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Grubs and Grub Damage in Field Crops

By Christina Difonzo
 
Revised field identification sheet for grubs will assist in identification and gives information about typical crops or situations affected by different species.
 
I have already had a few calls about grubs and grub damage appearing as the weather warms. This included a visit to a winter wheat field in Isabella County that had thinning stand in some areas, caused by European chafer. It has been 15 years since the last European chafer outbreak in central Michigan. Grub survival seems good, at least in the places I’ve dug around in, and there are often multiple species present. 
 
To assist in grub identification, I revised a field ID sheet for grubs with pictures and additional hints about typical crops or situations affected by different species.
 
In summary, at this time of the year:
  • Lighter-textured fields and parts of fields (e.g., sandy knolls) often are a focal point for grubs and their damage compared to other locations.
  • Very large grubs (size of a quarter) are almost certainly June beetle. Because of their multi-year lifespan, they may feed one more year before pupating.
  • Grub damage to winter wheat is almost always caused by European chafer in central Michigan. This species will pupate in the next month, so damage to the wheat stand will end.
  • Asiatic garden beetle is the easiest species to identify by sight based on a white “bulb” on its mouthparts. To date, Asiatic garden beetle as a pest is limited to the southern two tiers of counties bordering Indiana and Ohio. Thus far, the grubs I’ve collected appear to be second instar, meaning they have one more growth stage to go. This lengthy feeding period may be why they are so troublesome in corn. I am looking for Asiatic garden beetle fields to sample this summer.
  • If you know a field is infested, options are limited and there are no rescue treatments available. In winter wheat: nothing to be done now; chafers will stop feeding and pupate in a month. In soybean: seed treatments have little impact; delay planting as long as practical to avoid as much feeding as possible before grubs pupate. In corn: a high rate of seed treatment or a soil insecticide can help, but from experience, heavy grub populations eat through insecticide barriers; tillage can also help, but I’ve been in tilled fields with grub problems; as in soy, delaying corn planting can reduce the feeding window open to grubs