Application of a semiochemical-based bail to silking corn (Zea mays L.) with a high clearance sprayer effectively reduced the numbers of northern, Diabrotica barberi Smith & Lawrence, and western, D. virgifera virgifera LeConte, corn rootworm adults during studies conducted in 1994 and 1995. Bait was applied using water volumes of 19 and 37 L/ha above canopy nozzles vs nozzles on drop lines, and nozzles on drop lines on every row vs nozzles on drop lines on alternating rows. There were no significant differences among the high clearance sprayer nozzle configurations in reducing corn rootworm beetle populations; however, nozzles on booms above the canopy were more likely to deliver discrete droplets and were less likely to break due to entanglement of the drop lines with foliage. Semiochemical insecticide-baits had minimal effects on predaceous adults of the family Coccinellidae. Seven coccinellid species were observed in the plots in 1994 and 4 species in 1995.

During the 1993 growing season, extensive rainfall fell upon field plots that had been manually infested with western corn rootworm (Diabrotica virgifera virgifera LeConte) eggs. The topography of the plot area and extensive precipitation combined to saturate the topsoil in a portion of the field during egg hatch. In this area, larval feeding (root-damage ratings) and survival to adulthood (beetle emergence) were significantly reduced when compared with the portion of the field that had unsaturated soil. Root pull resistance was also lower in the saturated portion of the field, suggesting that the soil moisture conditions that reduced western corn rootworm populations also inhibited root growth. These findings suggest that, while water saturated soil at specific times of the life cycle may reduce corn rootworm populations, the magnitude and duration of this treatment needed to achieve this reduction may adversely affect plant growth and development.

The bioactivity of soil insecticides applied at planting time to protect roots of corn from feeding damage by larvae of the Mexican corn rootworm, Diabrotica virgifera zeae Krysan and Smith, was established by removing soil samples from field plots biweekly from time of application until samples no longer induced larval mortality. This study was conducted for two consecutive years that were distinctly different. The first year, virtually no rainfall occurred from time of insecticide application through the larval feeding period, and the topsoil remained very dry. During the second year, ample rainfall occurred during this period and topsoil moisture was abundant. Even though the bioactivity of each insecticide during the ''dry year'' was significantly longer, all insecticides had significantly shorter periods of bioactivity in the alkaline soils from Texas than for the same formulations in slightly acidic midwestern soils. Results from this study may provide an explanation for the highly irregular performance of soil applied insecticides and lack of target insect control in south and central Texas.

Toxicity of insecticides typically used to protect corn roots from feeding by Mexican corn rootworm, Diabrotica virgifera zeae Krysan and Smith, larvae were established using a soil bioassay. There was a 23-fold difference in toxicity of insecticides for larvae of the Mexican corn rootworm. Carbofuran had significantly lower LC(50) and LC(95) values in an alkaline soil from south central Texas compared to a nearly neutral pH silty clay loam soil from South Dakota. Lack of ''control'' by soil applied insecticides was not due to higher tolerance by larvae of this Diabrotica spp.

Studies were conducted in maize fields in eastern South Dakota in 1990 and 1992 to compare the attractiveness of several volatile, non-pheromonal semiochemicals to adult northern corn rootworms (Diabrotica barberi Smith and Lawrence). In 1990, during the late stages of maize development, eugenol and cinnamyl alcohol increased the capture of males relative to unbaited traps. Captures of females were greatest on traps baited with eugenol or cinnamyl alcohol; trans-cinnamaldehyde and 4-methoxyphenethanol also increased female capture relative to unbaited traps. In 1992, sticky traps baited with eugenol or cinnamyl alcohol captured more beetles than those baited with 4-methoxyphenethanol, except during silking, when no compound significantly attracted males and the capture of females averaged less than 1 beetle/trap. 4-Methoxyphenethanol did significantly increase catch relative to unbaited traps until the late stages of maize development. These results contrast with previous evidence that 4-methoxyphenethanol is a superior attractant for D. barberi, and suggest that there may be regional differences in the relative attractiveness of volatile non-pheromonal compounds to D. barberi.

Using controlled infestations, the efficacy of progeny from laboratory-reared Diabrotica virgifera virgifera was compared with those from field collected D. v. virgifera over three seasons. Efficacy was determined through com root damage ratings. Root damage ratings indicated that progeny from field collected adults and from laboratory-reared adults produced similar damage. Survival to adulthood was determined with emergence cages. Emergence for each egg density by origin was the same. This study demonstrates that: 1) larvae from laboratory reared eggs are as damaging as those from field collected origins; and 2) that studies using controlled infestations of com rootworms should evaluate adult survival.

A semiochemical-based toxic bait for adult Diabrotica virgifera virgifera LeConte and D. barberi Smith and Lawrence was broadcast over three plots (1-3 ha) of maize, Zea mays L. The bait contained an insecticide (0.3% carbaryl), a feeding stimulant (cucurbitacin), and several nonpheromonal volatile attractants (totaling 0.5% of the formulation) in a dry, bran-based carrier. Bait was applied at 7-14 kg/ha (20-40 g [AI] carbaryl/ha). Relative to paired plots that were not treated with bait, numbers of D. v. virgifera and D. barberi counted on maize plants were reduced 77-85% and 55-92%, respectively, 48 h after bait applications. Likewise, relative numbers of D. v. virgifera and D. barberi captured on unbaited yellow sticky traps declined 58-91% and 67-88%, respectively, after applications of bait. Numbers of dead beetles on the ground 24 or 48 h after applications were roughly comparable to reductions in numbers of live beetles counted on plants. Activity of the bait was short-lived in the field, and populations of Diabrotica beetles in treated plots generally returned to near pretreatment levels within 1-2 wk after applications of bait. Although formulations must be improved to enhance activity and longevity, results of this study indicate that semiochemical-based baits for Diabrotica beetles are a potential tool for greatly reducing the amount of insecticide applied in maize production systems.

Survival of adult Diabrotica virgifera virgifera LeConte was monitored in field cages that contained maize plants, Zea mays L., treated with semiochemical-based toxic baits. Baits consisted of starch granules that contained a feeding stimulant (cucurbitacins) and an insecticide (0.5% carbaryl); they were sprinkled by hand over plants at a rate of 5 kg/ha. In cages of silking maize, granules of bait that were 2-4 mm in diameter were significantly less effective (27% of beetles survived 48 h) than granules from three smaller size categories (13-18% survival). In another test, however, granule size appeared to influence efficacy of baits in cages of dent-stage maize but not in cages of silking plants. Efficacy did not differ significantly among baits that were formulated to contain 0, 0.5, 2, or 8% of the "TIC" attractant (a 1:1:1 mixture of 1,2,4-trimethoxybenzene, indole, and trans-cinnamaldehyde). Granules containing TIC attracted large numbers of D. v. virgifera in open cornfields when placed in traps constructed from 59-ml pill vials. In laboratory assays, TIC initially reduced the proportion of beetles that consumed a lethal dose of bait, but the effect was not evident after granules were exposed to open air for 24 h. Starch-granule bait and a similar cereal-based bait produced 87-94% mortality of D. v. virgifera when applied carefully to leaves of maize plants, but did not measurably affect survival when an equal amount was distributed on the soil surface. The distribution of bait particles within a maize field will profoundly affect efficacy and must be considered when developing formulations and application methodology. Adding moderate amounts of TIC to a bait will probably not affect mortality of beetles that are present in treated portions of fields.

The influence of corn plant phenology on the dynamics of adult western corn rootworm, Diabrotica virgifera virgifera, populations was studied during 1988 and 1989 in com fields artificially infested with eggs. Fifty percent of adult emergence from the soil occurred by day 194 in 1988 and day 203 in 1989. In both years, adult emergence was synchronized with corn flowering, eggs were recovered in soil samples approximately four days after reproductive females were first observed in the population, and oviposition was essentially complete about 25 days after it began. The number of reproductive female beetle-days accumulating per m2 was similar in both years. Approximately two times as many eggs were laid in 1988 (1239 eggs 1m2) as in 1989 (590 eggs 1m2). The difference in egg density may have been caused by differences among years in the temporal synchrony of reproductive beetles with flowering corn. Daily survival rates of adults were high while corn was flowering; exhibited a gradual decline during grain filling; and decreased rapidly during the grain drying stage.

The effects of three different planting dates of corn and three times of infestation on the survival and subsequent fecundity, egg viability, and longevity of Diabrotica virgifera virgifera LeConte were examined under field conditions in eastern South Dakota. Earlier planting and infesting dates resulted in greater survival of adults. Male-female survivorship was affected by the treatments because only the second infestation date, despite planting date, produced a 1:1 male/female sex ratio. When corn was planted and infested at the same time on the first or third date, the sex ratios were 1:1.5 and 2:1, respectively. Female longevity was significantly affected by the latest corn planting and infestation treatment. The various treatments had little effect on fecundity of individual adults derived from the treatment plots or on the viability of their eggs.

In a 4-yr study, granular soil insecticides were applied at planting time to plots of maize, Zea mays L., infested with population densities of eggs of western corn rootworms, Diabrotica virgifera virgifera LeConte, to determine how insecticides affected survival and development of immatures and female fecundity. Survival to adulthood differed among plots treated with different insecticides as well as among years of the study. Survival was generally highest in plots treated with insecticides that were the least water soluble (chlorpyrifos and fonofos) and lowest in plots treated with compounds that were the most water soluble (carbofuran and ethoprop). Survival was highest in years when precipitation and topsoil moisture were below normal during egg hatch and larval development and lowest when these conditions were above normal. Carbofuran, ethoprop, and chlorpyrifos delayed the mean emergence date in 1981. Only carbofuran delayed emergence in 1982, and none of the insecticides affected emergence in 1985. Mean percentage of survivors that were female differed each year (range, 23.2-45.3%). Egg density did not affect sex ratio. As reflected in the significant-year-by-insecticide interaction, some insecticides were inconsistent in their effect on sex ratios. In 1981, fecundity of survivors from plots treated with carbofuran, chlorpyrifos, terbufos, and isofenphos was significantly greater than that of survivors from plots treated with phorate, fonofos, and ethoprop or that were left untreated. Insecticides did not affect timing of oviposition, which began 2 wk after emergence and peaked when beetles were 4 wk old.

AbstractUnder field conditions in eastern South Dakota, USA three different planting dates of corn and three times of egg infestation were used to imposed synchronous and asynchronous timing of corn growth and Diabrotica virgifera virgifera LeConte egg hatch and subsequent development. Median occurrence of each of the three larval stages and of the adult stage for each of the infestation‐planting treatments was determined by relating occurrence to thermal units and to days after infestation. No significant differences were found among the treatments for median occurrence of each life stage when the thermal unit approach was used. However, significant differences were found among treatments for median occurrence when the day after infestation approach was used. Consistent parameters are needed for prediction of the occurrence of life stages of this insect. Despite the various imposed conditions, this study indicates that the least variable method of predicting life stage occurrence and adult emergence of D. v. virgifera was the use of thermal unit accumulations (base 11 °C).ResuméInfluence de la date des semailles du maïs sur le développement et la phénologie de Diabrotica virgifera virgiferaDans les conditions de l'est du Sud Dakota (USA), 3 dates de semailles de maïs et 3 dates de contamination avec des œufs de D. virgifera virgifera LeConte ont servi à imposer ou à empêcher la synchronisation des éclosions des œufs et par conséquent des développements ultérieurs. Les dates médianes d'apparition de chacun des 3 stades larvaires et des imagos ont été déterminées en relation avec les unités de température et le nombre de jours depuis la contamination. Aucune différence significative entre les dates médianes d'apparition n'a été mise en évidence par utilisation des unités de température; par contre, ces différences deviennent significatives lorsque l'on utilise le nombre de jours depuis la contamination. Des paramètres fiables sont nécessaires pour prédire la présence des différentes stades de cet insecte. En dépit des différentes conditions imposées pour cette étude, la méthode la moins capricieuse pour prédire l'apparition d'un stade larvaire et l'émergence des adultes est la somme de températures avec une base de 11 °C.

Journal Article Effect of Insecticide Treatments on Root Lodging and Yields of Maize in Controlled Infestations of Western Corn Rootworms (Coleoptera: Chrysomelidae) Get access G. R. Sutter, G. R. Sutter USDA-ARS, Northern Plains Area, Northern Grain Insects Research Laboratory, Brookings, South Dakota 57006 Search for other works by this author on: Oxford Academic PubMed Google Scholar J. R. Fisher, J. R. Fisher 1 USDA-ARS, Northern Plains Area, Northern Grain Insects Research Laboratory, Brookings, South Dakota 57006 1Current address: USDA-ARS, NPA, Rangeland Insect Laboratory, South 11th Avenue, Bozeman, Mont. 59717. Search for other works by this author on: Oxford Academic PubMed Google Scholar N. C. Elliott, N. C. Elliott 2 USDA-ARS, Northern Plains Area, Northern Grain Insects Research Laboratory, Brookings, South Dakota 57006 2Current address: USDA-ARS, SPA, Plant Science and Water Conservation Laboratory, P.O. Box 1029, Stillwater, Okla. 74076. Search for other works by this author on: Oxford Academic PubMed Google Scholar T. F. Branson T. F. Branson USDA-ARS, Northern Plains Area, Northern Grain Insects Research Laboratory, Brookings, South Dakota 57006 Search for other works by this author on: Oxford Academic PubMed Google Scholar Journal of Economic Entomology, Volume 83, Issue 6, 1 December 1990, Pages 2414–2420, https://doi.org/10.1093/jee/83.6.2414 Published: 01 December 1990 Article history Received: 06 September 1989 Accepted: 31 May 1990 Published: 01 December 1990

Adult western corn rootworms, Diabrotica virgifera virgifera LeConte, were offered baits containing toxicant (carbaryl), a feeding stimulant (the curcurbitacins in powdered root of buffalo gourd, Cucurbita foetidissima H.B.K. [BGRPl), and a nonpheromonal attractant (TIC; 1,2,4-trimethoxybenzene, indole, and trans-cinnamaldehyde, 1:1:1). Components were encapsulated into starch granules or were formulated into particles by Bio- Control (Warwick, Queensland, Australia). In laboratory assays, baits with approximately 0.5% carbonyl and 3-5% BGRP effectively killed D. v. virgifera. Baits with these components plus 0.3-1.5% TIC were broadcast over maize in walk-in field cages; adult D. v. virgifera beetles were released, and surviving beetles were counted after 24-72 h. In cages treated at 2-32 kg/ha of starch bait, numbers of beetles were reduced by 69-94%, respectively, relative to untreated cages. Starch granules containing only carbaryl (8 kg/hal killed few or no beetles, but granules with carbaryl plus BGRP reduced numbers of beetles per cage by approximately 80% in 24 h. Efficacy of baits was not affected by adding 1.5% TIC or by phenology of maize in the cages. Activity of baits declined after 2-3 wks in the field; 3-wk-old baits reduced numbers of beetles per cage by approximately 45%, whereas freshly applied baits produced reductions of approximately 85%. Although not ideal, these formulations appear suitable for testing the concept of semiochemical-based baits on a larger scale.

During a 5-yr study, recommended rates of soil insecticides were applied at planting time to plots of maize, Zea mays L., located on uniform soil. Plots were infested with known populations of eggs of western corn rootworm, Diabrotica virgifera virgifera LeConte, to determine the degree to which insecticides protected roots from larval attack. Moisture in the upper 10 cm of soil and precipitation were recorded each year. In the last two years of the study, we used laboratory soil bioassays to establish larvicidal activity for each insecticide application. Our results indicated that larval feeding damage for each egg density was consistent in the untreated plots every year except 1984, when excessive rainfall that coincided with egg hatch apparently prevented larval establishment. Root protection provided by insecticide treatments was highly variable. Significant interactions that occurred between root worm infestation rate, year, and insecticide treatments suggested that the level of root protection provided by insecticides was dependent on year (edaphic and environmental factors) and pest population density. The laboratory bioassays indicated dramatic differences in persistence of some insecticide treatments applied when maize was planted.

A field study with controlled infestations of eggs of the western corn rootworm, Diabrotica virgifera virgifera LeConte, was conducted over a four-year period. The percentage of insects surviving to adulthood was significantly reduced at an infestation of 2400 eggs per 0.3 m of row compared with rates of 300, 600, and 1200 eggs per 0.3 m of row. Survival to adulthood varied among years suggesting that biotic or abiotic factors in the soil, possibly soil moisture, may have influenced survival. The Julian date associated with mean adult emergence was significantly later at the 2400 egg infestation rate than at lower rates. These results suggest that immature survival is reduced and developmental time increased at high population densities.

The described rearing method approximates the natural environment for larvae of Diabrotica virgifera virgifera LeConte more closely than do previous methods: eggs are in close proximity to corn roots when they hatch, neonate larvae are not handled, and larvae are reared in soil. In addition, the method is less labor-intensive. Our quality-control samples indicate a mean (±SD) return of 53.0% ±5.0 from eggs to adults and a mean return of 72.7% ± 10.2 from neonate larvae to adults. Adult females have a mean head-capsule width of 1.22 mm ± 0.4. Moreover, our new method for handling adults, both before and during mating, has resulted in a significant improvement in mating; percentage of mated females increased from a mean of 66 to a mean of 95%.