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Western Corn Rootworm Damage Subtly Affects Corn Growth under Moderate Environmental Stress

^a Dep. of Crop & Soil Sci, Cornell Univ., Ithaca, NY 14853
^b Dep. of Entomology, Cornell Univ., Ithaca, NY 14853
^c Dep. of Animal Science; Cornell Univ., Ithaca, NY 14853

^* Corresponding author (wjc3{at}cornell.edu).

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
Western corn rootworm (Diabrotica virgifera virgifera Le Conte) is more prevalent because of its increased adaptation to crop rotation and increased continuous corn (Zea mays L.). A 2-yr field study using seed and soil-applied insecticide treatments evaluated growth responses of corn to rootworm damage. The control had moderately severe (1.40 on a 0–3 node-injury scale), whereas the 1.25 mg kernel^–1 clothianidin [(E)1-(2-chloro-1, 3-thiazol-5-ymethyl)-3-methyl-2 nitroguanidine] treatment had minor (0.18) rootworm damage at silking (R1 stage). The control vs. clothianidin had a lower leaf area index at the 12th leaf (V12) stage (2.80 vs. 3.18) but similar values at R1 (4.59 vs. 4.76) and early grain-fill (R3) stages (4.30 vs. 4.64 m² m^–2, respectively). The control vs. clothianidin had a lower mean crop growth rate from R1 to R3 (23.6 vs. 36.3) but similar rates from the V12 to R1 (34.9 vs. 36.4) and R3 to late grain-fill (27.2 vs. 25.3 g m^–2 d^–1, respectively) stages. The control vs. clothianidin had less kernels per square meter (4423 vs. 4751) but similar kernel weight (257 vs. 248 mg) and harvest index values (0.48 vs. 0.49 kg kg^–1, respectively). The control vs. clothianidin had lower yield (9.7 vs. 10.6 Mg ha^–1, respectively), but root damage ratings did not correlate with yield (r = –0.19, n = 48). The relationship between corn rootworm damage and corn growth was subtle and inconsistent across growth stages, but the control consistently yielded 8 to 9% less than the clothianidin treatment.

Abbreviations: CGR, crop growth rate • DM, dry matter • GDD, growing degree days • HI, harvest index • LAI, leaf area index • SDD, stress degree days

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
WESTERN CORN ROOTWORM (Diabrotica virgifera virgifera Le Conte) is the major insect pest in continuous corn fields, and crop rotation remains the major management strategy for control of this pest (Wright et al., 2000). Because one population of western corn rootworm has adapted to the corn (Zea mays L.)–soybean [Glycine max (L.) Merr.] rotation in many regions of the midwestern United States, some growers must now manage for corn rootworm in this cropping system (O'Neal et al., 2001). Also, some growers in the Midwest have adopted a corn–corn–soybean instead of a corn–soybean rotation since the early 2000s because corn yields increased more than soybean yields in the 1990s (Roe et al., 2006). Furthermore, a corn–corn–soybean rotation instead of a corn–soybean rotation will probably increase in some regions of the United States because of greater profit potential for this rotation (Tokgoz et al., 2007). The spread of western corn rootworm resistance to the corn–soybean rotation and the substitution of a corn–corn–soybean for a corn–soybean rotation has increased the importance of control of western corn rootworm.

Various root damage scales have been used to assess rootworm damage to corn, with all scales indicating economic yield losses with damage to one root node (1.0, 0–3 node-injury scale; 3.65, 1–6 scale; and 7.0, 1–9 scale, Oleson et al., 2005). Nevertheless, yield losses vary with damage to one or more root nodes because of the interaction between root damage and environmental conditions. For example, Kahler et al. (1985), Sutter et al. (1990), Spike and Tollefson (1991), and Riedell et al. (1996) reported yield losses of 6 to 9% with 1 to 2 root nodes damaged. In contrast, Godfrey et al. (1993), Davis (1994), and Roth et al. (1995) reported yield losses of 20 to 30% with one to two root nodes damaged.

Environmental conditions also influence the annual degree of root damage. Spike and Tollefson (1991) reported damage to more than one root node with yield losses of 9% in 1 yr and limited root node damage with no yield losses in the next year of a 2-yr study. Likewise, Godfrey et al. (1993) reported no yield loss in 1 yr with less than one root node damaged and a 30% yield loss with 1.5 root nodes damaged in the following year. Urías-López and Meinke (2001) also reported no yield loss with less than one root node of damage in the first year and an average 13% yield loss with more than one root node of damage in the following year. Researchers in these three studies artificially infested their experiments with a known quantity of western corn rootworm eggs under similar cropping conditions, so environmental conditions clearly influenced the degree of rootworm damage in these studies.

Corn rootworm damage may not be as severe under environmental conditions of the northeastern United States compared with the midwestern United States, as indicated by better performance of seed-applied and soil-applied insecticides for rootworm control in New York (Cox et al., 2007). The objective of this study was to evaluate how natural populations of western corn rootworm affect vegetative and reproductive growth processes of corn in an environment of moderate environmental stress in relation to corn rootworm damage. To the best of our knowledge, researchers have not documented how corn rootworm damage affects mean crop growth rates (CGRs) from the V12 to R1, R1 to R3, and R3 to the 1/2 milk-line stage of corn growth (Ritchie et al., 1993).

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
Field experiments were conducted in 2005 and 2006 on a Honeoye silt loam soil (fine-loamy, mixed, active, mesic Glossic Hapludalfs) at a Cornell University research farm near Aurora, NY (42°44 N, 76°40' W). The 240- by 33-m experimental area was subdivided into northern (120 by 33 m) and southern areas (120 by 33 m) in 1990. One area has been planted to corn in early May to conduct corn rootworm studies. The other area has been planted to a corn–pumpkin (Cucurbita pepo L.) mix in late June as a trap crop to increase western corn rootworm oviposition, which results in increased larval pressure for the following growing season. In 2005 the northern area was used for the corn rootworm study and the southern area was planted to the corn–pumpkin mix in late June. In 2006 the southern section was used for the study. Soil tests indicated a pH of 7.8 and high concentrations (Mehlich test) of P and K in the corn rootworm sections in both years.

The experimental design was a randomized complete block design with six treatments and four replications. Each plot measured 30 by 3 m with a 15 m length designated for grain and a 15 m length designated for silage studies. We discuss the grain study here.

Treatments included a control; the soil insecticide, tefluthrin (2,3,5,6-tetrafluoro-4-methylphenyl)methyl (1R,3R)-rel-3-[(1Z)-2-chloro-3,3,3-trifluoro-1-propenyl]-2,2-dimethylcyclopropanecarboxylate) applied at a 0.12 kg a.i. ha^–1 rate as an 18-cm band in front of the planter-press wheel over the open seed furrow and incorporated with the press wheel (T-band); 1.25 mg a.i. kernel^–1 of thiamethoxam (3-[(2-chloro-5-thiazolyl)methyl]tetrahydro-5-methyl-N-nitro-4H-1,3,5-oxadiazin-4-imine); 1.25 mg a.i. kernel^–1 of clothianidin; and combination seed–soil insecticide treatments of 0.25 mg a.i. kernel^–1 of clothianidin/tefluthrin (as described previously) and 1.25 mg a.i. kernel^–1 of clothianidin–tefluthrin (as described previously). The different combination of seed and soil-applied insecticides were used to attempt to obtain different degrees of corn rootworm damage in this study. Gustafson (Plano, TX) treated the Pioneer brand ‘34B23’, a 108-d relative maturity hybrid used in this study, with the seed-applied insecticides.

The experimental site was moldboard plowed on the day before planting and harrow-cultipacked on the day of planting in both years. The hybrid 34B23 was planted on 5 May 2005 and 2 May 2006 with a four-row planter (0.76 m row spacing) at 81,500 kernels ha^–1. A banded starter fertilizer, 10–20–20 (N–P–K), was applied at planting at a rate of 277 kg ha^–1. The insecticide hopper was turned on in the plots that received the soil-applied insecticide, tefluthrin, and turned off for the other plots. Preemergence herbicides provided excellent weed control in 2006, but in 2005 the plots were hand-weeded twice to achieve satisfactory weed control. All plots were injected (0.1 m deep between the center of each row) with 150 kg N ha^–1 as a 32% weight volume^–1 solution of urea [(NH2) Co] and ammonium nitrate (NH₄ NO₃) at the V5 stage.

Final plant densities of each plot were determined at the V4 stage by counting all the plants along the 30 m length of the two center rows. Five corn plants were harvested at the soil line from the two center rows (three consecutive plants from one center row and two consecutive plants from the other center row) on the south end of each plot (excluding two end border plants) at the V12 stage, from the north end of each plot (excluding two end border plants) at the silking stage (R1), from the interface of the grain and silage sections at the early-grain fill or milk stage (R3), and from five randomly selected plants from the two center rows from the silage study at silage harvest, which corresponded to the late grain-fill stage or 1/2 milk-line stage (R5.5). 34B23 attained the V12 stage on 7 July in both years, the R1 stage on 22 July 2005 and 28 July 2006, the R3 stage on 6 Aug. 2005 and 11 Aug. 2006, and the R5.5 stage on the 26 Aug. 2005 and 1 Sept. 2006. Green leaves from each five-plant sample were measured with an LI-3100 leaf area meter (LI-COR, Lincoln, NE) at the V12, R1, and R3 stages. All harvested plants were placed in a forced-air drier and dried at 60°C to constant moisture.

Leaf area index (LAI) and total dry matter (DM) accumulation were calculated on a land area basis determined from final plant densities in each plot. From the DM accumulation data, mean CGR, the gain in DM per unit area per unit of time, was calculated from the V12 to R1, R1 to R3, and R3 to R5.5 stages. Ten plants were also dug at the intersection of the grain and silage sections from each plot at the R1 stage. The roots were washed and rated for rootworm injury using the 0 to 3 node-injury scale (Oleson et al., 2005).

The inner 6 m of the two center rows in the northern half of each plot were harvested for grain with a two-row Almaco (Nevada, IA) plot combine when grain moisture was estimated to be about 250 g kg^–1 water (20 Oct. 2005 and 31 Oct. 2006). Yields were adjusted to 155 g kg^–1 of water. Also, the harvest index (HI), expressed as kilograms of grain per kilograms of total DM accumulation, was calculated from the grain yield and silage yield of each subplot.

Root lodging was calculated by counting the number of plants in the two center rows more than 30° from vertical on the day of harvest (Sutter et al., 1990). Immediately after harvest, the number of unharvestable plants, associated with severe lodging, was counted. Also, 10 ear samples were randomly selected from the two center rows of each plot on the grain harvest date. The ears were subsequently threshed, and the total number of kernels from the 10-ear sample was counted with a seed counter (Old Mill Co, Savage, MD). Kernel number was calculated on a per land area basis determined from the total kernel number from the 10-ear samples and the final plant densities of each plot. The total number of kernels were then weighed and divided by total kernel number to calculate kernel weight of each plot.

Precipitation and maximum and minimum temperatures were recorded daily at a weather station on the research farm. Growing degree days (GDD) were calculated according to the 30–10°C system (Cross and Zuber, 1972). Stress degree days (SDD), associated with corn rootworm damage, were also calculated by summing the number of heat units above a threshold temperature of 30°C during a 24-h period (Oleson et al., 2005).

Treatments were considered fixed and years and replicates were considered random effects in the analysis of variance using PROC MIXED (SAS Institute, 1998). The Shapiro–Wilk test indicated normality for all data. Least square means of the treatment were computed and the PDIFF option of the LSMEANS statement was used to determine differences among least square means at = 0.10. Simple correlations (Pearson) among variables were calculated using PROC CORR (SAS Institute, 1998).

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
Weather conditions differed markedly between growing seasons (Table 1 ). The 2005 growing season was exceptionally warm and dry. Monthly GDD exceeded the 30-yr average by 82°C in June, 47°C in July, and 60°C in August. Precipitation totaled 55 mm less than average in May, 32 mm less in July, and 21 mm less in August through 30 August (79 mm of precipitation was recorded on 31 August, the R5.5 growth stage). Stress degree days, as defined by Oleson et al. (2005), totaled 33.4 from 11 July through 4 August, 12 d before through 13 d following silking. Oleson et al. (2005) would characterize the 2005 growing season as one of high environmental stress in relation to corn rootworm damage.

When averaged across years, the control had a node injury damage rating at the R1 stage of 1.40 (Table 2 ), which indicates that corn rootworm pruned one full root node back to near the base of the root with significant feeding to a second root node. A 1.40 root node damage rating typically results in economic injury to corn under conditions of moderate environmental stress, even at low corn prices (Oleson et al., 2005). All seed-applied and seed- and soil-applied insecticide treatments had node injury levels of less than 0.40, indicating acceptable control of corn rootworm with conditions of moderate environmental stress (Oleson et al., 2005). The soil-applied insecticide, tefluthrin, however, had a root damage rating of 0.57, which was similar to all treatments, including the control. A 0.57 root node injury score could result in economic injury to a high-valued corn crop under moderate environmental stress (Oleson et al., 2005). Nevertheless, despite the use of different seed- and soil-applied insecticide treatments to induce different rootworm damage ratings in this study, only the control had significantly different root damage ratings compared with the other treatments. Furthermore, root lodging was limited and did not differ among treatments (Table 2).

When averaged across years, the 1.25 mg a.i. clothianidin and 1.25 mg a.i. clothianidin–tefluthrin treatments had a greater LAI at the R3 stage compared with the tefluthrin, 1.25 mg a.i. thiamethoxam, and 0.25 mg a.i. clothianidin–tefluthrin treatments (Table 3). All other treatment comparisons had similar LAIs at the R3 stage. The 1.25 mg a.i. clothianidin treatment with or without tefluthrin had only a 2.3% reduction in LAI between the R1 and R3 stages compared with a 7.8% reduction in the 1.25 mg a.i. thiamethoxam treatment, an 8.6% reduction in the 0.25 mg a.i. clothianidin–tefluthrin treatment, and a 13.9% reduction in the tefluthrin treatment. Valentinuz and Tollenaar (2004) have suggested that a longer maintenance of LAI after silking has contributed to yield increases of modern hybrids. In this study, however, all treatments had LAIs greater than 4.0 at the R3 stage. Such high LAIs at the R3 stage may negate any advantage for the greater LAI of the 1.25 mg a.i. clothianidin treatment with or without tefluthrin. Once again, root damage ratings had a significant but weak negative correlation (r = –0.28, n = 48) with LAI at the R3 stage. Apparently, other factors, in addition to corn rootworm control, contributed to the greater LAI at the R3 stage in the clothianidin treatments.

When averaged across years, the 1.25 mg a.i. clothianidin treatment had a greater mean CGR from the R1 to R3 stage compared with the control and the tefluthrin treatment (Table 3). The greater mean CGR of the 1.25 mg a.i. clothianidin treatment probably contributed to its greater LAI at the R3 stage compared with the tefluthrin treatment. More important, greater CGR from the R1 to R3 stage can result in greater kernel number and grain yield (Cirilo and Andrade, 1994). A mean CGR of 25 g m^–2 d^–1 is typically the plateau where further increases in CGR do not result in further increases in kernel number and grain yield (Uhart and Andrade, 1995; Andrade et al., 1999). The control and the tefluthrin treatments had mean CGR of 23.6 to 24.5 g m^–2 d^–1, close to the 25 g m^–2 d^–1 plateau where grain yields do not increase. Root damage ratings and mean CGR between the R1 and R3 growth stages had an insignificant negative correlation (r = –0.20, n = 48), which suggests that differences in CGR between the 1.25 mg a.i. clothianidin and control treatments may not be associated with corn rootworm damage.

When averaged across years, mean CGR from the R3 to R5.5 growth stage did not differ among treatments (Table 3). Differences in LAI among treatments at the R3 stage thus did not result in differences in CGR during the grain-filling period. Root damage ratings and mean CGR between the R3 and R5.5 growth stages had an insignificant positive correlation (r = 0.22, n = 48), indicating that corn rootworm damage was not associated with CGR during grain filling.

When averaged across years, the 1.25 mg a.i. clothianidin treatment yielded more than the control treatment (Table 4 ). All other treatment comparisons yielded the same. The 8.5% yield reduction in the control is of the same magnitude as reported in some studies (Kahler et al., 1985; Sutter et al., 1990; Riedell et al., 1996) but less than yield reductions reported in other studies (Godfrey et al., 1993; Roth et al., 1995) for corn with root damage ratings in the 1.0 to 2.0 range (node-injury scale). Root damage ratings and grain yield, however, had no significant correlation (r = –0.19, n = 48) probably in part because there was no significant lodging in the control treatment. In contrast, grain yield and LAI at the V12 stage (r = 0.64, n = 48) and R3 stage (r = 0.47, n = 48) had positive correlations. The yield difference between the 1.25 mg a.i. clothianidin and the control treatment was thus more closely associated with more rapid leaf area development and longer leaf area duration, which may or may not have been associated with corn rootworm control.

When averaged across years, kernel weight did not differ among treatments (Table 4) and had an insignificant negative correlation with root damage ratings (r = –0.12, n = 48). Godfrey et al. (1993) reported a reduction in kernel weight, whereas Kahler et al. (1985) and Urías-López and Meinke (2001) also reported no reduction in kernel weight with root damage to more than one node. Mean CGR from the R3 to R5.5 stage did not differ among treatments, which probably contributed to the similar kernel weights among treatments in this study. Dry matter partitioning also did not differ among treatments, as indicated by the similar HI values among treatments (0.47–0.49). Gray and Steffey (1998) reported differences in DM partitioning patterns. Gray and Steffey (1998), however, used 12 hybrids in their study with inherent differences in DM partitioning patterns.

	CONCLUSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
Corn rootworm damage did not consistently or dramatically impair corn growth processes in this study in which there was moderate environmental stress and minimum root lodging. The control compared with the 1.25 mg a.i. clothianidin treatment, which had the lowest numerical root damage rating, had a 12% lower LAI at the V12 stage but a similar LAI at the R1 and R3 stages. The control vs. the 1.25 mg a.i. clothianidin treatment had a 36% lower mean CGR from the R1 to R3 stage but a similar mean CGR from the V12 to R1 and R3 to R5.5 growth stages. The control vs. the 1.25 mg a.i. clothianidin treatment had 7% less kernels per square meter and 8.5% lower grain yield but similar kernels weight and DM partitioning patterns (HI). The control yielded less than the 1.25 mg a.i. clothianidin treatment probably in part because of lower CGR during the critical R1 to R3 growth stage, which resulted in less kernels per square meter and grain yield. Nevertheless, root damage ratings only explained 4% of the yield variability, which is consistent with the 7.5% variability in yield reported by Oleson et al. (2005) in an environment with moderate environmental stress and minimum root lodging. The relations between corn rootworm damage and corn growth is apparently subtle and somewhat inconsistent across growth stages under conditions of moderate environmental stress and minimum root lodging. Nevertheless, the control yielded a consistent 8 to 9% less than the 1.25 mg a.i. clothianidin treatment under these conditions.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
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Received for publication December 17, 2007.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 

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