For the second year in a row, we’ve been hearing multiple reports of fields with high grub activity in central Iowa, particularly between Ames and Ankeny. Typically, people were called out to the field for other reasons but found grubs once they started digging. In fact, a field I visited last week looked pretty healthy considering some other challenges farmers are facing, such as dry soil conditions, frost damage, and uneven emergence. There were some stunted plants and a few gaps, which could have a number of culprits. None of the plants we dug up had any obvious root feeding or damage to the seed, yet we could find at least one grub per plant.
The above photo (Photo 1) was taken during the initial look at the field, which prompted some questions by the agronomists as to which species of white grub was in the field. There are several species of white grubs in Iowa, but not all are pests, and they can co-occur within a field. Identifying the grubs you find is critical to ensure proper management. The two we think of the most in crop fields are Japanese beetle and May/June beetles, and although they look the same, their life cycles are completely different. Japanese beetles have one generation per year, while May/June beetles have a three-year life cycle and may need to be managed for multiple years.
We collected some grubs from the field the next day, and I brought them back to the lab. It is difficult to distinguish species in the field since it requires you to look at a pattern of hairs at the tip of the abdomen (called the raster). After taking some photos under the microscope (Photo 2), we determined that the grubs we collected were Japanese beetle.
The grubs we found were around ½ inch long and probably 6-8 inches deep in the soil since the topsoil was quite dry. Japanese beetle larvae are around 1 ¼ inch long fully grown, and we expect Japanese beetle adults to begin emerging in the next few weeks.
Although that particular cornfield did not seem to have any noticeable feeding, another field 30 miles southeast was confirmed to have Japanese beetle grubs in late May with evidence of feeding. Those grubs were closer to the roots at the time of inspection (Photo 3).
The ideal time to scout for white grubs is prior to planting; however, most people do not notice they have grubs until they observe stand loss or stunted, wilted, or discolored plants. If you are doing early-season stand assessments, make sure to check for white grubs or other early-season insect pests, such as wireworms, and properly identify them to understand management options. True white grubs (May/June beetles) and wireworms have long life cycles (3 years and 7 years, respectively) and would require a multi-season management plan.