Bryan Jensen, IPM Program, UW-Madison

 

The cool, wet weather we’ve been experiencing has the potential to increase the amount of secondary insect injury to corn seed and/or emerging seedlings. The longer it takes corn to develop the longer it will be susceptible. The end results may be reduced stands and poor seedling vigor which are often mistakenly attributed to planter problems or poor seed quality. Furthermore, rescue treatments are not reliable or suggested. Building accurate field histories is an important IPM practice so we can accurately choose preventive practices in future years.

 

Seedcorn Maggot: Corn is not as susceptible to seedcorn maggot injury as soybeans. However, cool, wet growing conditions can increase injury potential to both crops as can livestock or green manure. Seedcorn maggots are often overlooked as a potential cause of poor stands. When emergence is uneven, dig up skips in the row to make sure the planter delivered a seed. Suspect seedcorn maggots if the seed has been partially fed on or if the white, legless larvae are present. Larvae have a very short life cycle and may not be present when you troubleshoot the field. Also, similar appearing saprophytic maggots may be found feeding on seed which is decomposing from other causes. Symptoms of seedcorn maggots may also show up as small holes in the first emerged leaf (cotyledon) and is a result of below ground maggot feeding. Leaf injury is not considered economic because the plant has recovered, but it may help confirm below ground symptoms of maggot feeding. Maggot injury is also more uniform across the field compared to wireworms.

 

Wireworms: Cool/wet conditions also increases the potential for wireworm damage that may also be more common in corn after pastures or other crops which had grassy weed problems the previous year.  Like seedcorn maggots, wireworms can feed on ungerminated corn seed leaving skips in the row that result in similar misdiagnosis as seedcorn maggot (poor planter or seed health issues). Injured seed will appear hollowed out but the majority of the seed coat will be present. Larvae may be found nearby which are copper-colored and have 3 sets of jointed legs. Don’t confuse with millipedes (non-pest) which are also abundant in cool/wet weather. Millipedes are also hard-shelled but are dark gray and have a fringe of hair-like legs the entire length of the body. Wireworms will also tunnel into the below ground portion of the shoot causing holes in the newest emerging leaves if feeding is concentrated above the growing point. If feeding is at the growing point, symptoms called “dead heart” or “wilted whorl” are evident. 

 

True White Grubs: Don’t injure seeds but feed on corn roots and underground shoots.  Like wireworms, damage is more common in corn after pastures or other crops with a history of grassy weeds. Unlike wireworms or seedcorn maggots, white grubs will feed on corn roots. The above ground symptoms are stunted, discolored plants from lack of nutrient uptake. White grub injury also mimics wireworms when they feed on below ground shoots that result in wilting of newly emerged leaves or the entire plant.  White grubs are relatively easy to find in the soil around damaged plants.