Larval Rootworm Research Articles

Tolerance in Maize Landraces to Diabrotica speciosa (Coleoptera: Chrysomelidae) Larvae and Its Relationship to Plant Pigments, Compatible Osmolytes, and Vigor.

The rootworm Diabrotica speciosa (Germar) is native to South America and causes severe economic losses to several crops due to root feeding and disease spread. In maize (Zea mays L.), losses in production come from larval rootworm attack on plant roots resulting in plant health problems, including stalk lodging. More options for controlling this pest are needed to create well balanced, integrated pest management programs for farmers in this region. Natural sources of tolerance in maize genotypes are important for maize breeding programs, and this study investigated the expression of tolerance in several Brazilian maize landraces to D. speciosa. Plant vigor and compounds associated with plant health, including chlorophylls, carotenoids, glycine betaine, and proline were assessed for each landrace. Five landraces and one maize cultivar were selected based on their levels of antibiosis-resistance to D. speciosa that were determined in a prior screening. The percent reduction in plant growth was used as the measure of tolerance. The landrace Azteca was classified as tolerant to D. speciosa larval feeding, displaying less reduction in plant matter despite having lower plant vigor. This landrace also had higher amounts of chlorophyl and carotenoid pigments, suggesting a positive correlation between tolerance to D. speciosa and higher contents of these photosynthetic pigments. The compatible osmolytes glycine betaine and proline do not seem to be associated with tolerance in maize landraces to D. speciosa larvae. Landrace Azteca seems promising for plant breeding, and repeated field studies are needed to confirm its suitability in maize integrated pest management.

Effects of releasing a generalist predator (Acari: Gaeolaelaps aculeifer) on a subterranean insect herbivore (Coleoptera: Diabrotica virgifera virgifera)

Enhancing biological control of soil-dwelling insect pests is desirable, but due to the nature of the subterranean environment, it is challenging to elucidate interactions between pests and their natural enemies. Western corn rootworms (Diabrotica virgifera virgifera) are a major pest of maize, and the larvae can cause substantial damage to maize roots. Research has shown that numerous arthropods feed on rootworm immatures, and in some cases enhancing predator densities has led to rootworm suppression. We conducted a 2-yr field experiment and examined effects of releasing a predatory mite (Gaeolaelaps aculeifer) on densities of pest rootworms, root damage ratings, and plant growth parameters in conjunction with an insecticide control. In contrast to our expectations, larval rootworm incidence and root damage was generally higher in plots where mites were released. Differences in pest density and root damage did not translate into negative effects on plant biomass or grain yield at the end of the season, likely indicating that environmental conditions were favorable for plant compensation after rootworm damage. We also explored how rootworm and mite presence impacted densities of other soil invertebrates. Rootworm infestation had positive impacts on densities of carabid beetles and acarid mites during the time when rootworm eggs were hatching. Addition of predatory mites had both negative and positive effects on densities of soil invertebrates, with the latter potentially due to disruption of naturally-occurring biocontrol. This research indicates that the soil invertebrate community can impact densities of rootworm larvae and subsequent root damage.

The impact of Bt maize as a natal host on adult head capsule width in field populations of western corn rootworm

The success of the current resistance management plan for transgenic maize, Zea mays L. (Poaceae), targeting the rootworm complex hinges upon high rates of mating between resistant and susceptible beetles. However, differences in the fitness of adult beetles could result in assortative mating, which could, in turn, change the rate of resistance evolution. Adult head capsule widths of naturally occurring populations of western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), were examined in a variety of refuge configurations. Beetles were classified into treatments based on the hybrid – non-Bt refuge or Bt maize targeting larval rootworms (hereafter Bt-RW maize) – and location – proximity to other Bt-RW or refuge plants – of the natal host plant. Treatments included the following: a refuge plant surrounded by other refuge plants, a refuge plant located near a Bt-RW plant, a Bt-RW plant surrounded by Bt-RW plants, and a Bt-RW plant located near a refuge plant. The mean head capsule width of males emerging from Bt-RW plants was significantly smaller than the mean head capsule width of males emerging from refuge plants. These results indicate that males emerging from Bt-RW maize plants may be exposed to sublethal doses of the Bt toxin as larvae. No differences were detected between females emerging from refuge plants compared with Bt-RW plants. Overall mean head capsule width decreased as the season progressed, regardless of treatment. The diminished head capsule width of western corn rootworm males emerging from Bt-RW maize may act to enhance resistance management, particularly in a seed mix refuge system.

Characterization of bacteria isolated from maize roots: Emphasis on Serratia and infestation with corn rootworms (Chrysomelidae: Diabrotica)

Larval corn rootworms (Coleoptera: Chrysomelidae) are soil-dwelling insect pests that damage maize ( Zea mays L.) by consuming root tissues, thus lowering grain yield. Little is known about interactions between rootworms and soil bacteria, including potential impacts of maize rhizobacteria, such as entomopathogenic Serratia spp., on subterranean rootworm pests. We used selective growth medium (caprylate-thallous agar, CT) to quantify and isolate Serratia spp. from: (1) bulk soil, (2) roots of four field-grown maize genotypes, half of which were infested with rootworm eggs from a reared colony, and (3) non-diseased, larval rootworms from the same colony. Phenotypic testing and 16S rRNA gene sequencing were used to identify bacteria, including non- Serratia spp., that were successfully isolated on the CT solid medium. We also isolated and identified Serratia spp. associated with non-diseased and diseased Diabrotica adults. Serratia spp. associated with maize roots were more abundant than those associated with bulk soil, where they were undetectable with our methods except for Serratia grimesii. There was no impact of plant genotype on densities of bacteria isolated from maize roots. S. grimesii was frequently associated with maize roots, regardless of infestation with rootworm eggs. Serratia marcescens biotype A4, Serratia plymuthica and several other Enterobacteriaceae genera were also associated with maize roots. Infesting the soil with rootworm eggs enhance the occurrence of two strains of the S. marcescens biotype A1b, with comparable densities of these orange and pink strains within infested roots. However, both strains were associated with larval rootworms from the reared colony, which may indicate that rootworms were introducing these bacteria into infested maize roots. In addition, within larvae the orange strain was significantly more abundant than the pink strain, and was also associated with diseased rootworm adults. Our studies identified specific Serratia strains associated with diseased rootworms that may have potential as biological control agents, and additional Serratia biotypes associated with the maize rhizosphere that, based on the literature, may function as plant growth promoting agents via antagonistic action against plant-pathogenic fungi.

Toxicity and neurophysiological effects of fipronil and fipronil sulfone on the western corn rootworm (Coleoptera: Chrysomelidae)

Fipronil is a member of the relatively new phenylpyrazole insecticide class that is active at the neuro-inhibitory gamma amino butyric acid (GABA)-gated chloride channel/ionopore complex. The toxicity and neurophysiological effects of fipronil and its oxidative sulfone metabolite [5-amino-1-(2,6-dichloro-4-trifluoromethylphenyl)-3-cyano-4-trifluoromethylsulfonylpyrazole] were investigated by using an insecticide-susceptible western corn rootworm population. In topical bioassays using adult rootworms, fipronil was toxic at very low doses (LD50 = 0.07; LD90 = 0.33 ng/mg). At the LD90, pre-treatment with the cytochrome P450 monooxygenase inhibitor piperonyl butoxide led to mild antagonism of fipronil toxicity (LD90 = 0.42 ng/mg), while the sulfone analog had greater toxicity (LD90 = 0.22 ng/mg). In neurophysiological studies of spontaneous electrical activity, adult and larval rootworms were equally affected by fipronil and the sulfone analog at 10 μM (in the presence of 5 mM GABA) in comparison to GABA-treated baselines. Using larval rootworms, insensitivity of the GABA receptor to binding by picrotoxinin or dieldrin (10 μM ) was not apparent in the presence of 5 mM GABA. Further neurophysiological investigation using a range of concentrations (0.625–20.0 μM) on larval rootworms indicated concentration-dependent effects on bursting activity for both fipronil and the sulfone analog; however, subtle differences were observed between these two compounds. Results indicate that both fipronil and its oxidative sulfone metabolite have similar toxicological and neurological effects on rootworms. Arch. Insect Biochem. Physiol. 40:150–156, 1999. © 1999 Wiley-Liss, Inc.

Pest Management Practices of Crop Consultants in the Midwestern USA

A mail survey was conducted of independent crop consultants in 12 midwestern U.S. states working in corn (Zea mays L.), soybeans [Glycine max (L.) Merrill], wheat (Trilicum aestivum L.), and grain sorghum (Solghum bicolor L. Moench.) during 1993. The survey was conducted to document the acreage of crops monitored by crop consultants, and some of their operational characteristics. Kansas and Nebraska had the greatest reported acreage of crops scouted, over 3 million and 1 million acres, respectively. Total acres scouted were highest for corn, followed by soybeans, wheat, alfalfa (Medicago sativa L.), and sorghum. Acres contracted with crop consultants varied from 0.4 to 40.0% of the total 1993 harvested acres, depending on state and crop. The most common interval reported between field visits was once a week. Averaged over all states, the highest consulting fees were charged for alfalfa ($4.80), with less charged for corn ($4.31), sorghum ($4.26), soybeans ($4.07), and wheat ($3.74). There was a great range in fees within crops across the region (e.g., corn fees varied from $3.21-6.13/acre). Planting time application of insecticides was the most commonly used corn rootworm (Diabrotica virgifera virgifera LeConte and D. barberi Smith & Lawrence) control. Postemergence applications of insecticides for larval rootworms (cultivation time applications and chemigation) were used primarily in Kansas and Nebraska. Foliar sprays for adult corn rootworm control were also most common in these states. Use of crop rotation varied greatly across the region (23.8-83.7% of acres were corn after another crop). Additionally, across all states except Missouri, there was a significant correlation between frequency of rotation in corn and insecticide use against corn rootworms; ie., states with higher frequencies of rotated corn treated a lower percentage of corn acreage for corn rootworm controL The diversity of crop consultant activity across the region has implications for future adoption of IPM practices on midwestern field crops.

Control of Western Corn Rootworm Larvae in West Central Nebraska, 1995

Abstract Insecticides were applied to suppress larval rootworm populations in 2 fields near North Platte, NE. Both fields were composed of Cozad silt loam soils. In Field 1 ‘DeKalb 566’ hybrid field corn was ridge-planted with a John Deere 7100 Max-emerge planter, to a non-tilled seed bed on 19 May. At the time of planting, soil moisture was adequate for germination and the soil temperature at a depth of 5 cm was about 8° C. Several insecticides were applied at planting to plots 2 rows wide, 10 m in length, with a bicycle wheel type applicator. Four replications of each treatment were arranged in a RCB design. T-band applications were made by applying a 7 inch band of insecticide to an open seed furrow. In-furrow applications were made by applying insecticide directly into the seed furrow. After the insecticides were applied, furrows were closed with a rotary hoe and insecticides were incorporated with a rake. Field 2 was ridge-planted to ‘BoJac 577’ field corn with a John Deere 7100 Max-emerge planter, in a non-tilled seed bed on 18 May. At the time of planting, soil moisture was adequate for germination and the soil temperature at a depth of 5 cm was about 13° C. With the exception of 4 field length strips, 12 rows in width, Lorsban 15G was applied at planting with insecticide boxes on the planter at a rate of 1100 g Al/ha. Granular and liquid insecticides were applied to untreated strips in Field 2 immediately prior to cultivation on 22 Jun. The corn plants were in the early whorl stage (V3 stage) of development at the time of application. The plots for the granular treatments in Field 2 were 10 m in length and 4 rows wide. The granular treatments were applied in a 7 inch band, over the whorl of the plants, with a bicycle wheel type applicator. Furadan 4F was applied to 10 X 4 m plots in Field 2, either broadcast over the top of the plants or in 7 inch bands to the base of the plants. All liquid treatments were applied in a total volume of 120 liters/ha at 30 psi. All treatments were replicated 4 times and arranged in a RCB design. Treatments in both fields were evaluated 26 Jul by rating washed roots, using the 1-6 Iowa root damage rating scale, of 4 plants from each replicate.

Effect of Sulfonylurea Herbicide on Field Corn Following Application of Counter Insecticide at Planting, 1994

Abstract Counter 15G and Counter 20CR were applied to suppress larval rootworm populations in plots located near North Platte, NE. Field corn was ridge-planted in a Cozad silt loam soil with a John Deere 7100 Max-emerge planter, to a non-tilled seed bed on 12 May. At the time of planting, soil moisture was adequate for germination and the soil temperature was about 17° C. The insecticides were applied in a 7 inch T-band at planting to plots 4 rows wide, 10 m in length, with a bicycle wheel type applicator. Four replications of each treatment were arranged in a RCB design. After the insecticides were applied, furrows were closed with a press wheel and the insecticides were incorporated with a rake. Accent and Beacon herbicides were applied over the whorl of V3 stage field corn plants on 9 Jun. The applications were made with a CO2 charged backpack sprayer through 8001 flat fan nozzles at 30 psi, in a total volume of 19 gal/acre. Crop oil concentrate was added to each herbicide treatment at a rate of 1 qt/acre. The effect of the herbicide on corn height and condition was evaluated seven, 14, 28 and 60 DAT. Plant conditions which were considered herbicide affected included stunted height, yellowed leaves or twisted whorls. Yield was determined by hand harvesting and mechanically shelling all ears from 35 ft of row from the center of each plot.

Control of Western Corn Rootworm Larvae in West Central Nebraska, 1994

Abstract Insecticides were applied to suppress larval rootworm populations in 2 fields in west central NE. Both fields located at the University of Nebraska West Central Research and Extension Center were composed of Cozad silt loam soils.

Control of Corn Rootworm Larvae in West Central Nebraska, 1991

Abstract Granular insecticides were applied to suppress larval rootworm populations in 2 fields in west central NE. Insecticides were applied at planting to ridge-planted ‘Pioneer 3467’ hybrid field corn in Field 1 on 18 May. Application was made to a non-tilled seed bed with insecticide boxes on a John Deere Max-emerge ridge till planter. Four replications of each treatment were arranged in a RCBD. T-band applications were made by applying a 7 inch band of insecticide in front of the press wheel. Banded applications were made by applying a 7 inch band of insecticide behind the press wheel and incorporated with a chain. In-furrow applications were made by applying insecticide directly into the seed furrow. Insecticides were applied over the whorl of blocks of ‘Keltgen 1150’ and ‘Producers 870’ hybrid field corn plants in Field 2 at cultivation on 15 Jun. The application was made with International Harvester boxes on a KMC ditcher/cultivator unit. Each treatment in Field 2 was replicated twice and arranged in a RCBD. Corn plant hybrid was used as the blocking criteria. All insecticides in both fields were applied at a rate of 1114.5 g (AI)/ha. Treatments in both fields were evaluated 13 Jul by rating washed roots, using the 1-6 Iowa Root Damage Rating Scale, of 4 plants from each replicate.

Control of Western Corn Rootworm Larvae in West Central Nebraska, 1993

Abstract Insecticides were applied to suppress larval rootworm populations in 2 fields in westcentral NE. Both fields were located at the University of Nebraska West Central Research and Extension Center and were composed of Cozad silt loam soils. Hybrid field corn was ridge-planted in Field 1 with a John Deere Max-emerge planter, to a non-tilled seed bed on 20 May. At the time of planting, soil moisture was adequate for germination and the soil temperature was about 18° C. Several insecticides were applied at planting to a single row 145 m in length with insecticide boxes attached to the planter. Four replications of each treatment were arranged in a RCBD. T-band applications were made by applying a 7 inch band of insecticide in front of the press wheel. In-furrow applications were made by applying insecticide directly into the seed furrow. Field 2 was ridge-planted to ‘BoJac 520’ and ‘BoJac x438’ hybrid field corn with a John Deere Max-emerge planter, in a non-tilled seed bed on 6 May. At the time of planting, soil moisture was adequate for germination and the soil temperature was about 13° C. With the exception of 4 field length strips, 12 rows in width, Lorsban 15G was applied at planting with insecticide boxes on the planter at a rate of 836 g (AI)/ha. Granular and liquid insecticides were applied to untreated strips in Field 2 immediately prior to cultivation on 25 Jun. The corn plants were in the early whorl stage (growth stage 2) of development at the time of application. The granular treatments were applied over the whorl of the plants, on 10 m of a single row, with insecticide boxes attached to a tool bar. Furadan 4F was applied to 10 × 4 m plots in Field 2 either broadcast or in 7 inch bands to the base of the plants. All liquid treatments were applied in a total volume of 120 liters/ha at 206,786 Pa (30 psi). All treatments were replicated 4 times and arranged in a RCBD. Two blocks of treatments were applied to ‘BoJac 520’ hybrid field corn plants and 2 blocks of treatments were applied to ‘BoJac X438’ hybrid field corn plants. Treatments in both fields were evaluated 22 Jul by rating washed roots, using the 1-6 Iowa Root Damage Rating Scale, of 4 plants from each replicate. Data from each field were analyzed separately with analysis of variance and an LSD test was used for mean separation.

Control of Western Corn Rootworm Larvae in West Central Nebraska, 1992

Abstract Granular insecticides were applied to suppress larval rootworm populations in 2 fields in west central NE. ‘Pioneer 3394’ hybrid field corn was ridge-planted in Field 1 with a John Deere Max-emerge planter, to a non-tilled seed bed on 11 May. Insecticides were applied at planting with insecticide boxes attached to the planter. Three replications of each treatment were arranged in a RCB design. T-band applications were made by applying a 7 inch band of insecticide in front of the press wheel. In-furrow applications were made by applying insecticide directly into the seed furrow. All insecticides were applied at a rate of 1114.5 g (AI)/ha. Field 2 was ridge-planted to ‘Fontanelle 4435’ and ‘Fontanelle 6240’ hybrid field corn with a John Deere Max-emerge planter, to a non-tilled seed bed on 6 May. With the exception of 4 strips, 12 rows in width, Counter 15G was applied at planting with insecticide boxes on the planter at a rate of 1114.5 g (AI)/ha. A granular and a liquid insecticide were applied to untreated strips in Field 2 immediately prior to cultivation on 15 Jun. The field corn plants were in the early whorl stage (growth stage 2) of development at the time of application. The granular application was applied over the whorl of the plants with insecticide boxes attached to a tool bar. The liquid insecticide was applied in a 7 inch band to the base of the plants in total volume of 120 liter/ha at 206,786 Pa (30 psi). Cultivation treatments were applied at a rate of 1114.5 g (AI)/ha. Each treatment in Field 2 was applied to 10 × 4 m plots which were replicated 4 times and arranged in a RCB design. Two blocks of treatments were applied to ‘Fontanelle 4435’ hybrid field corn plants and 2 blocks of treatments were planted to ‘Fontanelle 6240’ hybrid field corn plants. Treatments in both fields were evaluated 13 Jul by rating washed roots, using the 1-6 Iowa Root Damage Rating Scale, of 4 plants from each replicate.

Corn Rootworm Larval Control, 1990

Abstract Granular insecticides were applied to suppress larval rootworm populations in two fields in west central Nebr. Insecticides were applied to Field I at planting on 18 May with application boxes on a John Deere Max-emerge planter. Two replications of each treatment were arranged in a randomized complete block design. T-Band applications were made in a 7.0 inch band in front of the press wheel. Banded applications were made by applying a 7.0 inch band behind the press wheel. In-furrow applications were made by applying insecticide directly into the seed furrow. Corn variety for Field I was undetermined at time of writing. Insecticides were applied to Field II at the time of irrigation furrow construction, 27 Jun, to the base of the plants, with International Harvester boxes on a Hawkins/Bridger ditching unit. Treatments in Field II were replicated 3 times and arranged in a randomized complete block design. All treatments were evaluated 13 Jul by rating washed roots, using the 1-6 Iowa State root rating scale, of 4 plants from each treatment.