Jon J. Tollefson

Development and Validation of Models for Predicting the Seasonal Emergence of Corn Rootworm (Coleoptera: Chrysomelidae) Beetles in Iowa

Abstract Effective management of adult northern and western corn rootworms, Diabrotica barberi Smith & Lawrence and D. virgifera virgifera LeConte, respectively, requires knowledge of their emergence pattern so that scouting and adult insecticide applications can be accurately timed. The objective of this study was to develop and validate species- and sex-specific models that reliably predicted adult corn rootworm emergence in Iowa. Prediction began from a biofix defined as the date of first beetle emergence in a field. The models were fit with a 3-parameter Weibull function using emergence data collected in 57 Iowa cornfields over 5 yr. Models were validated with emergence data collected in 21 additional fields from a separate year. A single Pherocon CRW Trap per field was as effective as 13 emergence cages per field at detecting the biofix. Air temperature degree-days accumulated from the emergence cage biofix explained 85% of the variability in total corn rootworm emergence over 5 yr. This model explained 89% and 83% of the variability in total beetle emergence observed in the validation year from the emergence cage and Pherocon CRW Trap biofixes, respectively. These models do not eliminate scouting for adult corn rootworms but should improve the scouting efficiency by allowing growers to focus scouting to key periods, such as peak beetle emergence, when populations should be at their maximum abundance in the field.

Spatio-temporal dynamics of corn rootworm, Diabrotica spp., adults and their spatial association with environment

Multiyear and multilocation studies were conducted to investigate the within‐ and the between‐year spatial dynamics of corn rootworms Diabrotica spp. (Coleoptera: Chrysomelidae), adults and their spatial associations with environmental factors in cornfields. Grid‐based spatial sampling was conducted by trapping adults emerging from the soil (i.e., ‘emergence‐trap counts’) and by counting the number of adults in the ear zone of corn plant (i.e., ‘ear‐zone count’). Spatial analysis with distance indices (SADIE) was used to determine spatial distribution patterns and to investigate spatial associations. Ear‐zone counts and emergence‐trap counts were spatially dynamic within a year and more frequently aggregated in the middle of growing seasons and random early and late in the season. However, during the peak population periods, spatial distribution of ear‐zone counts and emergence‐trap counts were significantly consistent between years, indicating predictability of future spatial distributions. Spatial distribution of emergence‐trap counts and ear‐zone counts were positively associated with soil moisture and presence of corn plants with silks, respectively. This study demonstrated that within‐year spatial distribution patterns were dynamic and that there was between‐year spatial consistency of corn rootworm adult distributions. Such information can be used to improve preventative management of corn rootworms.

Spatial Distributions of Corn Rootworm (Coleoptera: Chrysomelidae) Eggs and Larvae: Implications for Sampling

Abstract Experiments were conducted to determine distance between sample points to obtain independent samples of corn rootworm eggs and larvae. Spatial patterns and spatial dependence of egg and larval distributions were investigated using geostatistics at two different spatial scales: field scale with sampling distance of 20–25 m and plot scale with 0.2-m sampling distance. Field-scale egg distribution showed no spatial dependence, indicating a random distribution. In the plot-scale study, egg distributions of two plots were random but one plot exhibited spatial dependence that explained 89% of the spatial variability. Larval distribution showed spatial dependence at both field and plot scales and 49.7–99.7% of spatial variability was explained by spatial dependence. This study reported that the distance to obtain independent samples of egg densities would be >25 m for field-scale egg sampling and >0.4 m for plot-scale egg sampling. Independent larval estimates would be produced by spacing samples >46 m apart for field-scale larval sampling, and >1.0 m apart for plot-scale larval sampling.

Effects of Row Spacing and Plant Density on Corn Rootworm (Coleoptera: Chrysomelidae) Emergence and Damage Potential to Corn

Abstract Planting corn, Zea mays L., in row spacings less than the conventional width of 76 cm has been shown to increase grain yields. This study was conducted to determine if row spacing and plant density affected corn rootworm, Diabrotica virgifera virgifera LeConte and D. barberi Smith & Lawrence, adult emergence, larval injury to the roots, and plant tolerance to injury. Field experiments were conducted at Ames and Nashua, IA, in 1998, 1999, and 2000. Treatments were row spacings of 38 and 76 cm, and plant populations of 64,500 and 79,600 plants per hectare. Adult emergence was 31% greater in 38 cm compared with 76-cm rows. However, root injury was not significantly different between row spacings or plant populations. Row spacing alone did not significantly influence tolerance to injury, measured as root size and the amount of root regrowth. However, at one environment where precipitation was low, plants in 38-cm rows produced 25% more regrowth compared with plants in 76-cm rows. Root dry weight and regrowth were suppressed by 16 and 32%, respectively, at the high plant population. Although lodging was 51% lower in the 38-cm rows compared with the 76-cm rows, grain yields were not significantly different between row spacings. Reducing the row spacing of field corn from 76–38 cm should not increase the potential for injury from corn rootworm larvae.

Degradation of isofenphos in cornfields with conservation tillage practices

Abstract Isofenphos (Amaze®, Oftanol®, 1‐methylethyl 2‐[[ethoxy‐[(l‐methylethyl)aminol phosphinothloyl]oxy]benzoate) was applied at planting to study the effects of four different tillage treatments (no‐tillage, fall chisel plow, Paraplow®, and fall moldboard plow) on isofenphos degradation rates and routes in cornfields over two growing seasons. Soil samples were taken at intervals extending over 69 days each growing season. Tillage treatment had no significant effect on isofenphos degradation rates and products. However, the repeated application of isofenphos had a very significant effect on lsofenphos degradation. Degradation was much more rapid the second year. A laboratory experiment comparing sterile and nonsterlle soils, with and without isofenphos history, confirmed enhanced mlcrobial degradation resulting from two consecutive years of isofenphos application. In the first year, isofenphos oxon was found at greater amounts relative to the second year. Soil bioassays conducted on soils collected fro...