Early Season Pests of Corn-Sampling

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Early Season Pests of Corn- Sampling

04/24/2018
Pest Sampling
  • Damage by early season pests can be devastating to the successful establishment of a crop and its yield potential.
  • Preplant scouting is recommended especially when rescue treatments are not available.
  • In this article we will review sampling recommendations for some of the most important early pests of corn.

Early season pests can be devastating to the crop’s establishment, especially if the seed is unprotected and conditions are ideal for infestations (weather conditions, preceding crops, soil type, field history etc.). Preplant sampling is recommended for certain soil pests, especially those for which rescue treatments are not available. Sampling is necessary to diagnose insect problems in a field and to take appropriate measures of control. In this article we will review some of the most important early season pests of corn and their recommended sampling time and methods (Image 1). Sampling for these pests will take place from preplant to about V5.

Wireworms: 
These are slender, wire-like larva, orange- brown in color (Image 2). They damage corn by feeding on the germ of corn kernels, hollowing out the seeds completely or feeding on underground portions of the plant.  Wireworm damage can show up after corn emergence as gaps in the rows or stunted plants. Problems with wireworms are usually more common in fields where sod or small grains were previously grown. In these fields, it is advisable to set up “bait stations” before planting to have an idea of the potential for damage. The “bait” is a mix of equal parts corn and wheat seed buried 4” deep, covered with soil, a piece of black plastic and clear plastic on top. After 2 weeks of burying the bait (before planting), check the bait and count the number of wireworm s present. You can also use a shovel before planting to dig a 6” diameter hole x 10” deep examining each sample for the presence of wireworms (10-20 samples per field). After planting, keep a close eye on stand emergence and if you are finding stunted or missing plants, sample the field immediately. In those areas where damage is suspected dig up a 6” deep, 2 x 1 ft area along the row and set the sample on a dark background. Crumble the soil with your hands examining the soil for larvae and the plant roots, stems and seeds for signs of feeding. Wireworms tend to move deeper in the soil as temperatures warm up so if we suspect damage but we wait too long to sample we might miss them. 

Seed corn maggots (SCM):
SCM tends to be more of a problem in cool, moist soils with manure, green vegetation or reduced tillage. The maggot (larva) is a pale-yellowish maggot, legless with no heads. They burrow into seeds preventing them from emerging which results in gaps in the field (similar to wireworm damage although more widely distributed across the field). If damage is suspected after plant emergence, dig 2 linear foot of row in five suspicious areas in the field and examine the ungerminated seeds for damage (Image 3).

Black cutworm (BCW):
BCW adults (moths) can be recognized by a dark, dagger-shaped marking near the outer edge of each forewing (Image 4). BCW larvae are gray to nearly black in color with a greasy appearance, a pale band along top of the body and paired spots of uneven size along the sides of their bodies (Image 4). BCW larvae will feed on corn plants by either consuming leaf tissue (small larvae) or cutting plants right at the soil surface- large larvae (Image 5). They tend to be a bigger problem in fields with green vegetation or no-till or minimum till systems.  
BCW adult arrival to our area and flight activity can be monitored using light traps. Degree days can also help predict black cutworm development. Using this information and once the crop has emerged, walk fields from VE to V5 and look for signs of black cutworm damage (leaf feeding, wilted plants or cut seedlings lying on top of the ground). If damage is found, sample immediately to determine the percentage of BCW damaged plants. Examine 20 consecutive plants in 5 areas of the field (100 plants total) and record the number of plants that show signs of BCW damage. If 3-5% of plants show leaf feeding (have not been cut), control is recommended. 

White grubs:
Several species of grubs (the larval stage of scarab beetles) can attack corn seedlings. These include the May/ June beetle, masked chafer and Japanese beetle. White grubs are white with brown heads and six well developed legs (Image 6). White grub species can be identified by their raster pattern; a set of hairs found on the underside of their last abdominal segment. Damage to corn is caused by root pruning which may result in stunted, wilted or discolored plants. Corn planted following sod or a cover crop may be at greater risk for white grub damage. In high-risk fields, sampling can be done prior to planting by digging up a few areas of 2 x 1 ft (6” deep), and carefully sorting through soil looking for grubs. If two or more grubs/ cubic foot of soil are found prior to planting, a treatment is recommended.  After emergence, walk fields looking for stunted, discolored or wilted plants (Image 6). If damaged plants are found, dig the soil right below them (2 ft long x 1 ft wide x 6” deep) and examine the soil for the presence of grubs and the roots for signs of pruning. Do this in at least 5 areas of the field.

Occasional pests

Slugs:
Slugs can be an occasional problem of field crops especially in no-till or minimum till systems where high amounts of surface residue and moisture exist. They feed on corn leaves leaving narrow, irregular scars of various lengths and slime trails (Image 7). Damage can be confused with other insect feeding but the presence of slime trails is characteristic of slug damage. Cool wet weather and planting into wet soils leaving open furrows can create an ideal environment for slug problems. Slugs feed at night and hide in the soil or under crop residue during the day. Sample 20 consecutive plants in 5 areas of the field to get an estimation of percent damaged plants. Slug counts are more accurately recorded at night when they are active. Chemical options to manage slugs are very limited.

Garden Symphylans:
Adult garden symphylans are small (0.25” long) white and have 12 pairs of legs (Image 8). First instar nymphs have 6 pairs of legs and add a new pair of legs with each molt (total of 6 molts). Fields with high organic matter or manure are more at risk for damage by garden symphylans. They feed on germinating seeds and root hairs of corn resulting in stunted, purpled or dead plants. To sample, dig up plants and soil (4-6” deep) in areas exhibiting damage symptoms and carefully sort through the soil. Identify and record the number of symphylans found per plant. If garden symphylan was an issue in a field the previous year, dig a few samples in previous “hot spots” before planting. There is no treatment threshold for garden symphylans but preplant sampling in problem fields can help determine the need for chemical treatments prior to or at planting. Preplant and at plant applications of some OP’s and carbamates are labeled for control of garden symphylans.

Grape colaspis:
Grape colaspis larvae feed on root hairs and may cause wilting, stunting or purpling of the leaves and stems. Damage is usually noted in patches and it is more severe under stressful growing conditions. Grape colaspis larvae (grubs) are white, small, comma shaped with a tan head. Their legs are short and have a lot of hairs arising from bumps on the underside of the abdomen (Image 9). To sample, dig up plants in 2 linear ft of row in suspected damaged areas in the field. Inspect the roots for damage and carefully hand-sort through the soil and verify the presence of colaspis grubs.

References:
Purdue University Extension. Field Crops IPM

Corn-Pests.jpg

Image 1. Sampling schedule of early season pests of corn. (Corn stage diagram courtesy of Purdue University Extension).
Image 2. Wireworm larvae (Source: GROWMARK, Inc.)
Image 3. Seed corn maggot damage to seed. (Source: GROWMARK, Inc.)
Image 4.Black cutworm adult (left) and larvae (right). (Source: GROWMARK, Inc.)
Image 5. Black cutworm damage (cutting) to young corn seedling. (Source: GROWMARK, Inc.)
Image 6: White grub (larva) and damage symptoms on corn stands. (Source: GROWMARK, Inc.)
Image 7: Slug damage to corn. (Source: GROWMARK, Inc.)
Image 8: Garden symphylan (Source: GROWMARK, Inc.)
Image 9: Grape colaspis grub. Note bumps and hairs on the underside of the abdomen. (Source: GROWMARK, Inc.)


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