Grubs Damage Very Common this Spring

Dig up the grubs and look at the rasters or anal hairs of the larvae at the butt end (an enjoyable experience, I know). I've heard some pretty clever methods used by growers and reps to handle the grub and get it into position to look at the butt end. Some just cut the grub in half so that the grub stops trying to squirm, leaving you with just the bottom half to look at. Others have rolled up a piece of paper into a tube and let the grub crawl into it, then they pinch the tube off so that the butt end still sticks out but the head and sometimes biting part of the insect is left inside the tube. Regardless of how you wrestle with the critter, you will need a magnifying glass to get a good look at the rasters.

Figure 1. Grub Damage in Ontario Forages in 2008
Photo Credit: Robert Moloney, FS Partners

Figure 2a. European Chafer Grub - raster pattern. Note open "zipper"
Photo crecit: Art Schaafsma, U of G, Ridgetown Campus

If the rasters have a Y or deep, long V shaped pattern it is a European chafer grub (Figure 2a). Some say it looks like an open zipper. If the rasters are in an oval shaped pattern with the hairs running parallel to each other, then they are June beetle grubs (Figure 2b). If you live in the Niagara region, you may be dealing with Japanese beetle grubs, which have short V shaped raster patterns (Figure 2c).

Figure 2b. June Beetle Grub Raster Pattern
Photo Credit Art Schaafsma, U of G, Ridgetown Campus

Figure 2c. Japanese beetle grub - raster pattern

It is important to know which grub is present as they have different life cycles and feeding times. European chafer is a one year grub (Figure 3). The larvae feed into late fall and start up again in very early spring until they pupate by Mid May. Therefore, all of the feeding damage by chafer is done in late fall and early spring. This is why we tend not to see chafer damage in soybeans, because the crop is planted later when the larvae are no longer actively feeding. If they are June beetle grubs, these live for three years in the soil (Figure 4). Two of those years are spent as larvae which feed throughout the year on the crop. June beetle grubs tend to have more of an impact on soybeans (and sometimes forages) because those crops are still young and struggling to start when the June beetles grubs are still feeding. Japanese beetle grubs, common in the Niagara Region, similar to the chafer's life cycle, in that they are a one year grub, though they feed longer into the summer then chafer before pupating (Figure 5). Feeding damage by Japanese beetle grubs can continue until early June before they pupate to become an adult. Feeding begins again by these one year grubs in late summer, after the adults have laid their eggs and the small larvae hatch and begin feeding.

Figure 3. European Chafer - one year life cycle
Source: University of Nebraska

June Beetle cycle - live for 3 years in soil
Source: University of Nebraska

Figure 4. June Beetle cycle - live for 3 years in soil
Source: University of Nebraska

Figure 5. Japanese beetle life cycle
Source: University of Minnesota

It is important identify fields now that have issues and use an insecticide seed treatment on the crop going in those fields to protect it. Corn, soybeans and wheat do have Cruiser and/or Poncho registered to give some protection to the seedlings. If you are finding 2-3 chafer grubs per square foot in the sandy areas, an insecticide seed treatment or soil applied insecticide is needed. If there are higher infestations, Poncho 1250 may be necessary. If the issue is chafer and you plan to plant soybeans in late May, then you are probably safe from damage and won't need to use a seed treatment. However, if you plan to plant soybeans earlier then that or you have June beetle or Japanese beetle grubs, then you will need an insecticide seed treatment on that crop.

For forages, unfortunately we do not have any products that can protect the seedling crop nor rescue treatments that can help the second year crop that is also being impacted. For now, the best options we can suggest is to do some form of tillage in problem fields and replant the field into a crop that has an insecticide seed treatment available, like corn, soybeans or wheat to protect the crop and nuke some of the grub population. If you can not rotate to a different crop, increasing your ratio of alfalfa is safer, as the grubs are really only wanting to feed on the grass species. If you have chafer as the issue, then at least the feeding will stop in the next few weeks and won't start up again until early fall. But if they are June beetles, unfortunately the crop might have to struggle through the summer still.

In any of these grub fields, another scouting trip in late summer is recommended so that you can determine if grubs are still present and in high enough numbers to need seed treatments again on the next crop.