Check for rootworm damage
As mid-July approaches, it’s time to start assessing each of your cornfields for corn rootworm feeding and potential root injury. This process needs to be conducted every year and in every cornfield, regardless if it is a Bt or non-Bt traited hybrid, to determine if management strategies need to be changed the next growing season.
Optimum timing for scouting and evaluating cornfields should occur from the middle of July through early August. Continuous cornfields and areas with Bt performance issues should be the primary fields of concern and where to start first.
The beginning of corn rootworm hatch starts in late May and continues through mid-June, with the average peak hatching date on June 6. So far, the 2015 growing season has been similar to 2014, with cool and wet conditions, causing average hatching dates to be slightly behind.
Soil temperature is the driving force behind corn rootworm development. This is measured by growing degree days, where research has shown that 50% of egg hatch occurs between 684 to 767 accumulated degree days (base 52-degree-F soil). For 2015, the area around Muscatine in southeast Iowa was one of the first regions to reach peak corn rootworm egg hatch, with other areas of the state attaining it within the following seven to 10 days.
ISU site provides updates
To help growers and agronomists, Erin Hodgson, ISU Extension entomologist, writes an Integrated Crop Management article each year, to update corn rootworm development and expected dates for egg hatching. You can also monitor egg hatch for your area next year by going to the Iowa Environmental Mesonet on the ISU website. To create an accurate map, set the start date to Jan. 1 of the current year and the end date to the current date of when you’re generating the degree day accumulation. Set the plot parameter to “Soil growing degree days (base equals 52).”
In 2014, excessive rain in June and cooler temperatures through the growing season may have caused increased mortality rates of corn rootworm larvae, resulting in less root injury. Saturated soil conditions in June, caused larvae to drown, coupled with cooler temperatures that slowed growth (increasing mortality from insect-killing fungi). It is speculated these and other factors were responsible for the increased larval mortality in 2014. Although 2015 has been similar, it’s still important to assess your corn this year.
Both types of rootworms lay small white eggs. Western corn rootworms lay most of their eggs in the top 4 to 8 inches of soil; northern corn rootworms usually lay eggs in the top 4 inches, but eggs have been found as deep as 10 to 12 inches.
Here’s what to look for
Corn rootworms (western and northern) have only one generation per year and will go through three larval stages (called instars). Each of these stages will take seven to 10 days to complete, with the later instars primarily responsible for the injury caused to corn roots. Corn rootworm larvae are less than an eighth-inch long for the first instar, and will be about a half-inch when developed. Larvae are slender and white with a dark brown head and a dark plate on the top of the tail section.
Carbon dioxide is emitted from the root tips of corn, which attracts rootworm larvae to feed. The young, first instar begin to feed on smaller root branches, root hairs and outer root tissue. The older, more developed corn rootworms (later instars) will do the most feeding damage, as they enter the inner root tissue that’s responsible for movement of water and nutrients for the plant. Larval injury is usually most severe after the secondary root system is 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.
Estimate larval injury
To estimate larval injury for the next growing season, measure adult rootworm populations in cornfields of the current year. Place at least four unbaited Pherocon AM sticky cards throughout a field during the period of peak adult abundance (late July through mid-August). Replace cards weekly and count the number of adults collected per card. Based on recent economic analysis by Dunbar and Gassmann in 2013, if the average number of adult northern and western corn rootworm captured exceeds two per card per day, then the field should be managed for larval rootworm the next season.
The story below explains how to check corn for rootworm and rate rootworm damage. Of the seven steps, the final is critical: If you find an average injury rating above 0.5 on the three-point scale, you need to consider making some changes in your corn rootworm management.
Basol is an ISU Extension field agronomist based at Nashua in northeast Iowa. Contact him at [email protected]
This article published in the July, 2015 edition of WALLACES FARMER.
All rights reserved. Copyright Farm Progress Cos. 2015.
Photo by Erin Hodgson, ISU