Agronomy Journal 93:1043-1048 (2001)
© 2001 American Society of Agronomy
CORN
Responses of High-Oil and Hybrid Corn to Rootworm Beetles during Pollination
Stephen D. Strachan*,a and
Stuart L. Kaplanb
a Pioneer Hi-Bred Int., 7301 NW 62nd Ave., P.O. Box 85, Johnston, IA 50131
b Pioneer Hi-Bred Int., 7300 NW 62nd Ave, P.O. Box 1004, Johnston, IA 50131
* Corresponding author (steve.strachan{at}pioneer.com)
Received for publication September 11, 2000.
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ABSTRACT
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High-oil corn (Zea mays L.) may be produced as a physical mixture consisting of 91% male-sterile hybrid plants and 9% pollinator plants. Two field studies were conducted to determine if corn blends consisting of only 9% pollinator plants are more sensitive to high populations of rootworms (Diabrotica spp.) feeding on silks during pollination than hybrid blends consisting of 50 to 100% male-fertile plants. In one study, silks of hybrid corn and silks of the male-sterile hybrid within the high-oil corn blend were clipped daily during pollination to simulate silk clipping caused by rootworms. In a separate study, 165 locations of high-oil corn and 134 locations of hybrid corn were monitored for rootworm population and severity of silk clipping during pollination. Grain yield was significantly reduced (P 
0.05) if average lengths of exserted silks were <37 mm for high-oil blends with 9% pollinator plants and <25 mm for hybrid corn. Oil, protein, and starch concentrations of the high-oil grain were not significantly different from those of the untreated check if exserted silks were at least 25 mm during pollination. Severity of beetle infestation correlated poorly with daily average silk length (r = -0.06) and with grain yield (r = -0.01). Economic thresholds for grain yield were crossed, and appropriate pest control measures should be considered when rootworm beetles consistently clip exserted silks to <37 mm for high-oil blends with 9% pollinator plants or <25 mm for hybrid corn.
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INTRODUCTION
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APPROXIMATELY 400000 ha of high-oil corn, corn with oil concentration >7% in harvested grain (dry weight basis), was produced in the USA in 1999. Livestock producers worldwide continue to express interest in high-oil corn because the grain has 
4.5% more energy per kilogram than conventional corn grain (Alexander, 1988). Chickens (Gallus gallus domesticus) fed high-oil corn showed improved weight gain, gain/feed ratio, and skin and plasma pigmentation compared with chickens fed conventional corn (Han et al., 1987). In addition, the magnitude of response was highly correlated with the concentration of oil in the grain. Pigs (Sus scrofa domesticus) ranging in weight from 16 to 97 kg that were fed a properly balanced ration of corn with 7% oil showed 11% faster daily gain and 6% greater gain/feed ratio than pigs fed similarly balanced rations containing corn with 3.5% oil (Adams and Jensen, 1987).
The most common production method for high-oil corn is the TopCross grain production system. In the TopCross system, corn seed is planted as a physical mixture containing 91% high-yielding, male-sterile hybrid seed and 9% pollinator seed (Edge, 1997). The resulting hybrid or male-sterile grain parent plants contain agronomic traits necessary for high grain yields. The resulting pollinator plants produce essentially all of the pollen that fertilizes developing ovules on the grain parent plants. Through the biological process of xenia, the dominant or semidominant genetic traits for oil in the pollen produce grain on the male-sterile grain parent plants with increased concentrations of oil.
This percentage of pollinator plants in the TopCross production system (9%) is substantially less than the 50 to 100% of the plants capable of producing pollen in hybrid corn. In hybrid corn production, as few as five adult rootworms per plant during pollination may be adequate to significantly reduce grain yield (Leva, 1980). For seed corn production, approximately three adult rootworms per plant may be adequate to significantly reduce seed yield (Culy et al., 1992a; Leva, 1980). Under most production systems, fields of seed corn contain substantially fewer pollen-shedding plants per hectare than fields of hybrid corn. Rootworms reduce grain yield by eating pollen to levels insufficient to fertilize all available ovules and by clipping silks to lengths insufficient to intercept pollen. Economic thresholds of rootworm infestation in corn grown with the TopCross production system are not known. Is the TopCross production system more sensitive than the hybrid corn production system to high populations of rootworms or other silk-feeding pests during pollination?
The purpose of this research is to determine the economic threshold for adult rootworms in the TopCross production system and to compare this value with that established for normal hybrid corn production. This research is divided into two parts. First, we shall address minimum silk lengths necessary for proper pollination in both production systems. Then, we shall determine sensitivities of both production systems to various levels of adult rootworm populations during pollination.
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MATERIALS AND METHODS
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Silk-Clipping Study
Three physical mixtures of blended corn seeds were planted in each of three locations in 1997 and 1998. Test locations were near Bluffton, IN; Champaign, IL; and Van Horne, IA. All sites were monitored and managed according to best agronomic practices. Adult rootworm populations were very low (approaching zero beetles per plant) at each of these six test sites. The first of the three corn seed mixtures, labeled as the hybrid, was a popular commercial corn hybrid with a relative maturity of 112 d, LH198 x LH185, consisting of 50% male-sterile and 50% male-fertile seed. The second seed mixture, labeled as the 9% male-fertile hybrid, was LH198 x LH185; it was physically blended with 91% male-sterile and 9% male-fertile seed. Grain yield responses of the hybrid and the 9% male-fertile hybrid help to determine if the number of pollen-shedding plants per hectare affects grain yield under different silk-clipping regimes. The third blend, labeled as TC Blend, was a popular commercial high-oil blend produced with the TopCross grain production system. The TC Blend consisted of the male-sterile grain parent, LH198SDms x LH185, comprising 91% of the seed and the high-oil pollinator, P28.3, comprising 9% of the seed in the physical mixture. Grain yield responses of the hybrid and the TC Blend help to determine if hybrid and TopCross grain production systems respond similarly to silk clipping.
During the entire pollination period, we clipped silks once daily early in the day to simulate silk clipping caused by adult rootworms. The six treatments in this study consisted of untreated plants with no silks clipped and plants with silks clipped once daily to 0, 12, 25, 37, and 50 mm of silk exserted past the tip of the corn husk. Silks of 15 plants for each replication of each treatment were clipped for the hybrid, the 9% male-fertile hybrid, and the TC Blend. There were two replications of each treatment at each of the six locations. Within each corn blend, the locations where silks were clipped were isolated by at least 30 m from other corn blends to reduce contamination from foreign pollen. We hand-harvested mature corn ears; counted the number of kernels per ear; measured 100-kernel weight; estimated total grain yield; and measured oil, protein, and starch concentrations of the resulting grain with near-infrared transmission spectroscopy (Dyer and Feng, 1996).
Adult Rootworm Monitoring Study
Commercial fields of TC Blend and normal hybrid corn in central Illinois were monitored for adult rootworm infestation at or just before silk emergence in 1998 and 1999. Based on crop scouting reports, we selected fields with higher adult rootworm populations and monitored these fields for silk growth and beetle infestation. Approximately 80% of rootworm populations in infested fields were western corn rootworm (D. virgifera virgifera LeConte) while approximately 20% were northern corn rootworm (D. barberi Smith & Lawrence). Individual sample sites were approximately 0.25 ha and were widely dispersed throughout fields ranging in size from 32 to 130 ha. There were 10 to 12 sampling sites within each field. Over the 2-yr period, 13 normal hybrid fields containing 134 test sites and 15 TopCross fields containing 165 test sites were sampled. Each sample site was uniquely located with global positioning technology to better ensure that all measurements and grain samples were collected from appropriate locations. For each of the sampling sites, we recorded the length of exserted silks on 10 corn plants and monitored rootworm populations with Pherocon AM traps (Hein and Tollefson, 1985) every other day during the entire pollination period. We hand-harvested whole-ear samples from physiologically mature corn plants growing in 5.3 m of a single row within each of the specific sites. The resulting grain samples were measured for estimated grain yield and for oil, protein, and starch concentrations with near-infrared transmission spectroscopy (Dyer and Feng, 1996). All data from the small plot silk-clipping study and field rootworm-monitoring study were analyzed for analysis of variance using a Statgraphics software program (Manugistics, 1995). Least significant differences were determined with Fisher's LSD at P = 0.05.
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RESULTS AND DISCUSSION
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Silk-Clipping Study
Protein concentration, kernels ear-1, and estimated grain yield were not significantly different for the untreated check samples (plants with no silks clipped during pollination) collected from the normal hybrid, 9% male-fertile hybrid, and TC Blend (Table 1). Grain harvested from the TC Blend contained approximately 1.9 times more oil and 6% less starch than grain harvested from the normal hybrid and the 9% male-fertile hybrid. The TC Blend produced grain with significantly less 100-kernel weight than the normal hybrid and the 9% male-fertile hybrid. Part of this reduced kernel weight may result because lower-density oil (0.8 g mL-1) replaced higher-density starch (1.4 g mL-1). Total grain yield consists of the total number of kernels produced times individual weights of these kernels. Although not statistically significant, corn plants in the TC Blend produced the greatest average number of kernels ear-1. In this study, grain yields for the normal hybrid, 9% male-fertile hybrid, and the TC Blend are similar. The TC Blend may have at least partially compensated for the lighter kernel weight by producing more kernels ear-1.
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Table 1. Quality traits and physical characteristics of grain harvested from corn plants with no silks clipped during pollination (untreated checks).
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Silk length during pollination significantly affected all measured grain characteristics for each corn blend (Table 2). We observed significant interactions between silk length during pollination and the corn blend for oil, protein, and starch concentrations in harvested grain. These interactions occurred when silk lengths during pollination were clipped daily to 0 and 12 mm. Kernel set on these ears was highly variable across all locations. Under ideal growing conditions, silks can grow as much as 40 mm d-1 (Bassetti and Westgate, 1993). The rate of silk elongation decreases as silks mature and as the amount of available moisture decreases. Available moisture during pollination varied among the six locations. Ears with silks clipped daily to 0 and 12 mm under drier environmental conditions showed poor kernel set while ears subjected to the same silk-clipping regime in nearly ideal soil moisture conditions showed kernel set approaching that of ears with no silks clipped during pollination (Fig. 1). These results imply that grain yield loss resulting from feeding activities of adult rootworms during pollination may be more substantial as available moisture during pollination becomes more limiting.
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Table 2. Analysis of variance (P values) of selected grain traits at harvest resulting from silk clipping at pollination for all locations.
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Fig. 1. Hand-harvested ears of normal hybrid corn and TC Blend high-oil corn from plants growing under nearly ideal or very dry moisture conditions at pollination.
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Concentrations of oil, protein, and starch; 100-kernel weight; kernels ear-1; and grain yield of the normal hybrid and the high-oil TC Blend produced with the TopCross system did not differ from those of respective untreated checks (plants with no silks clipped during pollination) if daily silk length was at least 25 mm during pollination (Fig. 2). The 9% male-fertile hybrid was more sensitive than the commercial hybrid and the TC Blend to silk length during pollination. For the 9% male-fertile hybrid, protein concentration in the grain significantly increased while kernels ear-1 and grain yield significantly decreased if silk lengths were 25 mm during pollination. The remaining three measurements—oil concentration, starch concentration, and 100-kernel weight— were not significantly different from values for the untreated check if silk length was at least 25 mm during pollination. Reduced pollen density for the 9% male-fertile hybrid may explain this increased sensitivity because pollen production in the 9% male-fertile hybrid probably was approximately 18% of the amount of pollen produced in the commercial hybrid. With lower pollen densities, equivalent surface areas of exserted silks are less likely to receive adequate pollen to fertilize ovules, resulting in reduced kernel set and subsequent changes in grain characteristics.
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Fig. 2. Characteristics of grain harvested from corn plants with silks clipped daily during pollination. Within a corn blend for each grain characteristic, bars highlighted with the same letter are not significantly different based on Fischer's LSD (P = 0.05). Silk-clipping treatments highlighted with the letter a are not significantly different from the untreated check (no silks clipped during pollination) based on Fischer's LSD (P = 0.05). Daily silk clippings during pollination did not affect grain moisture content at harvest or grain test weight.
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As silk lengths were clipped daily to 12 mm, almost all grain characteristics changed significantly for all three blends (Fig. 2). The 9% male-fertile hybrid was most sensitive while the commercial hybrid was least sensitive to differences in silk length during pollination. The sensitivity of the TC Blend was similar to but slightly more sensitive than the hybrid. These data imply that the amount of pollen produced in the high-oil TC Blend was similar to or slightly less than the amount of pollen produced in the normal hybrid. In this study, the production method for the 9% male-fertile hybrid (low oil) was consistent with that of the TopCross production method for the high-oil TC Blend. The low-oil, 9% male-fertile hybrid may be more sensitive to silk clipping than the high-oil TC Blend because the larger-tasseled P28.3 pollinator in the TC Blend sheds substantially more pollen per plant than the smaller-tasseled LH198 x LH185 pollinator in the low-oil, 9% male-fertile hybrid blend. These data imply that the combination of the total amount of pollen produced plus the ability of exserted silks to intercept this pollen determines the degree of kernel set; subsequent grain yield; and oil, protein, and starch concentrations in the harvested grain.
The total number of kernels ear-1 and the physiological processes associated with seed development and grain fill may explain changes in oil and protein concentrations of grain harvested from plants with silk lengths 12 mm during pollination. After the ovule is fertilized, the developing seed evolves through two distinct phases until it reaches maturity (Ingle et al., 1965). For approximately the first 3 or 4 wk after fertilization, extensive cell differentiation occurs to form the embryo, endosperm, and other basic seed components. Subsequent seed growth results primarily from deposition of carbohydrates and other nutrients. Approximately 85% of the oil accumulates in the embryo (Glover and Mertz, 1987). The majority of embryo growth occurs during the third through fifth week after fertilization, and maximum embryo size is fixed by 6 wk after pollination (Ingle et al., 1965). The endosperm, a primary storage site for starch, and the aleurone, an additional storage site for protein, continue to grow until the seed is mature. Under most normal environmental conditions, black layer (maturity) occurs approximately 7 wk after fertilization. Plants with very short silks during pollination have fewer kernels on the ear at harvest (Fig. 2). These fewer kernels can increase to a greater size because the total amount of nutrients supplied by the plant is divided among fewer kernels as they develop. The majority of this increase in storage material occurs in the endosperm and the surrounding aleurone.
Adult Rootworm Monitoring Study
Across all test sites, the 134 hybrid sites yielded an average of 10.7 Mg ha-1 while the 165 high-oil TopCross sites yielded an average of 11.1 Mg ha-1. There was no significant difference (P = 0.14, F = 2.24) between grain yields of the normal hybrids and the high-oil blends. We selected fields based only on adult rootworm populations at or just before silk emergence. Corn variety, plant population, tillage, soil fertility, and agronomic practices varied among fields. These data imply that, in general, grain yields produced with either the TopCross or the hybrid method of production are similar for corn lines adapted to central Illinois.
Correlation analysis suggests a strong positive relationship between grain yield and individual ear weight (r = 0.75) with additional weaker, positive relationships between grain yield and plant population (r = 0.43), grain yield and 100-kernel weight (r = 0.45), and grain yield and length of exserted silks during pollination (r = 0.30) (Table 3). We observed no interaction between plant population at harvest and length of exserted silks at pollination on grain yield. The average length of exserted silks during pollination had a small, positive effect on percent oil (r = 0.17); no effect on percent protein (r = 0.02); and a small, negative effect on percent starch (r = -0.13) in the harvested grain. The average oil concentration of the high-oil TC Blend corn lines was 7.4% while the average oil concentration in the hybrid lines was 4.2% on a 0% moisture basis. Highest negative correlation of percent oil was with percent starch in the grain (r = -0.81). This relationship is consistent with the physiology of oil production in corn grain. As more oil is produced, the quantity of starch produced decreases. Higher populations of adult rootworms during pollination had no effect on daily average lengths of exserted silks during pollination and no effect on oil concentration, starch concentration, test weight of the harvested grain, and grain yield. However, higher adult rootworm populations did tend to increase percent protein (r = 0.36) and 100-kernel weight (r = 0.36) of the harvested grain. These data imply that higher populations of adult rootworms during pollination reduced the total number of successful fertilizations of ovules during pollination, resulting in fewer, but larger, kernels that are harvested as grain. In this field-monitoring study, we observed as many as 30 beetles d-1 averaged across all Pherocon AM traps within a location. Traps were observed every other day during the entire 8-d period of pollination. Although adult rootworm populations were very high during pollination in many of the hybrid and TopCross fields, based on beetle counts, corn growth under these environmental conditions seemed to be sufficient to withstand this degree of pest pressure.
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Table 3. Correlation coefficients (r) of characteristics of grain harvested from commercial hybrid and TopCross fields.
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Silk length during pollination affected grain yields of the normal hybrid and the TC Blend (Fig. 3). For hybrid production, grain yield was significantly reduced (P 0.05) if adult rootworms clipped silks to <25 mm during pollination. For high-oil corn produced with the TopCross system, grain yield was significantly reduced if adult rootworms clipped silks to <37 mm during pollination. Test results for the small plot silk-clipping study and the field-monitoring study are similar. Although not statistically significant, the small plot silk-clipping study also showed reduced grain yield for the high-oil TC Blend if silk lengths were between 25 and 37 mm during pollination (Fig. 2). Maximum grain yields for the high-oil TC Blends and the normal hybrids were not significantly different in this field-monitoring study. For all sites with average silk length of 
37 mm during pollination, the normal hybrid produced an average yield of 10.9 Mg ha-1 (114 sites) while the high-oil TC Blend produced an average of 11.3 Mg ha-1 (154 sites). Field sites were selected based only on the severity of adult rootworm infestation at pollination. These field sites contained several popular high-oil TC Blends and normal hybrids produced with varying agronomic and tillage practices common to central Illinois.
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Fig. 3. Grain yield as a function of average daily length of exserted silks during pollination. Within a corn blend, bars highlighted with the same letter are not significantly different based on Fischer's LSD (P = 0.05). ID, insufficient data.
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Silk lengths during pollination had no affect on oil concentrations in the harvested grain for either corn type. Grain harvested from the high-oil TC Blend contained an average concentration of 7.4% oil while grain harvested from the normal hybrid contained an average of 4.2% oil. All oil concentrations were calculated on a 0% moisture basis for all field studies.
Implications for Adult Rootworm Management
Based on the results of these studies, growers should consider appropriate pest control measures if silk length during pollination is <25 mm for the hybrid grain production system. This threshold is more restrictive than the commonly accepted threshold of 12 mm of silk length during pollination for hybrid corn production (Tollefson, 1998). A biological threshold of 25 mm of exserted silk is consistent with thresholds established in seed corn production (Culy et al., 1992b). Seed quality and the number of kernels harvested per hectare were not significantly affected if lengths of exserted silks were at least 25 mm long during pollination in seed corn fields. In addition, other research with hybrid corn shows similar results (Bergman and Edwards, 1986). These results further show that growers should consider appropriate pest control measures if the average length of exserted silk is <37 mm during pollination for the TopCross grain production system.
In these studies, silk length during pollination was a better indicator of yield loss than the number of rootworm beetles per plant. Environmental conditions during pollination substantially influence the number of adult rootworms necessary to reduce silk lengths sufficiently to affect kernels per ear and kernel yields in seed corn (Culy et al., 1992a). For seed production, economic thresholds in seed corn fields varied from one to three beetles per plant. In an additional study, also during pollination, three adult rootworms per plant and five adult rootworms per plant significantly reduced kernels ear-1, ear weight, and weight of harvested grain in inbred seed corn and hybrid corn production, respectively (Leva, 1980). Alternatively, populations of adult rootworms as high as 20 beetles plant-1 during pollination did not cause significant economic yield loss in hybrid corn (Capinera et al., 1986).
A proper scouting program for rootworms or other pollen- or silk-feeding insects during pollination should be considered as part of a management plan to help maximize yield and profit. Pest-monitoring programs for normal hybrid corn and high-oil corn produced with the TopCross system are similar. However, economic thresholds for silk-feeding pests for high-oil corn may be more restrictive than those currently accepted for hybrid corn. The economic threshold for high-oil corn may also be higher because of the additional value of the grain. Proper pest management for high-oil corn must therefore consider the economic value as well as the biological processes involved in deciding when to control rootworm beetles or other silk-clipping pests during pollination.
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ACKNOWLEDGMENTS
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The authors thank Holden's Foundation Seeds for supplying the corn seed for a portion of this study. We also thank Brad Close, Mike Mossburg, Eric Franzenburg, and the Alvey Agricultural Research Team for their assistance in conducting the field studies and for providing part of the data used in this work. The authors were employed by Optimum Quality Grains, L.L.C. when this research was conducted.
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NOTES
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TopCross and TC Blend are registered trademarks of E.I. Du Pont de Nemours and Company.
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REFERENCES
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TOP
ABSTRACT
INTRODUCTION
