Sampling Devices and Decision
Researchers suggested that a variant of WCR beetle, that prefers to oviposit in areas other than maize, had been selected through the wide- spread adoption of crop rotation (Edwards, 1996; Levine and Gray, 1996;
Sammons et al., 1997). To avoid economic losses, producers applied soil insecticides to first-year maize fields as well as to continuous maize fields (maize planted after maize). Consequently, the amount of maize treated with soil insecticides increased from 10% to 65% in northern Indiana and from 13% to 75% in east-central Illinois between 1994 and 1996 (Edwards, 2000). Because this increase was not the result of sampling decisions, it can be assumed that some fields may have been treated unnecessarily.
To reduce unnecessary insecticide applications, economic thresholds have been developed and implemented to trigger the control of pest species before economic injury levels are reached (Stern, 1973). The eco- nomic threshold, also known as the action threshold, is the number of pests at which treatment actions should occur to prevent the pest density from reaching the economic injury level. An economic injury level is the lowest number of pests that cause economic damage (the value of loss that equals the cost of avoiding the loss). Economic injury levels and/or eco- nomic thresholds have been established for corn rootworm eggs (Davis, 1994), larvae (Reed et al., 1991) and adults (Hein and Tollefson, 1985;
Kuhar and Youngman, 1998) infesting continuous maize.
To determine if economic injury levels or economic thresholds are met, sampling the targeted pest population is required. Tollefson (1975) demonstrated that sampling adult rootworms with sticky traps and count- ing adults on maize plants in continuous maize could predict rootworm larval damage the following growing season. Over the past three decades, a number of adult corn rootworm traps and techniques have been com- pared and tested as population density estimators in continuous maize, and as prediction tools for subsequent rootworm larval damage (Tollefson, 1975; Hein and Tollefson, 1984; Shaw et al., 1984; Levine and Gray, 1994; Kuhar and Youngman, 1998). Hein and Tollefson (1985) sug- gested that, in continuous maize fields, unbaited Pherocon® AM yellow sticky traps (Trécé Inc., Salinas, CA 93912) and the whole-plant count method were equally effective in predicting larval damage.
Beginning in the mid-1990s, monitoring tools and techniques for corn rootworms were tested and adapted for soybean production systems (Barna et al., 1998; Edwards et al., 1998; Spencer et al., 1998; O’Neal et al., 1999). In Indiana, economic injury levels and economic thresholds have not been determined for WCR adults infesting soybean preceding maize. The objectives of this study were to develop a sampling pro- gramme for adult WCR in the maize/soybean rotational system and to establish an economic injury level and an economic threshold based on WCR adult density estimates in soybean for the potential damage created by WCR larvae in subsequent maize fields.
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Materials and Methods
WCR sampling devices
Study sites
Six soybean fields in Benton County, Indiana, were used during 1997 and 1999, and six adjacent soybean fields were used in 1998 and 2000. These fields were selected based on the relatively high abundance of the variant WCR beetle (C.R. Edwards, L.W. Bledsoe, J.L. Obermeyer and R.L.
Blackwell, unpublished data). Field sizes ranged from 12 to 42 ha. Tillage, planting date and other crop inputs were not controlled in this experiment.
Sampling methods
In 1997 and 1998, four sampling devices were used to sample WCR adults:
the Pherocon®AM unbaited yellow sticky trap, the corn rootworm (CRW) non-lure trap (Trécé Inc., Salinas, CA 93912), a cucurbitacin vial trap (Shaw et al., 1984) and a standard 38.1 cm diameter muslin sweep net.
In each field, three transects were established running the length of the field, each approximately one-quarter of the field width apart. Three trap lines were established on these transects and each line consisted of one of the three trap types. Eight traps were placed equidistant in each line. Based on the length of each field, traps in each trapping line were placed approximately 50 to 100 m apart. It was assumed that traps were far enough apart for trap interference not to occur. At each Pherocon®AM trap and vial trap deployment site, 1.5 m wood lath stakes were driven approximately 0.3 m into the soil. Each trap was positioned at the top of the canopy, with adjustments each week to maintain height relative to the top of the soybean canopy. At each CRW trap deployment site, 1.8 m × 1.27 cm polyvinyl chloride (PVC) pipes were driven approximately 0.2 m into the soil. Traps were placed directly on the PVC piping located above the canopy. Trap adjustment relative to the soybean canopy was not pos- sible. Eight samples of 30 sweeps each were obtained in one of the trap lines in each of the soybean fields. Sweeps were made with a pendulum motion in the upper quarter to third of the soybean canopy.
In 1999 and 2000, sampling devices included only Pherocon® AM and CRW traps. Two transects were established running the length of each field, each approximately one-third of the field width apart. Each transect contained either eight Pherocon® AM traps or eight CRW traps.
Trap placement on stakes and piping, as well as the distance between traps, was identical to the protocol in 1997 and 1998.
Trapping periods
Traps were monitored over a 6–7-week period. Adult WCR peak activity in the eastern Midwest (Illinois, Indiana, Michigan, Ohio and Wisconsin)
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occurs during the first 3 weeks of August (Levine and Gray, 1996). In 1997 and 1998, trap deployment began on 22 and 14 July, respectively. Trap deployment in 1999 began on 20 July and in 2000 deployment began on 12 July. All traps were replaced every 7–8 days during the trapping period. The number of WCR beetles collected per week was recorded. The 6–7-week trapping period ended by 10 September in each of the years WCR beetles were sampled.
Data analysis
For the data set of each sampling device for each year, a linear regression of log s2on log x– was computed. Minimum sample sizes required for 20%
and 25% levels of precision were determined for each sampling device using Taylor’s power law (Ruesink, 1980):
n= ax–b – 2 c2
where nis the number of sampling devices, x– is the mean, aand bare the values of the yintercept and the slope of the line, respectively, based on the linear regression equations of log s2on log x– for each sampling device, and cis the precision expressed as a fraction of the mean.
WCR economic injury levels and threshold Study sites
Sixteen pairs of adjacent first-year maize and soybean fields, in an annual maize/soybean rotation, in six north-west Indiana counties – Benton (seven pairs), Clinton (two pairs), Fountain (one pair), Jasper (two pairs), Newton (three pairs) and White (one pair) – were used in this study during 1998–2000. In 1998, WCR adult populations were sampled in each soybean field. Maize was planted into these fields in 1999, and WCR larval damage was determined on maize roots from each field. In that same year, sampling WCR adult populations was conducted in the paired soybean fields. In 2000, maize was planted into these soybean fields, and again WCR larval damage was determined on maize roots from each field.
Field sizes ranged from 12 to 47 ha. Fields were selected based on the rel- atively high abundance of the variant WCR beetle (C.R. Edwards, L.W.
Bledsoe, J.L. Obermeyer and R.L. Blackwell, unpublished data). The 16 pairs of maize and soybean fields were considered replicates. Tillage, planting date and other crop inputs were not controlled in this experi- ment.
Population sampling
Adult WCR were sampled using the Pherocon®AM unbaited sticky traps in 32 soybean fields. Twelve of the 32 soybean fields were used in the pre-
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viously described WCR sampling study. In the remaining 20 soybean fields, a single transect was established running the length of each field, approximately one-half of the field width apart. A trap line consisted of six equidistantly placed Pherocon®AM traps. Based on the length of each field, traps in each trapping line were placed approximately 50 to 150 m apart. Establishment of traps at each deployment site was identical to the protocol in the WCR sampling study. Even though eight traps were used in 12 fields, and six traps in 20 fields, we believe the data were compara- ble and therefore they were pooled.
Trapping protocol
Trap deployment began on 13 and 14 July, 1998, and 19 and 20 July, 1999. Traps were replaced every 7–8 days and the number of WCR beetles collected per week was recorded. The 6-week trapping period, during the 1998 and 1999 growing seasons, ended by 2 September.
Subsequent WCR maize root damage assessment
To examine rootworm larval damage, roots were extracted from rows in which no soil insecticides were placed, approximately along the same transects where trap deployment occurred the preceding year in all fields.
Maize roots were dug up between 9 July and 18 August in 1999 and 6 July and 28 July in 2000. Fifty roots were dug up equally spaced throughout each untreated row in the 20 fields where a single trap line had been established in soybean in the preceding year. Twenty-five roots were dug up in each untreated row in the 12 fields that contained the two trap lines in the preceding year. Each root was labelled, washed and rated using the Hills and Peters (1971) 1 to 6 root damage rating scale.
Data analysis
Linear regression was used to determine the coefficient of determination between mean number of beetles collected per trap, during the sampling dates in 1998 and 1999, and the mean root rating values obtained in 1999 and 2000 (Supernova, 1989).
Validation study of the proposed economic threshold
A follow-up study was conducted from 2000 to 2002 to validate the pro- posed economic threshold.
Twelve and 11 soybean fields, located in north-west Indiana and east- central Illinois, were monitored in 2000 and 2001, respectively.
Deployment of the Pherocon® AM traps in each field was similar to the protocols in the WCR sampling device and the WCR economic injury level and threshold studies. However, in 12 of the 23 soybean fields, the
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trapping scheme was established as two rows of four traps, uniformly dis- tributed throughout each field. Even though two trapping schemes were used, we believe the data were comparable and therefore they were pooled.
In 2001 and 2002, WCR larval damage was assessed on maize roots extracted from rows located approximately along the same transects where trap deployment occurred the preceding year in all fields. Fifty roots were dug up equally spaced throughout each untreated row in the 11 fields where a single trap line had been established. Twenty roots were dug up in each untreated row in the 12 fields that contained the two trap lines. Damage on maize roots by WCR larvae was evaluated using the Hills and Peters (1971) root damage rating scale.
Data obtained from this study (23 fields) and from the WCR economic injury level and threshold study (32 fields) were pooled and a contin- gency table was developed to determine the accuracy of predicting WCR larval damage in first-year maize, based on the economic threshold of WCR adults in soybean.