CORN ROOTWORM MANAGEMENT
John Van Duyn, North Carolina State University, Entomology Extension SpecialistPrintable Version
Western and northern corn rootworms are recent arrivals in North Carolina. Currently they are spread across the Piedmont and Mountain regions, although western corn rootworm adults have also been detected in the Coastal Plain as far east as Martin county. These insects are the most important insect pests of corn in the Mid-West and have recently become troublesome to corn farmers in western North Carolina, as far east as Granville Co. It is the western corn rootworm that is most numerous and damaging. A third rootworm, the southern corn rootworm, is found throughout NC and can damage corn seedlings and roots. However, it attacks many hosts and is not a consistent pest of corn. This article concentrates on western and northern corn rootworms, which have similar life histories. Figure 1 shows adult corn rootworms of three species.
Figure 1. From left to right, western corn rootworm, northern corn rootworm, and southern corn rootworm.
Life History. Both western and northern corn rootworms have a single generation per year and overwinter as eggs that were laid during June and July in corn fields of the preceding year. Since beetles lay their eggs almost exclusively in corn fields, populations of rootworm larvae can be successfully avoided by following a good corn rotation plan. Rotation works well because the rootworm larvae cannot complete their life development on crops other than corn and die soon after hatching, if corn is unavailable. However, in the Mid-West, rootworm biotypes have been found that apparently have adapted to an every-year rotation. These biotypes remain in the egg stage over the following season and hatch the next season when corn is again in the field. Rotation remains an effective management tool for rootworms in North Carolina.
In non-rotated corn, rootworm eggs hatch in the mid-May to early June period and larvae move to corn roots and feed. Rootworms eat root hairs, the softer root tissue, and may tunnel into the roots as the insects become large. Feeding lasts for about three weeks and if rootworms are abundant, most of the plant roots can be eaten or damaged. Corn plants with root damage are easily drought stressed and are often blown over; lodged plants will grow back upright in a curved manner, a condition called "goose necking". Reduction of grain and silage yield can be substantial. Larvae pupate around the base of corn plants and adult beetles emerge in late June and July. The beetles feed on corn and are attracted to the pollen and silks. Western corn rootworm beetles will also feed on the soft epidermal tissue of leaves, especially tender leaves in the plant whorl, and can cause extensive leaf streaking. However, this leaf streaking is seldom of economic concern.
Damage to Corn. Corn rootworms can reduce yield in two ways. If adult populations are very high, and beetles are present at silking, feeding on the silks can interfere with pollination and ears may have missing grain. However, this is not the most common type of injury. Root feeding by larvae is the most damaging. Corn plants can tolerate some root feeding but plant performance is reduced as root damage becomes more intense. Plants are also more prone to both stress and lodging when populations of larvae are high. Grain yield and silage tonnage can be substantially reduced. Often rootworms have a greater economic impact in silage corn. A system for evaluating root injury has been develop at Iowa State University and the categories are shown in Table 1. A low root rating, usually a 3 or less, signifies very low impact by rootworm larvae. Ratings from 4 to 6 indicate that yield loss has occurred and management efforts are (were) warranted. Farmers can evaluate the impact of rootworms by digging and washing corn roots in late season (e.g. in late July).
Table 1. The Iowa corn root rating scale for corn root worm injury
Management. Rotation should be the first line of defense, however, many Piedmont farmers and dairymen may not be able to rotate all of their corn crop. Where rootworms are an economic problem, the remaining management option is the use of at-planting or lay-by soil insecticide. In most areas of the country a single application at planting will provide fair to good control of rootworms. Consult a current edition of the North Carolina Agricultural Chemicals Manual for rootworm insecticide recommendations.Using soil insecticide should be based upon some knowledge that corn rootworms are a problem in the area where corn is to be planted in the current year. This data would be gathered in the previous season and consist of: 1. root ratings, 2. surveys of beetle populations on the silks, 3. extensive lodging, and 4. yield information. If these data indicated the likelihood of high beetle populations then treatment may be in order. The information below gives a good indication of insecticide effectiveness and expected results. It was borrowed from the Iowa State University Cooperative Extension Service publications " Performance of Corn Rootworm Insecticides" Table 1 and "Corn Rootworm Insecticide Consistency", Table 2. Both publications were written by Dr. Marlin Rice and are used here with the author's permission. The articles can be retrieved from the internet at:
Data in Table 2 show that all products were applied at planting-time and gave a root rating of less than three (protected from damage). However, several products were statistically more consistent than others and those with the highest consistency scores generally produced lower (better) root rating scores. Thimet 20 G and Fortress 2.5 G, applied into the seed furrow at planting, showed poorer consistency ratings. Thimet (or the generic product Phorate) is not registered for use in the seed furrow.
Table 2. Five year summary of corn rootworm insecticide performance (1992-1996, Iowa State University 1.
1. 13 tests, 58 replicates;
The Iowa State University data presented in Table 3 show little yield advantage from using an insecticide when rootworm pressure is light (Untreated check root rating between 3 and 4). In 51 observations over 8 years where the check averaged a 3.3 root rating, treating provided an average yield increase of 4.6 bu/acre. Over the same time period, in 64 observations with moderate to heavy corn rootworm pressure, there was a 17 bu/acre yield advantage (Table 3). Also, the frequency of plant lodging can be related to the two corn rootworm levels. Over 10 years where pressure was light, corn lodging occurred 7 percent of the time in the untreated check. In moderate to heavy pressure tests, lodging occurred 57 percent of the time.
Table 3. Impact of two levels of corn rootworm larval injury on yield and lodging.
1 Check root ratings and yields based on 64 observations over 8 years; treatment data based on 384 observations (6 insecticides x 64 replications). Lodging based on 14 observations over 10 years.
2 Check root ratings and yield based on 51 observations over 8 years; treatment data based on 306 observations (6 insecticides x 51 replications). Lodging based on 14 observations over 10 years.
Corn and Soybean Insect Note (ENT/cs-03)
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Date Created 3/25/99.
Published by North Carolina Cooperative Extension Service
Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University at Raleigh, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.
CAUTION: The information and recommendations in these Notes were developed for North Carolina conditions and may not apply elsewhere.