Larry P. Pedigo

Effects of Weed Management Systems on Canopy Insects in Herbicide-Resistant Soybeans

Abstract The effects of transgenic herbicide-resistant soybean varieties and their corresponding weed management strategies on canopy insects were examined in studies at two locations in Iowa in 1997 and 1998. Weed management systems that allowed more weed escapes typically had higher insect population densities. However, systems with fewer weeds seemingly were preferred by potato leafhoppers. Bean leaf beetles and potato leafhoppers showed preferences for certain soybean varieties, but these effects were attributed to soybean plant height. These findings indicate that although the transgenic soybean varieties did not strongly affect insect populations, weed management systems can affect insect populations in soybean. However, this impact is likely related more to weed suppression effectiveness than to a direct effect of the herbicides on the insects.

Economic injury level concepts and their use in sustaining environmental quality

Abstract The economic injury level (EIL) concept is the basis for decisions in most integrated pest management (IPM) programs. These IPM programs are fundamentally different from control approaches to handling pest problems by focusing on tolerating pest effects. The EIL is essential for IPM programs because it indicates which levels of pests can be tolerated and which cannot. By increasing our ability to tolerate pests, it is possible to eliminate or reduce the need for management tactics. Moreover, we can improve and maintain environmental quality through better decisions on the use of those tactics. EILs help maintain environmental quality by reducing unnecessary use of management tactics, especially of pesticides. However, including environmental considerations explicitly in the decision-making process could greatly improve the ability of IPM to sustain environmental quality. To understand the current and potential roles of EILs in maintaining environmental quality, it is first necessary to examine the components of the EIL. These include economic damage, economic thresholds, and the EIL itself. Of these elements, the economic threshold has been the most problematic because it depends on predictions of pest population growth rates. Most approaches to economic threshold development can be grouped into subjective and objective categories, with the objective category being based on calculated EILs. Increased availability of calculated EILs and their related economic thresholds would reduce unnecessary use of management tactics. Additional attention to the EIL itself could further improve the responsiveness of IPM programs to environmental concerns. In particular, by including direct considerations of environmental factors, as well as economic and biological information, into EILs, it would be possible to develop environmental EILs. An environmental EIL evaluates a management tactic based not only on its direct costs and benefits to the user but also on its effects on the environment. Activities to support greater environmental responsiveness in the EIL include accounting for environmental costs in the C variable, reducing damage per injury by increasing plant tolerance in the D variable, and developing an effective, yet environmentally responsible, K variable by reducing pesticide application rates. Focusing research efforts on these aspects of the EIL concept offers the prospect of improved responsiveness to environmental concerns in IPM, without a reliance on new tactics.

Fungicidal Effects of Glyphosate and Glyphosate Formulations on Four Species of Entomopathogenic Fungi

Abstract Fungicidal effects of glyphosate and glyphosate formulations on the entomopathogenic fungi Beauveria bassiana (Balsamo) Vuillemin, Metarhizium anisopliae (Metchnikoff) Sorokin, Nomuraea rileyi (Farlow) Samson, and Neozygites floridana Weiser & Muma were evaluated under laboratory conditions. Media previously inoculated with entomopathogenic fungi were exposed to distilled water, glyphosate (active ingredient), seven glyphosate formulations, and five blank formulations (carrier only). The fungicidal activity was determined by measuring inhibition in mycelial growth in solid media (B. bassiana, M. anisopliae, and N. rileyi), and spore concentration in liquid medium (N. floridana). Glyphosate did not have fungicidal activity against any of the fungi tested. Fungicidal properties of glyphosate formulations varied among fungal species. Neozygites floridana and M. anisopliae were susceptible to all glyphosate formulations. RoundUp Ready-To-Use was consistently the glyphosate formulation with one of the strongest fungicidal properties. Fungicidal activity of some formulations had a synergistic effect with glyphosate. RoundUp Original was the only formulation that did not show any interaction on fungicidal activity between glyphosate and the formulation. The results showed that the four fungi tested are susceptible to various glyphosate formulations when exposed to field concentrations. RESUMEN Los effectos fungicidas de glyphosato y formulaciones de glyphosato fueron evaluados bajo condiciones de laboratorio en los hongos entomopatógenos Beauveria bassiana (Balsamo) Vuillemin, Metarhizium anisopliae (Metchnikoff) Sorokin, Nomuraea rileyi (Farlow) Samson, y Neozygites floridana Weiser & Muma. Medios de crecimiento previamente inoculados con los hongos entomopatógenos fueron expuestos a agua destilada, glyphosato (ingrediente activo), siete formulaciones de glyphosato, y cinco formulaciones sin ingrediente activo (solamente inertes). La actividad fungicida fue determinada en medios de crecimiento sólidos midiendo el área de crecimiento miceliar inhibido (B. bassiana, M. anisopliae, y N. rileyi), y en el medio de crecimiento líquido determinando la densidad de esporas (N. floridana). Glyphosato no tuvo ninguna actividad fungicida para ninguno de los hongos. Las propiedades fungicidas de las formulaciones de glyphosato diferieron entre las especies de hongos. Neozygites floridana y M. anisopliae fueron susceptibles a todas las formulaciones de glyphosato. RoundUp Ready-To-Use fue consistentemente una de las formulaciones de glyphosato con propiedades fungicidas mas fuertes. La actividad fungicida de varias formulaciones tuvieron un effecto sinérgico con glyphosato. RoundUp Original fue la única formulación que no mostró ninguna interacción en la actividad fungicida entre glyphosato y la formulacion. Los resultados muestran que los cuatro hongos en este estudio son susceptibles a varias de las formulaciones de glyphosato cuando expuestos a concentraciones de campo.

A Predictive Model for the Survival of Overwintering Bean Leaf Beetles (Coleoptera: Chrysomelidae)

Abstract Survival of overwintering bean leaf beetle, Cerotoma trifurcata (Förster), populations in woodland and soybean habitats was studied for three consecutive winters from 1996 through 1999. Four locations of soybean fields and three locations of woodlands were studied at Iowa State University farms near Ames, IA. In the winter of 1996–1997, woodlands and soybean fields were sampled monthly from mid-November through mid-April of the following year, whereas in the winters of 1997–1998 and 1998–1999, both habitats were sampled twice a month from mid-October through mid-April. On each sampling date, three sampling units (1 m2) of leaf litter or crop residue was randomly collected from soybean fields and woodlands at each location. The collected samples were bagged and returned to the laboratory for estimates of overwintering beetles. The beetles obtained from the samples were kept in individual sample bags at 24°C for 100 h to assess their survival. The beetles were then kept in a freezer at −15°C for investigation of potential fungal pathogens and ectoparasitic mites. During the three-winter study, the mortality of overwintering beetles in soybean fields (77.04–88.89%) was higher than those in woodlands (48.9–82.29%). The effects of winter temperature on beetle mortality and a predictive model for the overwintering beetle survivorship are discussed.

Effects of Transgenic Herbicide-Resistant Soybean Varieties and Systems on Surface-Active Springtails (Entognatha: Collembola)

Abstract The degree of abundance and diversity of springtails (order Collembola) often indicates the extent of disturbance by various agricultural practices. We examined how transgenic herbicide-tolerant soybean varieties and their associated weed management systems affect the abundance of 21 surface-active springtail species during three successive soybean growing seasons. With six soybean varieties (three transgenic, three nontransgenic), we tested three weed management systems: (1) targeted application of specific herbicides to the corresponding tolerant transgenic varieties; (2) conventional pre- and postemergence herbicide applications; and (3) mechanical cultivation. Each method posed its own potential costs and benefits to springtails. In targeted plots, springtail numbers were similar to or higher than those in conventional plots, suggesting that the later and repeated targeted applications to transgenic soybeans do not adversely affect springtail numbers in the short term. We attributed the observed treatment effect differences on springtail numbers to resultant differences in weed cover and degree of soil disturbance (indirect effects), rather than to any direct toxic effects of the herbicides. The treatments affected some species but not others; most of the affected species responded similarly to differences in weed treatment. Our results overall suggested no deleterious short-term effects of transgenic soybean targeted weed-management systems on abundance of the springtail species examined.

Population Dynamics of Bean Leaf Beetles (Coleoptera: Chrysomelidae) in Central Iowa

Abstract Linear regression and multiple regression stepwise selection procedure analyses were used to relate population fluctuations of bean leaf beetle, Cerotoma trifurcata (Förster), in soybean, Glycine max (L.) Merrill, to climatic factors. The study was conducted at Iowa State University Johnson farm near Ames, Iowa for 10 consecutive years (1989 through 1998, inclusively). Bean leaf beetle adults were sampled twice weekly from emergence to mature stages of soybean. Linear regression showed a strong relationship between first- and second-generation adult densities during the growing season, whereas multiple regression analyses indicated a high correlation between second-generation adult densities and environmental factors, including temperature and precipitation. The analyses demonstrated that field history of beetle density is important for pest management. The major factors that affected the population dynamics of bean leaf beetle and regression models for the beetle density prediction are discussed.

Potential Primary Inoculum Sources of Bean pod mottle virus in Iowa

A survey of Bean pod mottle virus (BPMV) in Iowa counties was conducted and the virus was found throughout the state. A long-term monitoring study (1989 to 2002) of the main BPMV vector, the bean leaf beetle, Cerotoma trifurcata, indicated that, in 2002, populations reached the highest abundance recorded in 14 years. Three potential sources for an early season primary inoculum source were found: (i) soybean (Glycine max) seed, (ii) overwintered bean leaf beetles, and (iii) alternate BPMV host plants. Examination of 5,804 and 8,064 soybean seedlings of two cultivars yielded 0 and 3 seedlings, respectively, infected with BPMV. In a separate test, BPMV was detected in mottled and nonmottled soybean seed. Some mottled seed did not contain BPMV, indicating that soybean seed coat mottling is an unreliable indicator for presence of the virus in seed. Of 194 naturally overwintered bean leaf beetles, only 1 transmitted BPMV to soybean. BPMV was detected serologically only in 1 alternate host, Desmodium canadense, out of 23 naturally occurring plant species collected from the field. The three inoculum sources discovered in Iowa in this study could be important primary sources when vector populations are high and indicate starting points for future epidemiological investigations.

Effect of Trichome Density on Soybean Pod Feeding by Adult Bean Leaf Beetles (Coleoptera: Chrysomelidae)

Abstract The role of soybean, Glycine max (L.) Merrill, pod trichomes on feeding by adult bean leaf beetles, Cerotoma trifurcata (Förster), was evaluated under laboratory and field conditions during 1997 and 1998. Three Clark isolines and ‘Corsoy 79’ were used to compare the feeding preference of the beetle on pods with different trichome densities. The three Clark isolines, including densely pubescent, sparsely pubescent, and glabrous, were isogenic except for trichome density. The trichome densities on the pods of Clark densely pubescent, Clark sparsely pubescent, and Corsoy 79 were significantly different. In no-choice tests, under both laboratory and field conditions, the feeding on pods of Clark densely pubescent was significantly lower than that on the other isolines. In the choice test of Corsoy 79 conducted in the laboratory and field, when the trichomes on one of the two pods were shaven off, the feeding on the shaven pods was significantly higher than that of the intact ones. In the choice test among Clark isolines under laboratory condition, the result was significant, with the lowest feeding on the densely pubescent pods. This study demonstrates that densely pubescent soybean has the potential to resist bean leaf beetle feeding on pods.

Cold Tolerance of Overwintering Bean Leaf Beetles (Coleoptera: Chrysomelidae)

Abstract Laboratory studies were conducted during the winters of 1996 and 1997 to determine the cold tolerance of overwintering adult bean leaf beetles, Cerotoma trifurcata (Förster). Second-generation adults were collected during early September from soybean fields near Ames, IA. The collected beetles were reared at 24°C with a photoperiod of 10:14 (L:D) h in the laboratory for 3 wk. The beetles were transferred to a chamber with high relative humidity at 5°C and photoperiod of 9:15 (L:D) h for evacuation of gut contents dangling. After 2 wk at 5°C, the beetles were selected randomly and subjected to cold baths maintained at temperatures, including −15, −10, −5, 0, and 5°C. At certain time intervals, beetles were retrieved from the cold baths for the observation of mortality. More than 50% of the adult beetles survived over hundreds of hours at −5, 0, and 5°C, whereas most of the beetles died after 15 min at −10 and −15°C. The results indicated that the critical temperature range causing significant mortality of the overwintering beetles would be between −5 and −10°C. Moreover, during the two-winter study the daily leaf litter mean temperature recorded in woodland mainly stayed at above −5°C. The results showed that hibernating in the leaf litter of woodland allowed the beetles to avoid extreme temperature fluctuations during winter.

Comparison of Estimated Costs and Benefits of Site-Specific Versus Uniform Management for the Bean Leaf Beetle in Soybean

Six soybean, Glycine max (L.) Merrill, fields were examined to compare estimated costs and benefits for uniform and site-specific management (SSM) programs for the bean leaf beetle, Cerotoma trifurcata (Forster). Beetle counts and soybean yield were field-collected, insecticide and sampling costs were estimated. In five fields, site-specific management produced only slightly greater return. The inclusion of sampling costs in each scenario resulted in higher return for the uniform scenario. The uniform management scenario estimated pest pressure as well as the site-specific scenario when beetle populations were high or low. In one field with moderate pest pressure, the SSM scenario would have increased insecticide use. The estimations in this study are based on hypothetical scenarios and the application equipment to target insecticides based on map coordinates is not readily available. The economic estimations provide examples of current limitations of site-specific management that need to be addressed before this technology becomes valuable for soybean insect management.