By Kelley Tilmon
There have been recent reports of high corn earworm populations in certain grain corn fields. Corn earworm is a pest with many hosts including corn, tomatoes and certain legumes. In Ohio it is typically considered a pest of sweet corn rather than field corn, but this past week substantial populations have been found in certain field corn sites. Corn earworm moths are most attracted to fields in the early green silk stage as a place to lay their eggs. These eggs hatch into the caterpillars that cause ear-feeding damage, open the ear to molds, and attract birds. With a wide range of planting dates this year, different fields may be at greater risk at different times.
It is open to debate how well corn earworm can overwinter in most parts of Ohio, and the majority of our population probably immigrates each summer from more southern states. Weather fronts from the south can help carry influxes of moths our way. Compounding the problem, many of these southern moths are resistant to some of the Bt hybrids used against them in the past. Dr. Celeste Welty, OSU vegetable entomologist, maintains a trapping network for corn earworm in sweet corn which can be found here:
Corn earworms are damaging as caterpillars laid by moths in the silks near the developing ear tip, and are all but impossible to find by scouting. They vary quite a bit in color – with individuals that are dark brown, brown, tan, green, or even pinkish. Typically only one caterpillar is found per ear but in heavy infestations more may be found. They enter corn ears at the tips where the majority of feeding occurs. This also opens the corn ear up to the potential development of ear rots. Unlike western bean cutworm caterpillars, corn earworm caterpillars do not spend any time out on the plant surface before migrating to the ears – they are protected in the ear structure from the beginning and so insecticide application does little good against the caterpillars. When corn earworm moths are immigrating, sweet corn growers rely on frequent sprays to kill adult moths, which is not economical in field corn.
The Bt protein Vip3A (in Viptera) is still deemed effective against corn earworm. For a current infestation in field corn, because chemical control is ineffective, the scouting emphasis should be on assessing mold and disease levels in infested corn.
Feeding sites or exit holes when the caterpillar matures and leaves the ear leave holes in the corn husk, which provide a potential entry wound for pathogens like Fusarium and Gibberella. Some of these organisms can then be a further source for mycotoxins, including Fumonisins and deoxynivalenol, also known as vomitoxin. In some cases, damaged kernels will likely be colonized by opportunistic molds, meaning that the mold-causing fungi are just there because they gain easy access to the grain. However, in other cases, damaged ears may be colonized by fungi such as Fusarium, Gibberella and Aspergillus that produce harmful mycotoxins. Some molds that are associated with mycotoxins are easy to detect based on the color of the damaged areas. For instance reddish or pinkish molds are often cause by Gibberella zeae, a fungus know to be associated with several toxins, including vomitoxin. On the other hand, greenish molds may be caused by Aspergillus, which is known to be associated with aflatoxins, but not all green molds are caused by Aspergillus. The same can be said for whitish mold growth, some, but not all are caused by mycotoxin-producing fungi.
So, since it is not always easy to tell which mold is associated with which fungus or which fungus produces mycotoxins, the safe thing to do is to avoid feeding moldy grain to livestock. Mycotoxins are harmful to animals – some animals are more sensitive to vomitoxin while others are more sensitive to Fumonisins, but it is quite possible for multiple toxins to be present in those damaged ears. If you have damaged ears and moldy grain, get it tested for mycotoxins before feeding to livestock, and if you absolutely have to use moldy grain, make sure it does not make up more than the recommended limit for the toxin detected and the animal being fed.