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Crop and Soil Environmental News, March 2001
Multi-Year Survey of European Corn Borer Damage in non-Bt Cornfields of Eastern Virginia
The Virginia Corn Board provided support from 1997-1999 to survey European corn borer damage in non-Bt cornfields in eastern Virginia. The main goal of this project was to determine the damage potential of European corn borer in non-Bt corn in order to better understand the role that Bt corn hybrids might play in eastern Virginia.
Briefly, the European corn borer (ECB) is a moth with a large host range of over 300 plant species. It has two primary generations per year on field corn in Virginia, and overwinters as a fifth instar larva inside pieces of corn stalks, cobs, and stems of other host plants. The main damage to corn is caused by larvae which tunnel into the stalk about two weeks before silking, or bore into the ear shank later in the season. Cornfields which average one or more tunnels per stalk of over one inch in length can expect a grain yield reduction of about 5%. Research conducted by Dr. Dennis Calvin in Pennsylvania has indicated that one ECB larva per stalk at about the mid- to late-vegetative stage of growth will reduce grain weight about 6%. Although it is relatively easy to scout for ECB during the growing season, it has been our experience that few growers in Virginia do. The main reason given by growers for not scouting is based on years of observations of low stalk breakage and ear drop at harvest. In an effort to address the pest potential of ECB in eastern Virginia, we began a regional survey of ECB damage in selected counties of the state.
1997-1999 Survey Methods
In late August 1997, 1998, and 1999, we surveyed 54, 58, and 60 non-Bt cornfields in eastern Virginia, respectively, for damage caused by second generation ECB larvae. The counties surveyed in the northern neck included Westmoreland, Northumberland, Richmond, and Lancaster; the counties surveyed in the middle and lower peninsulas included Essex, Middlesex, King and Queen, Charles City, James City, and New Kent; and the counties surveyed south of the James River included Prince George, Surry, Sussex, Isle of Wight, Southampton, and Suffolk. The sampling procedure used to evaluate corn borer damage consisted of thoroughly inspecting five sets of 20 consecutive stalks per field for a total sample size of 100 stalks. Plants were destructively sampled if evidence of feeding or tunneling activity was detected so that detailed records on the location and extent of the damage could be made. The total number of plants surveyed in 1997, 1998, and 1999 was 5,400, 5,800, and 6,000, respectively.
European corn borer pressure was generally very light in 1997. Damage ranged from a low of 0 tunnels per stalk in Essex and Middlesex Co.'s, to a high of 0.29 tunnels per stalk in Isle of Wight Co. (Fig. 1). Despite the low pest pressure, however, we did detect a north-south gradient of increasing corn borer damage across the survey area. The percentage of stalks with corn borer tunnels in the northern, central, and southern counties averaged 0.6%, 3.6%, and 13.2%, respectively. Although we cannot be certain why this trend occurred, it may be related to differences in planting dates among the three regions, which in turn led to different stages of crop susceptibility during the time when ECB egg-laying activity was at its peak. Nevertheless, when averaged across the 54 fields, only 5.2% of the plants exhibited any kind of tunneling activity at all. This includes all tunnels ranging in length from less than 0.5 inch to more than 3 inches. Overall, none of the cornfields surveyed in 1997 exceeded the economic threshold (i.e., none averaged one or more tunnels per stalk of over one inch in length) for ECB damage. These results indicate that corn growers in eastern Virginia would not have benefited from planting Bt hybrids in 1997.
Figure 1. 1997 survey of European corn borer damage in 54 non-Bt cornfields of eastern Virginia. Numbers represent county averages of corn borer tunnels per stalk.
Unlike our 1997 survey results which indicated low levels of ECB damage, our 1998 results indicated relatively higher damage levels across all 15 counties. Damage ranged from a low of 0.19 tunnels per stalk in Northumberland Co., to a high of 1.2 tunnels per stalk in Isle of Wight Co. (Fig. 2). The percentage of stalks with corn borer tunnels in the northern, central, and southern counties were similar, averaging 53.2%, 60.4%, and 53.5%, respectively. When averaged across the 58 fields, 54.7% of the plants exhibited some level of tunneling activity, but only 15.5% of the fields (9 of 58) were above threshold for ECB damage. These results again indicate that the majority of corn growers in eastern Virginia would not have benefited from planting a Bt hybrid in 1998.
Figure 2. 1998 survey of European corn borer damage in 58 non-Bt cornfields of eastern Virginia. Numbers represent county averages of corn borer tunnels per stalk.
Our 1999 survey results indicated low levels of ECB damage across all 15 counties in eastern Virginia. Damage levels ranged from a low of 0.04 tunnels per stalk in Northumberland Co. to a high of 0.84 tunnels per stalk in Suffolk (Fig. 3). The percentage of stalks with corn borer tunnels in the northern, central, and southern counties were similar, averaging 23.9%, 25.8%, and 30.5%, respectively. When averaged across the 60 fields, 26.6% of the corn plants had tunnels ranging in size from less than 0.5 inch to more than 3 inches. However, only 5% of those fields (3 of 60) exceeded the economic threshold for ECB damage. For the third year in a row, the results indicate that most corn growers in eastern Virginia would not have benefited from planting a Bt hybrid in 1999.
Figure 3. 1999 survey of European corn borer damage in 60 non-Bt cornfields of eastern Virginia. Numbers represent county averages of corn borer tunnels per stalk.
Of the 172 cornfields surveyed from 1997-99, only 12 (7%) exceeded the economic threshold for ECB damage (i.e., one or more tunnels per stalk of over one inch in length). The fact that so few cornfields experienced economic damage in our survey raises the question of the importance of ECB as a pest of corn in eastern Virginia. One explanation for the low ECB pressure across our survey years may be the relationship between time of planting and peak egg-laying activity of ECB moths. In general, most of the surveyed fields were planted from early- to mid-April. The planting dates for the 1999 survey fields ranged from Mar. 29 to May 1, with 75% of the fields planted by Apr. 14. There appears to be a trend for greater ECB damage in late planted cornfields, indicating a closer synchrony between increased activity of ECB egg-laying moths and their preference for younger corn plants. We know from doublecrop corn hybrid studies conducted in eastern Virginia during the same period as our surveys, that late-planted corn (i.e., corn planted after the first week in June) is at severe risk to ECB damage. Overall, our survey results indicate that about 90% of the cornfields planted in April in eastern Virginia will not benefit from a Bt corn hybrid because of low ECB pest pressure. Nevertheless, because it is not possible to predict the severity of ECB pest pressure at the time of planting, we recommend that growers consider planting about 20% of their corn acres to Bt hybrids to begin gaining experience with those Bt hybrids reported to be the most promising for their area. Also, because the risk of ECB damage is much greater in late-planted corn, we strongly recommend growers plant a Bt corn hybrid anywhere late-planted corn is grown in eastern Virginia.
Virginia Cooperative Extension