Lead Author: Adrian Pekarcik
The grubs of Asiatic garden beetle (AGB, Maladera castanea) have been causing annual early-season problems in field crops (predominately field corn) of Northwest and North Central Ohio since 2012, and this year is no exception. Historically a pest of turf grasses and ornamentals, this species has gained a hunger for corn seedlings, and to a lesser extent, young soybean plants and occasionally wheat. Since the arrival of warmer temperatures this spring, AGB has been reported as early as April 25 this year from 5 Ohio counties including Erie, Fulton, Henry, Lucas, and Williams.
How did AGB get in my field? AGB is an annual white grub species meaning it completes the four stages of its life cycle (i.e., egg, larva/grub, pupa, and adult/beetle) in one year. The adults, which are about ½” long and a dull chestnut brown, are nocturnal and active during the summer months when they feed on the leaves and flowers of more than 100 plants. The beetles mate and lay their eggs in the soil which quickly hatch into very tiny grubs. There are three developmental stages, or instars, that the grubs must complete before becoming adults. The grubs feed on the roots of many plant species but have recently shown preference for field corn, and sometimes soybean in sandy soils of Ohio, Michigan, and Indiana. As of May 23, a majority of grubs were in their third and final instar and should enter the pupal stage, which does not feed, very soon; in 2017 this occurred during the last week of May. Problems arising early season are caused by the same grubs that hatched in the field last summer. Large infestations of AGB in fields almost always occur in sandy soils, common in Northwest Ohio.
Image of second (bottom) and third (top) instars of AGB next to a dime. Notice the enlarged “cheek” structure near the mouth.
What is AGB doing to my field crops? The grubs feed solely on the roots of plants, often clipping the main root of seedlings. Grub feeding on roots reduces water and nutrient uptake of the plants and results in stunted, discolored (typically yellowing and purpling), wilted plants that may ultimately die. Damaged fields often have gaps in row. Oftentimes these symptoms do not begin until after grubs have been feeding for several days. Plant stand losses can exceed 40% when populations are large enough.
AGB feeding causes plants to wilt, stunt, discolor, and ultimately die, resulting in noticeable gaps in the rows.
How can I identify and scout for AGB? AGB is most prevalent in sandy soils, and in Ohio most often occur in the northwest, although there have been occurrences in lake sands along the northern coast, and in isolated areas near sandy river beds. The most efficient way to scout is to dig around plants in the field, especially any that may be stressed, and look for the presence of root feeding and/or white grubs (see the video here:http://www.oardc.ohio-state.edu/ag/pageview3.asp?id=3787). AGB seems to show preference for weed species like marestail and giant ragweed, so make sure to check these plants, if present. Unlike other white grub species, such as Japanese beetle, AGBs are typically smaller and more active. They will immediately run when unearthed and picked up. AGBs can be distinguished from all other white grub species by the presence of large, bulbous maxillary palps (cheeks), near the mouth. The adults, which are nocturnal and present from June through September, can be found near outdoor lights in the evenings where they feed on the leaves and flowers of nearby garden plants.
Strategies for Control? While there are no good treatments for controlling AGBs, here are some thoughts to consider. Be sure to manage weed populations (e.g. marestail, giant ragweed, chickweed, etc.) to prevent adult beetles from laying eggs, as they seem to prefer these species. Planting corn late to miss the most active early feeding of grubs (minimizing the overlap between grub feeding and small roots) appears to be helpful. While there was no statistical difference in yield after using in-furrow insecticide in 2014 On Farm Research (https://agcrops.osu.edu/sites/agcrops/files/ofr_reports/2014%2520Fulton%2520AGBproducts.pdf), we can find dead or dying grubs that appear more yellow when soil insecticides are used in furrow at planting. Fields with low to moderate pressure may benefit from a high rate insecticidal seed treatment, although more data is needed for both control tactics. Finally, if stand loss occurs, consider replanting corn (See replant calculator: http://web.extension.illinois.edu/csrec/eb260/entry_6903/). If you have questions on this new pest, please contact Eric Richer (firstname.lastname@example.org), Adrian Pekarcik (email@example.com), Andy Michel (firstname.lastname@example.org), or Kelley Tilmon (email@example.com