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Background Field-evolved pyrethroid resistance has been confirmed in western corn rootworm (WCR) populations collected from the United States (US) western Corn Belt. Resistance levels of WCR adults estimated in lab bioassays were confirmed to significantly reduce the efficacy of foliar-applied bifenthrin. The objective of the present study was to investigate the impact of WCR pyrethroid resistance levels on the performance of common soil-applied insecticide formulations (23.4% tefluthrin, 17.15% bifenthrin, and 0.1% cyfluthrin + 2.0% tebupirimphos). Field trials were conducted in 2016 and 2017 in three Nebraska, US, counties (Saunders, Clay, and Keith) where distinct levels of WCR susceptibility to pyrethroids (susceptible, moderately resistant, and highly resistant) had been previously reported in adult and larval bioassays. Results All soil insecticide treatments effectively protected maize roots from a pyrethroid-susceptible WCR population at Saunders. In contrast, the efficacy of bifenthrin and tefluthrin soil insecticides was significantly reduced at Clay and Keith, where pyrethroid-resistant WCR populations were reported. At Keith, where an additional failure of the cyfluthrin + tebupirimphos soil insecticide was observed, WCR laboratory dose-response bioassays showed a consistent ∼5-fold resistance level to the active ingredients bifenthrin, tefluthrin, and cyfluthrin. Conclusion The efficacy of common soil insecticides used in the US for WCR management was significantly reduced in populations exhibiting relatively low levels of WCR pyrethroid resistance. Using a multitactical approach to manage WCR within an integrated pest management framework may mitigate resistance evolution and prolong the usefulness of WCR insecticides within the system. © 2019 Society of Chemical Industry.

Maize, Zea mays L., is an economically important crop grown throughoutthe world. Corn rootworm, Diabrotica spp. (Coleoptera: Chrysomelidae),larvae constitute a significant economic threat to maize production in theUnited States, where yield losses and management costs associated withcorn rootworm species exceed $1 billion annually. Furthermore, theintroduction of the western corn rootworm, D. virgifera virgifera LeConte,into maize-producing regions of Europe has made managing corn root-worm larval injury an international concern. Larvae injure maize plantsby feeding on root tissue and are the primary target of management activi-ties. Products commonly used to protect root systems from injury includechemical insecticides (seed or soil applied) and genetically modified maizehybrids expressing toxins derived from Bacillus thuringiensis Berliner (Bt).The confirmation of field-evolved resistance to various Bt toxins in popu-lations of the western corn rootworm presents a significant managementchallenge. We performed a meta-analysis to provide a broad understand-ing of the relative efficacy of the primary products currently being used tomanage corn rootworm larval injury, including insecticidal seed treat-ments, soil insecticides and Bt hybrids (with and without the addition ofsoil insecticide). Our analysis is unique in the breadth of locations andyears included – we analysed 135 individual trials conducted from 2003through 2014 at multiple sites in both Illinois and Nebraska. Panel datawere produced by pairing the mean node-injury rating for each treatmentof a given trial with the mean node-injury rating for untreated maize. Lin-ear regression models were developed to estimate the relationshipbetween the potential for corn rootworm larval injury and product perfor-mance. For a given level of injury potential, the parameters estimatedreveal differences in the degree of root protection offered by the variousproduct categories analysed. Implications for developing long-term, inte-grated, and sustainable practices for managing this important pest ofmaize are discussed.IntroductionMaize, Zea mays L., is an economically important cropgrown throughout the world. An estimated 177 mil-lion ha of maize were planted in 2013 – the UnitedStates accounted for approximately 20% of this area(USDA NASS 2013). Corn rootworm larvae, Diabroticaspp., represent a significant and widespread economicthreat to maize production in the United States (Grayet al. 2009). In the Midwest, the corn rootworm com-plex is dominated by two species – the western cornrootworm, Diabrotica virgifera virgifera LeConte, and

Western bean cutworm, Striacosta albicosta (Smith) (Lepidoptera: Noctuidae), is a native pest of dry beans (Phaseolus vulgaris L.) and corn (Zea mays L.). Historically, the western bean cutworm was distributed in the western United States, but since 1999 eastward expansion has been observed. In corn, economic impact is caused by larval ear feeding. Information on western bean cutworm biology, ecology, and economic impact is relatively limited, and the development of economic injury levels (EILs) and economic thresholds (ETs) is required for more effective man- agement. Studies during 2008 Ð2011, across three ecoregions of Nebraska, sought to characterize western bean cutworm survival and development of EILs and ETs. Calculations of EILs and ETs incorporated the dynamics of corn price, management cost, and pest survival. The results from the current study demonstrated low larval survival of this species (1.51Ð12.82%). The mean yield loss from one western bean cutworm larva per plant was 945.52 kg/ha (15.08 bu/acre), based on 74,100 plants per ha. Economic thresholds are expressed as a percentage of plants with at least one egg mass. This study is the Þrst study that explicitly incorporates variable management costs and crop values into western bean cutworm EIL calculations, and larval survival into ET calculations.

The alfalfa weevil, Hypera postica (Gyllenhal), is a serious, yet sporadic defoliator of alfalfa, Medicago sativa L., in Nebraska. A 2-yr study was conducted in 2005 and 2006 to test for variation in degree-day requirements by location in eastern Nebraska. Sampling took place along a latitudinal gradient in three regions of eastern Nebraska. Three fields were sampled in each region during the 2 yr of the study. Alfalfa weevil larval degree-day requirements were found to vary by latitude in eastern Nebraska. Alfalfa weevil larvae were discovered in southern regions after fewer developmental degree-days had accumulated than in fields in the northern regions. Alfalfa weevils may be more damaging to alfalfa in southern regions than in northern regions of eastern Nebraska because they emerge earlier relative to alfalfa growth. Management implications of this shift in alfalfa weevil phenology are discussed.

European corn borer Ostrinia nubilalis H . ubner (Lepidoptera: Crambidae) larvae that have completed development on Event 176 Bt corn hybids have either survived exposure to sublethal doses of Cry1Ab Bt toxin or exploited plant tissues that do not express the toxin. To evaluate the impact of such exposure on larval establishment and survival, Event 176 plants with and without tassels and a non-Bt isoline were infested with O. nubilalis egg masses during anthesis. On the non-Bt plants, larval establishment occurred primarily on pollen collecting in the leaf axils, silks and ears. In contrast, almost no larvae were recovered from leaf axils of the Bt treatments and at least 50% fewer larvae were recovered from the silks and ears during the first 2 weeks after infestation relative to the non-Bt plants. The larvae recovered from Bt treatments weighed significantly less than those observed in the non-Bt isoline at 4 weeks after infestation. By the eighth week, the larval weights of all three treatments were similar in three of four different field tests, suggesting that second generation larvae have the ability to compensate for initial exposure to sublethal doses of Bt toxin. In laboratory assays involving exposure of neonate larvae to silks of Event 176 and non-Bt corn, survival of neonate O. nubilalis was not different although larval weights were significantly reduced (2- to 6-fold). These results suggest that second generation larvae completing development on Event 176 corn do not completely avoid exposure to the Bt toxin, although those that do survive are able to compensate for these sublethal effects. The implication of these results is that Event 176 hybrids do not appear to satisfy requirements for high dose that are recommended for resistance management purposes. r 2002 Elsevier Science Ltd. All rights reserved.

The small-scale spatial distribution of European corn borer, Ostrinia nubilalis (Hub- ner), larvae and damage in whorl stage corn, Zea mays L., was characterized using geostatistics. Spatial distribution of O. nubilalis larval feeding damage was studied at Clay Center, North Platte, and Concord, NE, during JuneÐJuly 1992Ð1994, and spatial distribution of O. nubilalis larvae and damage was studied at Clay Center in June 1997. Semivariograms were calculated to model the change in spatial correlation with increasing distance between samples. Spatial distribution of larval damage during 1992Ð1994 was best described using a spherical model. Damage was spatially correlated among plants at distances up to 2.84 m apart. The spatial distribution of larvae in 1997 was best described using an exponential model for three of seven data sets, a spherical model for one of seven data sets and no model Þt three of seven data sets. Larvae were spatially correlated among plants at distances up to 3.05 m apart. These data have implications for developing sampling plans for management of O. nubilialis, and for site-speciÞc agriculture.

Western bean cutworm, Striacosta albicosta (Smith), has undergone a recent eastward expansion from the western U.S. Corn Belt to Pennsylvania and parts of Canada. Little is known about its ecology and behavior, particularly during the early instars, on corn (Zea mays L.). There is a narrow treatment window for larvae, and early detection of the pest in the field is essential. An understanding of western bean cutworm larval feeding and early-instar dispersal is essential to understand larval survival and establishment in corn. Studies were conducted in 2009 through 2011 in Nebraska to determine the feeding and dispersal of early-instar western bean cutworm on corn. The treatment design was a factorial with three corn stages (pretassel, tassel, and posttassel) and five corn plant zones (tassel, above ear, primary ear, secondary ear, and below ear) in a randomized complete block design. The effects of different corn tissues on larval survival and development were investigated in laboratory studies in a randomized complete block design during 2009 and 2011. Treatments were different corn tissues (leaf alone, leaf with developing tassel, pollen, pollen plus silk, and silk alone). Results demonstrated that neonate larvae move to the upper part of the plant, independent of corn stage. Larval growth was optimal when fed on tassel tissue. Overall results indicated a selective benefit for movement of the early instar to upper part of the plant.

Near isoline corn rootworm (CRW), traited and refuge corn hybrids in combination with soil insecticides were evaluated for effectiveness of larval CRW control near Clay Center, NE during 2015. Trial site was late-planted corn and pumpkins (insecticide free) during 2014. Experimental design was a RCB with four replicates. Plot size was 4 rows × ∼70 ft length with 30-inch row spacing. Soil type was a Crete silt loam. ‘NK N68B-3122’ (contains AgriSure 3122 insect and herbicide traits), ‘NK N68B-3000GT’ (contains AgriSure 3000GT insect and herbicide traits), and ‘NK N68B-GT’ (contains AgriSure …

Single corn rootworm (CRW) traited and refuge hybrids in combination with Aztec 4.67G insecticide at planting were evaluated for effectiveness of larval CRW control near Clay Center, NE during 2015. The trial site was late-planted corn and pumpkins (insecticide free) during 2014. The experimental design was a RCB with four replicates. Plot size was 4 rows × ∼70 ft length with 30-inch row spacing. Soil type was a Crete silt loam. ‘DeKalb DKC62-95’ (contains Roundup Ready 2 herbicide trait [refuge hybrid]), ‘DeKalb DKC62-97RIB’ (contains 90% GENVT3PRIB traited seed and 10% refuge seed), ‘Pioneer P1395AM1’ (contains 90% Herculex XTRA traited seed and 10% Herculex I traited seed), and ‘NK N68B-3111’ …

SmartStax traited and refuge corn hybrids in combination with soil insecticides were evaluated for effectiveness for larval corn rootworm (CRW) control near Clay Center, NE during 2015. The trial site was late-planted corn and pumpkins (insecticide free) during 2014. Experimental design was a RCB with four replicates. Plot size was 4 rows × ∼150 ft length with 30-inch row spacing. Soil type was a Crete silt loam. ‘DeKalb DKC62-77 RIB’ (contains GENSSRIB [SmartStax] insect and herbicide traits) and ‘DeKalb DKC62-98RIB’ (contains GENVT2PRIB [refuge] insect and herbicide traits) corn hybrids were planted on 30 Apr with a two-row JD 7100 Maximerge planter with finger pickup seed units. A …

Corn hybrid family isolines possessing stacked rootworm traits, a single rootworm trait, or refuge traits, in combination with soil insecticides, were evaluated for effectiveness of larval CRW control near Clay Center, NE, during 2014. Trial site was in late-planted corn and pumpkins (insecticide free) during 2013. Experimental design was an RCB with three replicates. Plot size was 4 rows by 75 ft in length with 30-inch row spacing. Soil type was a Crete silt loam. ‘Golden Harvest G11U58-3122’ (3122, contains Agrisure RW and Herculex RW rootworm traits), ‘Golden Harvest G11U58-3111’ (3111, contains Agrisure RW rootworm trait), and ‘Golden Harvest G11U58-GT’ (GT, contains no rootworm …

A study was conducted at the University of Nebraska South Central Agricultural Laboratory to evaluate the effect of Prevathon insecticide on several pest and beneficial arthropods in 2014. The study was conducted using an RCB design with four replications and two treatments, Prevathon (treated), and check (untreated). Areas within the field were treated as blocks. Plots were 24-rows wide by 100-ft long in 30-inch row spacing. The …

We validated a stochastic model of the evolution of resistance to adulticidal sprays of methyl-parathion in western corn rootworm, Diabrotica virgifera virgifera LeConte, populations in Nebraska. The population dynamics predicted by the model resembled that reported for field populations, and time until control failures occurred closely matched reports by commercial crop consultants. We incorporated uncertainty about the values used for 18 model parameters by replacing default values with random draws taken from a normal distribution. One parameter, the initial resistance allele frequency, was no longer measurable because of the evolution of resistance. We therefore proposed five candidate initial allele frequencies and developed probability distributions for the time to resistance for each by running 1000 simulations with parameters randomly varied. These distributions included variation because of stochastic effects as well as parameter uncertainty. We used Bayesian inference to estimate the candidate frequency most likely, given reported times to field control failures. The initial allele frequency of 10(-4) was most likely (29%), 10(-3) was less likely (28%), whereas 10(-6) was relatively unlikely (5%). Results from sensitivity analysis depended upon how evolution of resistance was measured. When resistance was examined as a genetic phenomenon, the rate of increase of the resistance allele depended almost entirely on genetic factors (LC50 values), the characteristics of the pesticide (residual activity), and the variance associated with emergence of adults. When resistance was measured as failure of methyl-parathion to reduce populations below threshold levels (0.5 gravid females per plant), parameters that contributed to population growth rate (mortality and fecundity) were also important. These data suggest two important phases in resistance evolution in corn rootworms: a genetic phase associated with negative growth rates and rapid changes in resistance allele frequencies and a rebound phase associated with positive growth rates and near fixation of the resistance allele.