Semivariance analysis
Representative semivariograms are shown in Fig. 7.1 for egg, larval and adult stages of northern corn rootworms. Semivariance was plotted as a percentage of sample variance so that the figures could be compared on a common scale (Rossi et al., 1992).
Distributions of northern corn rootworm eggs were best described by linear semivariogram models that were horizontal or nearly so for both the Moody County and Brookings County field sites. A typical semivari- ogram is shown in Fig. 7.1A for rootworm eggs sampled from the Brookings County field. The horizontal form of this semivariogram indi- cates little spatial dependence in distribution of northern corn rootworm eggs at the time of sampling. This interpretation is supported by mean egg densities that varied little from lowest (footslope and toeslope) to highest (summit and shoulder) landscape positions (Tables 7.1 and 7.2).
Oviposition by corn rootworms thus may occur more or less randomly on a whole-field scale but may also be variable at a given location within a field.
It should also be noted that the pure nugget effect seen in the semi-
148 M.M. Ellsbury et al.
variograms for egg samples indicates that it may not be possible to obtain enough samples to adequately describe the field-scale spatial variability of rootworm eggs (Krajewski and Gibbs, 2001).
Data from larval sampling produced semivariograms that were also best described by a linear model (Fig. 7.1B) but with positive slopes and a definite nugget effect. The nugget effect suggested relatively high vari- ability between samples at short distances (Krajewski and Gibbs, 2001)
Within-field Spatial Variation of Northern Corn Rootworm Distributions 149
0 100 200 300 400 500 600
Separation distance (m)
0 100 200 300 400 500 600
0 100 200 300 400 500 600
Fig. 7.1. Representative semivariograms for eggs (A), larvae (B) and adults (C) of the northern corn rootworm. Semivariance is expressed as a percentage of the sample variance to allow comparison on a common scale. Parameters: Co, nugget; C, silk; Ao, range; RSS, residual sum of squares.
1.2 1.0 0.8 0.6 0.4 0.2 0.0
1.2 1.0 0.8 0.6 0.4 0.2 0.0 1.2 1.0 0.8 0.6 0.4 0.2 0.0
Eggs, Brookings Field, 1996
Larvae, Moody Field, 1999
Linear model:
Co = 1.019; Co + C = 1.019;
Ao = 514.17; r2= 0.069;
RSS = 0.0494
Linear model:
Co = 0.590; Co + C = 1.020;
Ao = 360.73; r2= 0.976;
RSS = 0.593
Exponential model:
Co = 0.546; Co + C = 1.092;
Ao = 178.00; r2= 0.943;
RSS = 0.0227
Semivariance
Adults, Moody Field, 1999
A
B
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150 M.M. Ellsbury et al.
Table 7.1.Northern corn rootworm egg and adult population densities, soil electrical conductivity (EM-38) and larval injury to maize roots in relation to landscape position for the Moody County, South Dakota study site. Adult emergence was the mean total for the season at each site and eggs were sampled in the autumn. Numbers in parentheses are standard errors; means followed by the same letter were not significantly different, Fisher’s protected LSD (P< 0.05).
Landscape Elevation EM-38 Eggs per Adults Root
position interval (m) (mS/m) l soil per 0.5 m2 injury
Footslope/toeslope < 525 41.08 a 5.1 a 66.8 a 3.9 a
(0.18) (1.8) (9.4) (0.2)
Backslope 525–530 35.90 b 5.0 a 107.0 b 4.0 a
(0.17) (1.2) (13.2) (0.2)
Shoulder/summit > 530 31.02 b 4.8 a 115.5 b 4.5 a
(0.07) (1.4) (18.1) (0.3)
LSD, least significant difference.
and the positive slope indicated more spatial dependence among larval sample locations than was found for egg data. For the semivariogram shown in Fig. 7.1B the intercept with the vertical axis indicates that about 50% of the variation in larval numbers cannot be attributed to spatial cor- relation. We hypothesize that edaphic variability associated with the effects of mortality factors such as temperature or moisture acting on egg populations may be responsible for the differences in degree of spatial variation between egg and larval rootworm populations.
Semivariograms for adult emergence densities from both study sites showed stronger spatial dependence than that seen for eggs or larvae, with a nugget effect in most cases. Semivariograms for adult emergence
Table 7.2. Northern corn rootworm larval densities, soil electrical conductivity (EM38), and larval injury to maize roots in relation to landscape position for east and west halves of study area in Brookings County, South Dakota. Adult emergence was mean total for the season at each site, eggs were sampled in the autumn. Numbers in parentheses are standard errors; means followed by the same letter were not significantly different, Fisher’s protected LSD (P< 0.05).
Landscape Elevation EM-38 Eggs per Adults Root
position interval (m) (mS/m) l soil per 0.5 m2 injury
Footslope/toeslope < 509 33.85 a 2.5 a 41.6 a 3.3 a
(0.54) (0.9) (5.0) (0.3)
Backslope 509–513 31.14 b 1.6 a 45.3 a 3.7 a
(0.25) (0.2) (3.9) (0.2)
Shoulder/summit > 513 30.67 b 1.4 a 30.6 a 4.1 a
(0.34) (0.4) (20.2) (0.6)
were best described by exponential or spherical models with distinct sills, suggesting a higher degree of spatially dependent variation for adult emergence than was evident in distributions of the egg or larval stages.
Figure 7.1C shows a semivariogram of exponential form for northern corn rootworm adult emergence density from the Brookings County field. The low slope is indicative of a relatively gradual change in variability with increasing distance between samples (Krajewski and Gibbs, 2001).
Distribution Maps
Distributions of northern corn rootworm eggs are shown as contour maps in Plate 1. Mean egg densities varied from 1.4 to 2.6 eggs/l in the Brookings County field and 4.8 to 5.1 eggs/l of soil in the Moody County field. Differences in the appearance of the contour maps reflect the fact that numbers of eggs in soil samples were higher in the Moody County field than in the Brookings field, even though spatial dependence was not strong at either site.
Larval sampling produced generally lower numbers than did egg sampling at both sample sites (Plate 2). As with the egg samples, consis- tently more larvae were recovered from plant samples taken in the Moody County field than in the Brookings County field.
Adult emergence densities for northern corn rootworm are shown as contour maps in Plate 3. Adult densities in the Brookings field were highest in an area apparent as a band of higher values running diagonally from north-west to south-east through an area that coincided with the location of a subsurface drainage system. This is most evident in Plate 3B.
Similarly higher densities of adult rootworms occurred in better-drained areas of the Moody field at shoulder and backslope areas. The mean number of northern corn rootworm adults emerged per cage (0.5 m2) was 44.6 ± 3.5 in the Brookings County field (Plate 3A–C) and 91.5 ± 7.5 in the Moody County field (Plate 3D–F). As with the egg and larval samples, populations of adult northern corn rootworms were consistently higher at the Moody site than at the Brookings site. It should be noted that com- parison among variograms for egg, larval, and adult samples may be ques- tionable because of the differences in support (Krajewski and Gibbs, 2001) among the data sets in terms of methodology by which data for the different life stages were obtained.
Landscape effects
Landscape position significantly affected EMI readings at the Moody County site (F= 811.73, degrees of freedom (d.f.) = 2, 5277, P < 0.0001) and at the Brookings County site (F= 18.74251, d.f. = 2, 622, P< 0.001).
In both fields, the highest EMI readings were associated with moisture- laden soil at the footslope/toeslope landscape positions (Tables 7.1 and
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7.2). In the Moody County field (Table 7.1), adult emergence appeared to be negatively correlated with the higher EMI readings, at the footslope/toeslope positions. In contrast, adult emergence in the Brookings County field was highest at the footslope/toeslope positions, where EMI readings were also high, and yet root injury was low at these positions. In the examples from our Moody and Brookings County study sites, adult rootworm populations appeared to behave quite differently in the two fields with respect to field topography, root injury and soil EMI properties.
Comparison of the two study sites suggests that it will be difficult to develop generalized guidelines for site-specific management of corn root- worms based on any single sampling protocol or indicator variable.
Conclusions
Distribution of adult emergence for northern corn rootworms is affected by soil-mediated changes in the distribution of the soil-dwelling imma- ture stages from the time eggs are deposited until the damaging larval populations occur. Because of the influence of the soil environment on survival of the immature stages of corn rootworm, we hypothesize that measurable soil properties, such as soil electrical conductivity, may be used as ancillary variables to predict where corn rootworms are most likely to survive and cause economic loss.
Intensive grid-sampling, georeferred by the use of a global positioning system (GPS), provides potentially valuable knowledge in geographical information system (GIS)-managed data layers incorporating rootworm distribution in relation to soil properties, fertility, weeds, landscape and yields in maize fields. However, it should be noted that the usefulness of the information presently is limited by the complexity of data layer inter- pretation and by the cost of sampling for corn rootworms. The necessary GIS/GPS capabilities are available but have not yet been effectively com- bined into systems incorporating map-driven application technology with economical scouting methods or real-time monitoring and mapping of corn rootworm variability.
The complexity and field-to-field variability of insect pest/soil inter- actions is such that a multivariate approach rather than use of a single predictor such as soil electrical conductivity may be necessary for site- specific management of corn rootworms in the soil environment. The focus of continuing research will be to identify relations among soil char- acteristics and yield-limiting factors in order to define boundaries of zones for site-specific management.
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