Examine corn for rootworms
It’s hard to believe we’re already approaching the halfway mark of summer. The corn is planted, weed control is under wraps, and the fertility management practices and decisions have all been made. Another sound agronomic practice is to assess rootworm feeding and possible damage on corn roots.
Evaluating for corn rootworm injury is essential to help determine if any management strategies need to be changed for the next growing season. All corn hybrids with and without Bt proteins should be rated for possible injury. The optimum time to do this is usually from mid-July to early August.
Corn rootworm begins to hatch in late May to the middle of June, with June 6 as the average hatching date in Iowa. Soil temperature drives the development of corn rootworm, and is measured by soil growing degree days, using a base of 52 degrees F. The 50% egg hatch occurs between 684 to 767 accumulated degree days.
For 2014, this was occurring in the Muscatine area and the southern parts of Iowa the first week of June, with northern areas of the state reaching 50% egg hatch June 15 to a week afterward. This is why we want to begin scouting and looking at fields mid-July to early August.
Again, I want to emphasize you should check all corn hybrids, even if they have the Bt rootworm resistance trait. You may be surprised at what you find.
Both northern and western corn rootworm beetles lay very small white eggs that have the shape of a football. According to entomologists, western corn rootworms usually lay most of their eggs in the top 4 to 8 inches of soil; northern corn rootworms usually lay their eggs in the top 4 inches. However, eggs have been found as deep as 10 to 12 inches below the soil surface.
3 larval stages
Corn rootworms (western and northern) have only one generation per year and will go through three larval stages (called instars). Some northerns have extended diapause; it’s not common in Iowa but still present in some areas.
Each of these three stages will take seven to 10 days to complete, with the later instars primarily responsible for the damage caused to corn roots. Corn rootworm larvae are less than an eighth of an inch long for the first instar, and will be about half-inch long when fully developed. Corn rootworm larvae are slender and white with a dark brown head and a dark plate on the top side of the tail section.
Carbon dioxide is emitted from the root tips of corn, which attracts rootworm larvae to feed. The first instars begin to feed on smaller root branches, root hairs and outer root tissue. The older, more developed corn rootworm (later instars) will do the most of the feeding damage, as they enter the inner root tissue that is responsible for movement of water and nutrients for the plant.
Larval damage is usually most severe after the secondary root system is well established and brace roots are developing. Root tips will appear brown and are often tunneled into and chewed back to the base of the plant.
The article below explains how to check for and rate corn rootworm damage.
For more corn rootworm information, Aaron Gassmann, ISU corn entomologist, has a very good webpage that includes an interactive node-injury scale that helps you visually assess corn rootworm damage. It can be found at www.ent.iastate.edu/dept/faculty/gassmann/rootworm.
Basol is an ISU Extension field agronomist based at Nashua in northeast Iowa. He can be contacted at firstname.lastname@example.org.
• 0 — no feeding damage
• 1 — one node (circle of roots) or the equivalent of an entire node, eaten back to within about 1.5 inches of the stalk (soil line on the seventh node)
• 2 — two complete nodes eaten; about 20 roots pruned to within 1.5 inches of the stalk
• 3 — three nodes eaten (highest rating); about 30 roots are pruned to within 1.5 inches of the stalk
This article published in the July, 2014 edition of WALLACES FARMER.
All rights reserved. Copyright Farm Progress Cos. 2014.