprayers101.com/spraying-ginseng/).
European Chafer Grubs
Ginseng growers will lose the use of imidacloprid insecticide (Admire, Alias) as a drench for grub control in May of 2025, meaning that it can only be applied at seeding this year and next. This is a longer transition than most other crops, for which the imidacloprid soil drench is already discontinued. There are no alternative insecticides registered for use on ginseng to control grubs and it is unlikely any new ones will be registered before the loss of imidacloprid due to a lack of viable alternatives. It is important to start to determine what impact the loss of imidacloprid will have on your operation.
European chafer grubs are large white grubs frequently found in soil under turf in lawns (Figure 1). Adults fly from mid-June to mid-July of each year in Ontario, depending on location. It is probably around now for this area of Ontario. They congregate around trees in open areas and then lay eggs into suitable soil habitat (e.g., turf or field crops).
Figure 1. A European chafer grub.
Based on this, the best guess for how the life cycle works on ginseng is:
- Adults mate on trees near the edges of gardens or the posts of newly erected shade structures.
- The females lay eggs in nearby habitat which could include weedy areas of the future ginseng gardens.
- The eggs hatch soon after and the grubs feed on the roots of weeds in the garden.
- Once beds are formed, ginseng is seeded and straw mulch applied, they then feed on the volunteer rye that germinates shortly after seeding.
- The grubs attack the ginseng in the seedling year after the rye is killed off.
Feeding damage from grubs is mostly confined to the seedling year. The grubs feed on the young seedlings from within the soil. Initially the top wilts as the grub severs the vascular system (Figure 2) and then the grub pull the tops down through the straw as it feeds (Figure 3). A single grub will eventually kill enough seedlings to leave an empty patch about 30 cm (1 ft) in diameter, within which all stages of damage can be seen. This damage occurs in May and June until they pupate and adults emerge in June. It is likely that due to the shade structure, the adults are unable to fly and mate within the garden and they exit the garden and lay eggs elsewhere. The damage is most obvious in year 2 when the canopy closes and the missing holes are noticeable, but the grubs are gone by that point.
Figure 2. A seedling wilting due to grub feeding below ground. The actively feeding grub is usually found under plants with these symptoms if the symptoms in Figure 3 are also present within a patch.
Figure 3. Seedling tops pulled into the soil from beneath due to recent grub feeding.
If the egg-laying and early life cycle discussed above is correct for ginseng, then the most important factor for preventing grubs from damaging the ginseng is to prevent weeds from growing between fumigation and seeding. Without the weed cover in the empty field during the construction of the shade structure, it is unlikely that the adults would lay eggs in the field in the first place.
Fumigation would also likely kill the eggs and young larvae if they were present in the soil at the time of fumigation. Fields fumigated in May could be more at risk because the egg laying is several weeks after fumigation. Fumigating later in the spring or in early summer may lead to fewer grub problems.
Some growers never apply imidacloprid and have no major issues with grubs. Some growers had issues with grubs before they started to apply imidacloprid and have applied it ever since. Consider a couple of possibilities to explain this, 1) growers that never had issues with grubs may have subtly different production practices such as fumigation timing and weed control to prevent grubs from surviving to ginseng seeding, or 2) growers that experienced grub issues had one or two bad years due to either differences in production practices, differences in the egg-laying timing that year, or fields that were in proximity to substantial European chafer habitat (e.g., hay fields, turf). It is possible that in most years the imidacloprid application doesn’t actually do anything because the grubs are not there. Nevertheless, grub issues do exist in some years and can be locally severe. Imidacloprid provided the assurance that grubs would not develop, and growers will no longer have that option.
At seeding later this year, consider leaving some beds untreated with imidacloprid so you know how big a problem grubs could be without treatment. If grubs do cause damage in those areas, consider the agronomic practices used during garden construction this year (e.g., time of fumigation, weed pressures etc.). Over time, you can start to identify the practices and conditions that lead to grub damage and avoid those that allow them to infest and survive in the garden.