Final Report for GS05-050
Project Information
European corn borer infestation level had a significant negative effect on whole-plant yield (P = 0.0086), whereas the main effect of plant growth stage as well as the interaction between plant growth stage and infestation level had no significant effect on whole-plant yield. Of the five infestation levels, only the infestation level of 5 larvae per plant resulted in a significantly lower whole-plant yield (282.3 ± 10.8 g/plant) than the uninfested control (315.3 ± 7.5 g/plant). Economic injury levels are presented for each of the growth stages, where significant regressions were found between whole-plant yield and infestation level. In addition, plant growth stage and infestation level had no effect on percent acid detergent fiber, percent neutral detergent fiber, and percent crude protein values.
Introduction
Field corn is one of the most important crops grown for grain and silage worldwide (Youngman and Tiwari 2004). The U.S. accounts for nearly one-fifth of annual worldwide corn production and ranks as the number one corn-growing nation in the world (Youngman and Tiwari 2004). In 2001, Virginia corn farmers harvested 133,550 hectares of grain and 54,630 hectares of silage, which yielded over 40.5 million bushels of grain and nearly 2.1 million tons of silage.
European corn borer has posed a potential risk to corn farmers in the Mid-Atlantic and southeastern U.S. for decades. However, most farmers have elected not to aggressively control this pest because of years of infrequent and sporadic infestations, and the cost and effort associated with traditionally managing this insect. Biological control has been investigated for managing European corn borer on field corn, but it has yet to prove economically successful in North America (Youngman and Tiwari 2004).
Essentially, the traditional way of managing second generation European corn borer infestations has consisted of aerially-applying granular insecticides on cornfields identified with above threshold eggmass counts. Although the traditional method has been proven to be cost effective on corn grown for grain, it is not considered an economically viable option in many areas where corn is grown for silage (Thompson and White 1977, Myers and Wedberg 1999). Now, the simple convenience of planting Bt corn hybrids that impede European corn borer feeding injury has dramatically changed the pest management scenario for this insect. Although planting Bt corn seed is straightforward, the main drawback is that it is not possible to predict at the time of planting whether a field will sustain enough European corn borer pressure to benefit from the additional cost of $17-25 per ha for the Bt corn seed (Hyde et al. 1999).
Surveys conducted by Youngman et al. (1998, 1999, 2000), and Youngman and Laub (2002) on second generation European corn borer damage in conventional (i.e., non-Bt) cornfields in eastern (1997-99) and western Virginia (2000-02) have provided partial insight on the role for Bt corn in Virginia. Less than 2 percent of the 172 fields surveyed annually in eastern Virginia from 1997-1999 experienced economic damage from European corn borer. The western Virginia survey findings, however, revealed a different picture (Youngman and Laub 2002). Nearly 25 percent of the 126 fields surveyed annually from 2000-2002 experienced economic damage. For the fields in these surveys, economic damage was based on a threshold of 1 or more tunnels of > 1.3 cm in length per plant. It is important to realize, however, that this economic threshold was developed on corn grown for grain; not corn grown for silage. The significance of this is that this threshold can be applied with confidence to the eastern survey results because the vast majority (> 90 percent) of corn grown in eastern Virginia is grown for grain. In contrast, the majority of corn grown in the western half of the state is largely grown for silage and kept on-farm as feed for dairy animals. Therefore, it was important that research be done to determine the level at which European corn borer causes economic damage in corn grown for silage. This study has improved our understanding on the appropriate role for Bt corn hybrids in managing European corn borer populations in fields planted for silage production.
Objective 1. Determine effects of European corn borer infestation on whole-plant yield and forage quality of corn grown for silage
Objective 2. Determine economic injury levels of European corn borer infestations at the 10-leaf, 16-leaf, and blister stages of corn grown for silage
Cooperators
Research
Objective 1. Determine effects of European corn borer infestation on whole-plant yield and forage quality of corn grown for silage
A controlled experiment consisting of Pioneer 34B23 (non-Bt hybrid) and Pioneer 34B23 (Bt corn hybrid) (Pioneer Hi-Bred International, Johnston, Iowa) was conducted in spring 2005 at the Virginia Tech Kentland Research Farm in Montgomery Co. (800 25’ W, 370 14’ N; elevation ≈ 640 m), Virginia. The corn was planted on a 76.2-cm (30-inch)row spacing and at a plant population of 64,493 seeds/ha (26,100 seeds/ac) on May 9, 2005. The seeds were treated with Kernel Guard® hopper box treatment at the rate of 56.7 g per bushel of corn seed. In addition, Force ® 3G granular insecticide was applied at planting at the rate of 113.4 g per 1000 row feet to eliminate complications from soil pests such as western corn rootworm, white grubs, wireworms, and seedcorn maggot. The experimental design consisted of a split-plot randomized complete block design, with blocks being replicated eight times. Each block consisted of three main plots with each main plot representing one of the following plant growth stages: 10-leaf, 16-leaf, and blister. Each main plot further consisted of 7 subplots with each subplot being 4 rows wide and 7.6 m (25 ft) long. Each subplot consisted of one of the following levels of ECB infestations: 0 (control); 1; 2; 3; 4 and 5 third instar larvae per plant on the 34B23 non-Bt corn. The Bt corn hybrid, Pioneer 34B24 (7th subplot) was planted at the same time as the non-Bt hybrid. The Bt corn hybrid served as a backup control against the possibility of heavy European corn borer infestation in the non-Bt control. Sixteen plants from the middle two rows of each subplot were used for each infestation level. The total number of plants artificially infested per subplot experiment-wide was 384 (16 per subplot x 3 main plots x 8 blocks). Plants used for artificial infestation were selected on the basis of uniformity in size and spacing. Any plant found to be naturally infested with European corn borer was not used for artificial infestation. Of the 384 plants in the non-Bt control subplots experiment-wide, 105 plants (25%) were found to be naturally infested.
Laboratory-reared third instars were purchased from French Agricultural Research, Inc. (Lamberton, Minnesota). The intended number of larvae per plant was introduced at the rate of one larva per internode, using a modified wire-nut technique (Youngman et al. 2007 unpublished data). Artificial infestation using the wire-nut technique involved the WingGard™ (Gardner Bender, Milwaukee, Wisconsin) plastic wire connector (hereafter referred to as wire-nut) (size 10-086). The wire-nut is shaped like a capsule, with an open end (1.1 cm inside diameter) that is slightly wider than the closed end (0.8 cm diameter). The wire-nut measures 3.0 cm in length and has two side flanges on the open end. In order to fasten the wire-nut to a cornstalk, an approximate 0.25-cm notch was cut into each flange using a Dremel® rotary drilling tool (Dremel, Robert Bosch Tool Corporation, Racine, Wisconsin). In addition, the flat circular open end of the wire-nut was slightly curved with a 1.90 cm diameter drum sander tool (Ali GatorGrit Inc., Fairborn, Ohio) to better conform to the natural curvature of the cornstalk. This was necessary to minimize larvae from escaping at the point of attachment on the stalk. Prior to infestation and transportation to the trial site, larvae were placed individually into the wire-nuts and capped with corks. The capped wire-nuts were then placed in plastic ice coolers on top of a cardboard barrier above a layer of cubed ice. The wire-nut was secured onto a cornstalk with size 10 or 12 rubber bands (Alliance Advantage, Hotsprings, Arizona). In general, size 10 rubber bands were used for 10-leaf stage plants, and size 12 rubber bands were used for 16-leaf and blister stage plants.
For 10-leaf stage plants, all larvae were introduced into the internodes below the whorl at the rate of 1 larva per internode. For the 16-leaf and blister stages, half the number of total larvae were placed on the internodes above the primary ear with the remaining half placed on the internodes below the primary ear, at a rate of 1 larva per internode. At the infestation level of 1 larva per plant on 10-leaf stage plants, the larva was placed on the internode just below the whorl. Likewise, for the 16-leaf and blister stages at the infestation level of 1 larva per plant, the larva was placed on the internode just above the primary ear. All infested plants were marked with colored vinyl tape (non-adhesive) tied loosely around the base of the plant. After 24 hours, all wire-nuts with dead or missing larvae were replaced with a fresh larva.
Harvest
Infested plants were cut manually approximately 5 cm above the soil surface at the half kernel milkline stage. Each plant was split longitudinally from the tassel to the base to record the number of tunnels per plant and length of each tunnel. Given that some plants may be naturally infested by European corn borer, plants found with tunnels in internodes other than the area of infestation were not used.
Percent dry matter and forage quality was done by arbitrarily selecting four complete corn plants from within each infestation level subplot and then chopping them into approximately 2.5 cm pieces using a Tomahawk® Chipper/Shredder (Troy-Bilt Manufacturing Co., Troy, NY). A 1 kg subsample of the chopped material was placed in a cotton harvest bag (50 x 100 cm) and then dried in a forced air dryer at 700C (1580F) at a Virginia Tech forage drying facility, Blacksburg, VA. After the plant material was completely dry, it was removed from the dryer and weighed. Dry matter values were then used to determine the percent moisture content of the subsamples at harvest. All yields were adjusted to 100% dry matter. In addition, a 20 g dried portion of each subsample was taken to the Virginia Tech Forage Analysis Laboratory, Blackstone, VA, for forage quality analysis. Near infrared reflectance spectroscopy analysis procedures were performed to determine the percent acid detergent fiber, neutral detergent fiber, and crude protein in each sample.
Statistics
Two-way analysis of variance (ANOVA) was used to analyze for differences in whole-plant yield with growth stage and infestation level as main effects (SAS Institute 2001). When significant main effect interactions were found, separate ANOVAs, and Fisher’s Protected LSD mean separation tests were performed to inspect for whole-plant yield differences among infestation levels within a growth stage (SAS Institute 2001). Unless specified otherwise, a P ≤ 0.05 was used for assigning significance. The general linear model procedure (SAS Institute 2001) was used for performing ANOVA and Fisher’s Protected LSD mean separation tests. Linear regression models for each plant growth stage were generated by regressing whole-plant yield against infestation level. The general linear model procedure (SAS Institute 2001) was used for all regression analyses.
Objective 2. Determine economic injury levels of European corn borer infestations at the 10-leaf, 16-leaf, and blister stages of corn grown for silage
The economic injury level (EIL) for each plant growth stage was determined based on regression analysis using varying crop values and control costs. Economic injury levels were calculated based on the following formula of Bode and Calvin (1990):
NL = TC/(CV x PL x PC)
Where NL is the number of larvae per plant; TC is the total control cost of planting Bt corn seed (dollars per hectare); CV = MV x EY, where CV is the crop value, MV is the expected market value (dollars per kilogram); EY is the expected yield (kilograms per hectare); PL is the expected average proportional yield reduction per larva per plant (PL values were calculated from regression equations developed for each growth stage); PC is the expected proportional control by planting Bt corn seeds. A PC of 1 signifies 100% control. When PC is less than 1, the EIL can be obtained by dividing the EIL for 100% control by the percent control achieved from planting Bt corn. It should be noted that the Bt hybrid (event MON810) used in this study provides 96% control of European corn borer (Ostlie et al. 1997).
Although two controls (Pioneer 34B23 [non-Bt hybrid] and Pioneer 34B24 [Bt hybrid]) were used in this study, for purposes of analysis only the non-Bt control was used. Infestation level had a significant negative effect on whole-plant yield (F = 3.26; df = 5, 119; P = 0.0086), whereas the main effect of plant growth stage (F = 1.56; df = 2, 119; P = 0.2148) as well as the interaction between plant growth stage and infestation level (F = 1.39; df = 10, 119; P = 0.1932) had no significant effect on whole-plant yield. The infestation levels for 0, 1, and 3 tunnels per plant had significantly greater whole-plant yield than the infestation level of 5 tunnels per plant. Although there was no significant effect of growth stage on whole-plant yield, mean separation tests were performed for each growth stage separately to determine the infestation level effect on whole-plant yield within each growth stage. The experimental unit used in all analysis was the mean whole-plant yield of the number of plants per subplot with the requisite number of tunnels. Therefore, at the most, twenty-four yield means per infestation level were available for the overall analysis, and eight yield means per infestation level were available for the growth stage analyses.
Based on the results of our 2005 study, it can be concluded that significant losses in whole-plant yield occurred only after 4 tunnels per plant. This differed from the results of a study (Thompson and White 1977) conducted in Prince Edward Island, Canada, where whole-plant yields were not affected by level of European corn borer infestation. Thompson and White (1977) studied the effect of different insecticides on the European corn borer population and subsequent effects on whole-plant and grain yields. They found that although insecticide treatments significantly reduced European corn borer levels, whole-plant yields were not significantly affected. They did find, however, increased grain yields in some insecticide treatments (carbofuran, ethyl parathion, carbaryl, and diazinon).
In our study, growth stages at which European corn borer tunneling was initiated were not found to have a significant effect on whole-plant yield. However, results of a Wisconsin study by Myers and Wedberg (1999) showed that the artificial infestation of first generation European corn borer resulted in greater whole-plant yield loss than from second generation European corn borer infestations.
In our study, regression analysis revealed significant relationships between whole-plant yield and infestation level for the 16-leaf (P = 0.0169) and blister growth stages (P = 0.0008), but not for the 10-leaf stage (P = 0.8223). The r-square values for 16-leaf and blister stages were 0.12 and 0.22, respectively. These low r-square values are likely due to the lack of slope between the infestation level and whole-plant yield in the regression analyses. Further regression analyses revealed no significant quadratic or cubic component involved in either relationship.
Separate linear regression equations (Table 2) were used to calculate mean percent yield reduction per larva per plant, for each growth stage. The mean percent yield reduction per larva per plant was 0.32, 2.35, and 3.59 at the 10-leaf, 16-leaf, and blister stages, respectively. The mean percent yield reduction per larva per plant was used to calculate EILs for each of the growth stages at varying control costs and crop value levels. Only growth stages with a significant regression relationship between whole-plant yield and infestation level were used for calculating EILs (Table 3).
Economic injury levels (number of ECB larvae per plant) were calculated using varying control costs (cost of planting Bt hybrids per hectare) and crop values (dollars per hectare). The EILs were calculated assuming 100% control achieved by planting Bt hybrids; however, the EIL for 100% control can be adjusted for lower levels of protection. For example, the EIL for a Bt corn hybrid that provides 96% control, can be obtained by dividing the EIL value for 100% control by 0.96 (Table 3). The EIL decreases as crop value increases, but increases as control costs increase. The EILs calculated in our study for corn whole-plant yields were higher than those calculated by Bode and Calvin (1990) for corn grain yields. At a control cost of $20 per hectare and crop value of $250 per hectare, Bode and Calvin (1990) calculated an EIL of 1.35 larvae per plant. Assuming the same control cost and crop value for corn silage, the EIL for our study is 3.40 larvae per plant. This suggests that corn grown for silage can withstand higher numbers of larvae per plant than corn grown for grain.
Forage analysis indicated no significant effect of growth stage, infestation level, and the interaction between growth stage and infestation level on percent ADF, percent NDF, and percent CP values. This suggests that the quality of the forage remains largely unchanged even at an infestation level of 5 larvae per plant. Similar results were found in a study conducted by Myers and Wedberg (1999), where ECB infestation had no effect on silage quality when measured in terms of milk per ton. A study on the effect of western corn rootworm, Diabrotica virgifera vergifera, on corn silage quality also revealed no significant losses in silage quality in terms of digestibility factors (Davis 1994).
Table 1. ANOVA table for average whole-plant yield at 3 growth stages and different levels of European corn borer.
Source df F Value P > F
Block 7 0.97 0.4545
Growth stages 2 1.52 0.2233
ECB levels 5 3.15 0.0105
Growth stages x ECB levels 10 1.42 0.1812
Table 2. Linear regression equations relating whole-plant yield (dry matter in g) to different levels of European corn borer infestations, at 3 growth stages.
Plant growth stage Regression equation r2 P > F
10-leaf Y = 313.3 – 1.0X 0.0011 0.8223
16-leaf Y = 327.7 – 7.7X 0.1179 0.0169
Blister Y = 326.0 – 11.7X 0.2179 0.0008
Table 3. Economic injury level values for European corn borer larval populations at various control costs and crop values on whole-plant yield (PC = 1, assuming 100% control).
Crop Value ($/ha) Control cost ($/ha)
20 25 30 35 40 45 50
16-leaf stage
100 8.51 10.64 12.77 14.90 17.02 19.15 21.28
150 5.67 7.09 8.51 9.93 11.35 12.77 14.18
200 4.26 5.32 6.38 7.45 8.51 9.57 10.64
250 3.40 4.26 5.11 5.96 6.81 7.66 8.51
300 2.84 3.55 4.26 4.96 5.67 6.38 7.09
Blister stage
100 5.57 6.96 8.36 9.75 11.14 12.53 13.93
150 3.71 4.64 5.57 6.50 7.43 8.36 9.29
200 2.79 3.48 4.18 4.87 5.57 6.27 6.96
250 2.23 2.79 3.34 3.90 4.46 5.01 5.57
300 1.86 2.32 2.79 3.25 3.71 4.18 4.64
Educational & Outreach Activities
Participation Summary:
Results of this study have been presented at the following National and Branch meetings of Entomological Society of America.
1. Tiwari, S., R. R. Youngman, C. A. Laub, and T. A. Jordan. 2006. Effect of European corn borer, Ostrinia nubilalis (Hübner), on corn whole-plant yield and forage quality. ESA Annual Meeting, December 11, Indianapolis, IN.
2. Tiwari, S., R. R. Youngman, C. A. Laub, and T. A. Jordan. 2006. Effect of European corn borer on corn whole-plant yield and forage quality. In 39th Northeast Regional Field Crops Insects Conference. Eastern Branch ESA Meeting, March 13, Charlottesville, VA.
Project Outcomes
Our overall 2005 results indicate that a significant reduction in whole-plant yield occurs after 4 European corn borer larvae per plant. The importance of this finding for Virginia corn growers is that this level of infestation rarely occurs in Virginia (Youngman, unpublished data). Another important outcome of this research concerns the non-significant effect of European corn borer infestation level on forage quality.
Economic Analysis
From an economic point of view, a major outcome of this study is that Virginia growers who grow corn for silage should strongly consider planting non-Bt corn hybrids given that infestation levels of more than 4 European corn borer larvae per plant are rarely found in Virginia (more than 4 ECB larvae per plant). Planting non-Bt hybrids can cut down production costs by $17-25 per ha.
Farmer Adoption
We anticipate that the results of the present finding will be adopted by the corn growers in Virginia, after the results have been published and made available to the corn growers.