Conducting plant population checks is the first step to determining if there are pest problems in a corn field. Missing, stunted, or wilting plants could indicate a field has an insect pest problem. The pressing question is “How do I check my corn plant populations?” We suggest you sample in units that are one-thousandth of an acre.
|1/1000 of an acre|
|30||17 ft. 5 in.|
|32||16 ft. 4 in.|
|36||14 ft. 6 in.|
|38||13 ft. 9 in.|
|40||13 ft. 1 in.|
Sampling rows as they were laid down by the planter is helpful too. For instance, if the field was planted with a six-row planter, sample the six rows that represent one pass of the planter. Watch tractor wheel tracks and choose your next six-row sample that corresponds with the same six-planter units. This way, you’ll notice variations or patterns between the units of the planter. Make sure to sample the proper length for your row width. Take at least 5 samples as you cross the field.
After you’ve finished counting plants in each sampling location, take the average number of plants and multiply it by 1000. This will give you the number of plants per acre. For example, if you had an average of 30 plants, multiply it by 1000—which gives you 30,000 plants per acre. If your plant population is more than10% lower than that expected, it may indicate a problem in the field. While many things could cause reduced populations, several pest-related issues might be involved:
- early season seed or seedling blights
- seed corn maggot
- white grub
Note also that those symptoms might suggest problems with your planter:
- seed depth
- seed furrow closure
- other insect pests, or even bird
Proper diagnosis requires some digging in the gaps within a row to check for seeds and seed health.
Early Season Soil-Born Insect Pest Identification
Seed corn maggot (SCM) is an insect pest that feeds on large-seeded crops like corn and soybean. An adult female fly (SCM resembles house flies) searches for moist soil cracks, high organic matter, decomposing plant material, and fields with manure, laying their eggs there early in the spring. These tapered, legless, ¼” long maggots appear to be headless and are pale yellowish-white. They burrow inside the germinating seeds to feed. Symptoms of SCM damage may show as skips in the corn rows or seedlings without cotyledons.
Wireworms are soil-dwelling larvae of click beetles. Adults are brown to black, bullet-shaped, hard-shelled beetles about 1½” long. Larvae are hard, smooth, slender and yellow to reddish-brown. These worms resemble wire; thus their name. They vary from ½” to 1½” long.
White grubs are the soil-dwelling larvae of several types of scarab beetles: Japanese beetles, May (or June) beetles, or masked chafers. Adults lay eggs in the soil near perennial grasses. These larvae prefer to feed on roots of grasses. White grubs are normally a problem in 1st year corn after sod. They are thick, white soft-bodied larvae about 1/4 to 1 inch long, curling into a C-shape when disturbed.
Early Season Soil-Borne Insect Pest Damage
As corn seed germinates, it gives off carbon dioxide—thus attracting SCM and wireworms. SCM only feed inside germinating seeds; wireworms feed on both seeds and roots. These seeds will be hollowed-out, soft and starting to rot. White grubs feed on the roots of newly germinating corn seed and young plants.
Specifics on SCM: Incorporating residue and tilling more frequently increases possible damage. Plowed and disked fields are more attractive for adults than chiseled fields, though chiseled fields won’t be maggot-free. Reduced- and no-tillage fields have the lowest density of adult flies. Incorporating cover crops, alfalfa, and grass field in spring increases the potential for SCM infestations. In that case, it’s best to wait at least 2 ½ to 3 weeks to plant your corn— otherwise you’ll dramatically increase your chance of SCM damage. Crimping a winter rye cover crop will see some increase in damage, but less than incorporating it.
Scouting for and Managing Early Season Soil-Borne Insect Pests
For scouting, refer back to “Conducting Plant Population Checks” and double-check your results and your pest ID. Examine plants and seed closely.
For managing early season soil-borne insect pests, know that they have no economic threshold. Prevention is key to managing them. You have several choices:
Avoid SCM damage by using a degree day model to predict the peak flight of adult flies. Start your degree model on a biofix date of January 1st. The base temperature for this model is 39°F.
360 degree days marks peak flight by the 1st generation. You’ll want to plant 450 degree days after peak flight for a fly-free crop, giving yourself 810 total degree days. By now, most of the larvae will have pupated in the soil.
Do the same for the second and third generation:
|Base Temp = 390 F||Peak 1st Generation||Seed Corn Maggot fly-free degree days||Peak 2nd Generation||Seed Corn Maggot fly-free degree days||Peak 3rd Generation||Seed Corn Maggot fly-free degree days|
Source: Insect IPM for Organic Field Crops: Seed Corn Maggot by Katelin Holm and Eileen Cullen
You can also use the first day you till or disk as peak flight. This assumes the field is most attractive at this point. Plant 450 degree days after this point.
Planting into soil at 50°F degree or warmer encourages corn to germinate quickly, promoting rapid emergence and reducing exposure time to SCM, wireworm and white grubs.
Another option: use insecticide pretreated seed for controlling all three pests. Note that these are neonicotinoid insecticides. Make sure that you use the low rate unless you have an infestation of corn rootworms. The high rate is ONLY for corn rootworm.
If you use these seed treatments, consider the effect on pollinators like bees. Use bee-safe seed lubricants in planters to reduce dust that can drift off-target. Hundreds of bee species are native to New York and provide invaluable services; neonicotinoids can kill these important members of our agroecosystems.
A soil-applied insecticide can be use at planting to control wireworm and white grub. Consult your Cornell Guide for Integrated Field Crop Management to select one for use. If losses are severe, consider the practicality and economics of replanting a field.
Natural Enemies for Early Season Soil-Borne Insect Pests
Many diseases, insects, and nematodes can attack seed corn maggot, wireworm and white grub larvae. For instance, ground beetle larvae and adults feed on their eggs and larvae. But such natural enemies can be affected by soil applied insecticides.