The above-normal rainfall that we’ve had on the Eastern Shore has brought some pest problems to corn growers in Virginia.
Periods of cool wet weather after planting resulted in slow germination in many fields, which left seeds and seedlings more vulnerable to soil insect pests such as seedcorn maggot, white grubs, and wireworms. The longer the seed remains in the ground the greater the chance of it being discovered by the aforementioned soil pests. At this point, most corn has been planted and there isn’t anything a grower can do postplanting for these soil pests. The majority of corn was probably treated with an insecticide seed treatment such as Poncho or Cruiser, which undoubtedly helped improve stand in many fields this year.
Another problem that the spring rainfall brought was slugs in no-till fields. This is an age old problem, and the solution has not changed in decades. Dry weather will eliminate the slug problem. However, if seedlings are small (less than 3 leaves), and the slug infestation is heavy, economic damage can occur from this pest. Below is the recommendations taken from the Virginia Cooperative Extension Pest management Guide for Field Crops http://www.ext.vt.edu/pubs/pmg/fc/InsectsCorn.pdf
Slug, Snail Sampling/Decision Making
Slugs can become serious pests in no-till fields during spring periods of cool, wet weather. Fields with heavy layers of manure, crop refuse, or thick weed cover are at higher risk from slugs. Because slugs feed at night and hide during the day in the mulch and surface trash near the seedlings, they often are not suspected of being the cause of the shredded leaves on the young corn seedlings. Yet slugs can be found during the day by turning over clods of dirt and surface trash near the seedlings. It is suggested that samples be taken from the area around 5 plants in 10 locations of the field to determine the average number of slugs associated with each plant. Populations of 5 or more slugs around each plant at the spike through the 3rd-leaf stage may be economic, especially if injury is heavy, plant growth is slow, and cool, wet conditions prevail. During dry, warm weather, 10 or more slugs per plant may be tolerated. Also, corn seedlings that have reached the 3rd-leaf stage of growth generally are able to outgrow feeding damage by slugs.
Cultural practices which may help reduce slug populations include reduction in the use of manure, shift to conventional tillage practices for at least one season, and minimum tillage to reduce the amount of surface trash.
There is basically one registered product for control of slugs, and it works well – Metaldehyde (product = Deadline M-Ps, or Deadline Bullets)applied at 10.0-40.0 lb per acre. For best results, apply product in evening. Especially beneficial if applied following rain or watering. It should be noted that most corn-producing states are suggesting an application rate of 12 to 15 lb/A, if banded over or along side the row after the plants have emerged. Recent Delaware field trials indicate good results against slugs using 10 lb Deadline M-Ps/A broadcast with a cyclone spreader. Spreader must be calibrated to deliver at least 5 pellets/sq ft. Slugs generally stop feeding in 2-3 hours and die within 2-3 days.
Another potential spring pest problem in corn is armyworm infestations on seedlings or early-whorl stage plants. Armyworm is a common early season pest that can cause occasional losses in corn and should be monitored for in the spring. Infestations usually first develop in fields of small grains or in other grass cover crops. In conventional tillage systems, partially-grown larvae can migrate into corn fields from grassy waterways or wheat fields. Armyworms chew irregular holes on the edges of corn leaves. Damage is usually first noticeable around the field margins adjacent to these areas. The name armyworm derives from its behavior of migrating in large numbers into fields similar to invading armies. In no-till or reduced tillage systems, infestation may cover the entire field. In these systems, eggs may be laid on grasses within the field prior to planting and herbicides may force armyworms to feed on corn as the weeds or cover crop dies. Cool, wet, spring weather usually favors armyworm development. Armyworms overwinter as partially grown larvae in grasses or small grain fields. When warm spring temperatures return, armyworm feeding resumes. Armyworms may move onto corn during this period. Moths also begin flying in spring and deposit new eggs. Eggs are small, greenish-white, globular, and laid in clusters of 25 or more on the leaves of grasses.
True Armyworm Sampling/Decision Making (taken from the VCE Pest Management Guide Pub. No. 456-016).
No-till fields planted into a small grain cover crop, pastures, or weedy fields all have a high risk for armyworm infestation. Survey field edges where margins border small grains or large grassy areas and watch for damaged plants. If armyworm damage is seen, examine 20 plants at each of 5 locations within the field and record the percentage of damaged plants, the average size, and the severity of injury.
Armyworms usually migrate from small grains starting in late May. Spot treatments may be warranted if infestations are confined to small areas. Control for armyworms is recommended if 35 percent or more of the plants are infested and 50 percent or more defoliation is seen on the damaged plants, provided that larvae average less than 0.75 inch long. Worms greater than 1.25 inches in length usually have completed their feeding. A number of insecticide products provide effective control of true armyworm including: most pyrethroids such as Baythroid, permethrin, Asana XL, Mustang Max, Warrior/Karate, Hero, as well as other insecticides such as Lorsban, Lannate, Sevin, Pencap-M, and Bt products.