Sid Bosworth, Associate Extension Professor,
Plant & Soil Science Department, University of Vermont firstname.lastname@example.org
The Northern Corn Rootworm (Diabrotica barberi) and Western Corn Rootworm (Diabrotica virgifera) are considered the most serious insect pests found in field corn in Vermont. The Northern (NCRW) was first detected in Vermont back in the early 1970's and the Western corn rootworm (WCRW) was first detected in Vermont in 1991. Both species have been found in almost every county in the state since first detected. Although both insects can cause damage to corn, the Western is considered far more aggressive than the Northern corn rootworm and is, therefore, a greater threat.
Damage first occurs by root-feeding larvae. Pruning of roots causes plants to be more susceptible to dry conditions and reduces water and nutrient uptake. Visual damage is sometimes observed when plants lodge due to a weak root system. However, research in New York found that corn silage yields can be reduced even when there are no above ground visual signs of damage.
One reaction by corn growers to the recent concerns of WCRW has been an increased use of insecticides applied at planting. According to Vermont Department of Agriculture pesticide use records, about 5 percent of the corn acreage was treated with an insecticide for this purpose in 2000. Yet, there has been very little quantitative data on actual WCRW populations and impact in Vermont.
Over the past four years, the University of Vermont has been involved in a project to evaluate the presence and impact of the Western and Northern Corn Rootworm on field corn grown in Vermont. The objectives of the project were as follows:
1. Assess the population levels of Western and Northern corn rootworm adult beetles;
2. Determine the acreage of field corn that is above economic threshold for corn rootworm;
3. Compare two scouting methods for assessing CRW;
4. Provide information about IPM for corn rootworm to producers; and
5. Assess attitudes of Vermont farmers concerning IPM methods for managing CRW.
Results of the Project
Insect Population and Threshold
Levels (Objectives 1- 2)
In order to assess population levels and determine how many fields were above economic threshold, data was collected over a four-year period (2000 to 2004) in six counties (Addison, Caledonia, Chittenden, Franklin, Orange, and Windsor) by various independent and dealer crop consultants. All of these fields had been in corn for two or more years prior to evaluation.
What we found – Of the 128 fields monitored, approximately 20% were at or above economic threshold (a population level that would warrant a pest management decision). Most of the fields were far below threshold. In addition, this low percentage seemed to be consistent every year across the various parts of Vermont in which we monitored except in Franklin County where there has been a higher percentage of fields above economic threshold.
Most of the adult beetles were the Northern type. Less than 20% of the adults monitored were Western. This is good news since the Western species is more aggressive and usually causes greater injury than the Northern species.
Scouting Methods (Objective
In 2000 and 2001, two different scouting methods were evaluated: 1) the standard “visual” method as recommended by Cornell University Entomologists and 2) a sticky trap method. The standard method (Sutton and Ireland, 2001) relies on visual assessment during a four week period from corn tasseling to silk drying. Scouting begins at silking because this is time when adult beetles emerge as pupae from the soil). The scouting protical normally recommends that a field be monitored approximately once per week for three or four more weeks (until either and economic threshold is reached or, if below threshold, when the population peaks and starts to decline). A common visual method is to evaluate up to 55 plants throughout randomly choosen in the field and calculate an average number of beetles per plant to determine threshold. We used the threshold recommended by Cornell University of 1 WCR or 2 NCR per plant. For large uniform fields, a modified method called “Sequential Sampling” can be used which reduces time needed in the field.
The second method we evaluated was a sticky trap method (Kuhar et.al., 1998). We were interested in this method to see if it would be more rapid and accurate as compared to the visual method. In addition, since the beetles stick to the trap, we assumed it would be easier than the visual method in assesing and counting adult beetles (with the visual method, one has to “sneak” up to the plants or else the beetles drop to the ground as soon as they feel any unusual vibrations). Yellow sticky traps were placed throughout each field just before silking and beetles are counted once per week for three or four weeks. A typical 10 to 15 acre field would have 8 to 10 traps placed just above the ear. The threshold for this method was based on 20 western corn rootworm equivalence per trap per week.
What we found – We thought that once the traps were up, it would be quicker to move through the field counting beetles compared to the standard visual method. However, in many of the fields, traps had to be replaced after a week or two due to intense rainstorms which washed away the sticky substance. Also, many non-rootworm insects stuck to and cluttered the traps making it difficult to accurately count corn rootworm beetles. As a result, total scouting time was about twice as much using the sticky trap method as compared to the Cornell Sequential Sampling method. Our conclusion was that, although the sticky trap method required less skill on the part of the scout, the increased costs due to both the purchase of the traps and the additional time to properly used them was greater than compared to the visual method.
Outreach Education (Objectives
4 and 5)
During the four year period, presentations about these pests and management strategies were made at winter pest management meetings, Certified Crop Advisor trainings and summer field days. During winter IPM meetings held throughout Vermont in February 2004, we assessed farmer awareness of these insects as well as their management practices. Generally, farmers were asked to answer a series of questions as part of a presentation concerning corn rootworm and other corn pests.
What we found – About 48% of the farmers responding
to the survey indicated that they “scouted” their cornfields for either
Northern and/or Western Corn Rootworm the previous year (representing approximately
27% of the corn acreage). Only 11% of the farmer respondents indicated
that they had applied an insecticide the year before on some of their acreage
(5% of the corn acreage represented in the survey received an insecticide
for treating corn rootworm in 2003). Of the farmers that applied
an insecticide, 57% based their decision on past symptoms (ie., goosenecking)
in those fields and 43% also based their decision on scouting information
of the previous year. Eighty seven percent if the farmers surveyed
stated that they would be scouting for goosenecking symptoms and seventy
six percent would be scouting for adult beetles during the upcoming season.
|We know that one of the most effective and economical methods of controlling corn rootworms is crop rotation. These insects only have corn as a host and they lay their eggs at the base of the plant; therefore, planting another crop the next year breaks the cycle dramatically. Based on our 2004 survey, of the total acreage of field corn, about 18% is rotated each year. If extrapolated to the approximately 95,000 acres grown in Vermont, there are 17,100 acres rotated each year helping to break the CRW life cycle.|
This project has been sponsored by a grant from the USDA Extension IPM program. Appreciation is made to the following individuals who collected rootworm data for this project: Sarah Cushing, Paul Stanley, Sue Hawkins, Abby Willard and Carl Majewski.
Kuhar, Thomas P., Roger R. Youngman and Curt Laub. Entomology Fact Sheet - Western Corn Rootworm. Publication Number 444-266, Virginia Tech and Cooperative Extension. (http://www.ext.vt.edu/pubs/entomology/444-266/444-266.html)
Sutton, Philip and
Katie Ireland. 2001. Corn Rootworm Management in Field Corn.
New York State Integrated Pest Management Program, Cornell Cooperative
This site is maintained by Sid.Bosworth@uvm.edu, Plant & Soil Science Department, University of Vermont.
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