Jeffrey A. Davis

Sweetpotato Viruses: 15 Years of Progress on Understanding and Managing Complex Diseases

Sweetpotato is a member of the morning glory family that is thought to have originated in Central or South America but also has a secondary center of diversity in the southwest Pacific islands. It is grown in all tropical and subtropical areas of the world and consistently ranks among the 10 most important food crops worldwide on the basis of dry weight produced, yielding about 130 million metric tons per year on about 9 million hectares. Sweetpotato is an important crop for food security. It has been relied on as a source of calories in many circumstances. Vines and/or storage roots can be used for direct human consumption or animal feed. Growing awareness of health benefits attributed to sweetpotato has stimulated renewed interest in the crop. Orange-fleshed cultivars, a source of vitamin A, were introduced to developing countries with hope that they would replace the white-flesh varieties and help alleviate vitamin A deficiencies. In East Africa, sweetpotato virus disease, which is caused by the synergistic interaction of the whitefly-transmitted crinivirus and the aphid-transmitted potyvirus, can cause losses of 80 to 90% in many high-yielding genotypes. During the past 15 years, as molecular methods have been adopted, much has been learned about the composition of the sweetpotato virus complexes, the effects of virus diseases on production systems, the biology of the virus–plant interaction, and management approaches to sweetpotato virus diseases. This article is intended to summarize what has been learned since earlier reviews, integrate knowledge gleaned from experiences in tropical and temperate production systems, and suggest courses of action to develop sustainable management programs for these diseases.

Effects of High and Fluctuating Temperatures on Myzus persicae (Hemiptera: Aphididae)

Abstract Development of green peach aphid, Myzus persicae (Sulzer), was modeled under high and fluctuating temperatures to determine what effects an increase in mean summer temperatures of 2.5–3.5°C would have on green peach aphid populations in Minnesota and North Dakota. Green peach aphid developed faster and had greater fecundity under fluctuating conditions. The constant temperature model failed to predict observed development under fluctuating temperatures (R2 = 0.01). Optimal temperature for green peach aphid population growth was 26.7°C. The lower and upper developmental thresholds were 6.5 and 37.3°C, respectively. Under optimal conditions, intrinsic rate of increase was 0.356, and population doubling time was 1.95 d. At optimal fluctuating temperature conditions, one female aphid produced 12.2 progeny each week while under the most favorable constant temperature conditions; each female aphid produced only 5.9 progeny. Green peach aphid was able to survive 1 h each day above its calculated lethal death point of 38.5°C. It seems that, in midwestern North America, green peach aphid would benefit from an increase in mean summer temperature (20°C) of 2.5–3.5°C.

Soybean Aphid, Aphis glycines Matsumura, a New Vector of Potato Virus Y in Potato

Soybean aphid (Aphis glycines Matsumura), an exotic species first discovered in the North Central region of the United States in 2000, is a competent vector of severalPotyviridae. Soybean aphid has high fecundity and produces alatae (winged morphs) readily, characteristics typical of proficient virus vectors. When soybean aphids were exposed toPotato virus Y (PVY)-infected potato plants and then clip-caged on healthy potato plants in groups of five or as single aphids, PVY transmission ranged from 14% to 75% across all experiments. PVYo, PVYn, and PVPntn strains were transmitted by soybean aphid.ResumenEl áfido de la soya (Aphid glycines Matsumura), especie exótica que fuera descubierta en la región norcentral de los Estados Unidos en el año 2000 es un vector de variosPotyviridae. El áfido de la soya tiene una alta fecundidad y produce rápidamente formas aladas, característica típica de los vectores de virus. Cuando los áfidos de la soya se pusieron en contacto con el virus Y (PVY) en plantas de papa infectadas y luego se insertaron en plantas sanas en grupos de cinco o como áfidos solitarios, la trasmisión de PVY fue de 14% a 75% en todos los experimentos. Las variantes PVYo, PVYn y PVYntn fueron trasmitidas por el áfido de la soya.

Species Composition and Seasonal Abundance of Stink Bugs (Hemiptera: Pentatomidae) in Louisiana Soybean

ABSTRACT In Louisiana during the last decade, the redbanded stink bug, Piezodorus guildinii (Westwood), has become a significant and yield-limiting pest of soybean. The redbanded stink bug was previously reported in the United States in 1892, but was never considered an economically important pest until recently. Soybeans representing four maturity groups (MG) III, IV, V, and VI were sampled weekly from beginning bloom (R1) to physiological maturity (R8) during 2008–2010 at five locations across Louisiana to determine the Pentatomidae composition. In total, 13,146 stink bugs were captured and subsequently identified to species. The predominant species included the redbanded stink bug (54.2%); southern green stink bug (27.1%), Nezara viridula L.; brown stink bug (6.6%), Euschistus servus (Say); and green stink bug (5.5%), Acrosternum hilare (Say). Redbanded stink bug comprised the largest percentage of the complex collected at four of the five survey sites. Numbers exceeding action thresholds of this stink bug complex were only detected during R4 to R7 growth stages. Redbanded stink bug accounted for the largest percentage of the stink bug complex in early maturing soybean varieties (MG III [86%] and IV [60%]) and declined in later maturing soybeans (MG V [54%] and VI [50%]). The redbanded stink bug was initially identified in southern Louisiana during 2000 and had been reported in all soybean producing regions in Louisiana by 2006. This survey is the first to report the redbanded stink bug as a predominant pest of soybeans from locations within the United States.

Resistance to green peach aphid,Myzus persicae (Sulzer), and potato aphid,Macrosiphum euphorbiae (Thomas), in potato cultivars

Previous research suggests that extant potato cultivars offer little promise as sources of useful aphid resistance. However, few prior studies have critically measured the effects of host cultivar on aphid age-dependent life table statistics or related these measures to field performance. Therefore, a comprehensive field and greenhouse study was undertaken to assess 49 commercial potato cultivars, primarily of North American origin, for resistance to green peach aphid,Myzus persicae (Sulzer), and potato aphid,Macrosiphum euphorbiae (Thomas). Cultivars were found to show considerable differences in resistances to each aphid species, but these resistances were not significantly correlated (R2=0.032). In greenhouse life table studies, the intrinsic rate of increase (rm) of green peach aphid was lowest (0.167) on cv Russet Norkotah and highest (0.350) on cv Red La Soda. Potato aphidrm was lowest (0.122) on cv Aracy and highest (0.229) on cv Irish Cobbler. Among cultivars, year of release, maturity class and yield potential did not significantly influence number of progeny per female per day for either green peach aphid or potato aphid (P=0.987 and 0.954, respectively). In field trials (2004 and 2005), yield potential was significantly correlated with green peach aphid counts, (P=0.006 and <0.0001, respectively). Aphid/predator population models using aK value of 15.2 predicted that following colonization green peach aphid populations would remain stable for 20 days on Russet Norkotah (resistant), whereas on Red La Soda (susceptible) populations would reach over 54,000. With non-persistent foliar insecticides as the only control, population models indicated that three applications would be necessary to maintain green peach aphid below the Minnesota recommended action threshold on Red La Soda for 21 days, while just one application would be needed for green peach aphid on Russet Norkotah. In combination with biological control or insecticide use, this resistance could provide substantial control while reducing the reliance on pesticides.ResumenInvestigaciones previas sugieren que los cultivares existentes ofrecen muy pocas posibilidades como fuente útil de resistencia a los áfidos. Sin embargo, pocos estudios previos han medido crítica y estadísticamente los efectos del cultivar hospedante sobre la edad del áfido o han relacionado estas medidas con el comportamiento en el campo. Por consiguiente, se realizaron estudios detallados de invernadero y campo para probar 49 cultivares de papa, principalmente de origen norteamericano para resistencia al áfido verde del melocotón,Myzus persicae (Sulzer) y al áfido de la papaMacrosiphum euphorbiae (Thomas). Los cultivares mostraron considerables diferencias en resistencia a cada especie de áfido, pero estas resistencias no estuvieron significativamente correlacionadas (R2= 0.032). En estudios de invernadero sobre edad, la tasa intrínseca de incremento (rm) del áfido verde fue más baja (0.167) en el cv Russet Norkotah y más alta (0.350) en el cv Red La Soda. Larm del áfido de la papa fue más baja (0.122) en el cv Aracy y más alta (0.229) en el cv Irish Cobbler. Entre cultivares, el año de liberación, la clase de madurez y el potencial de rendimiento no influenciaron significativamente el número de progenie por hembra por día, tanto para el áfido verde como para el áfido de la papa (P=0.987 y 0.954, respectivamente). En pruebas de campo (2004 y 2005), el potencial de rendimiento fue significativamente correlacionado con el número de áfidos (P=0.006 y <0.0001, respectivamente). Los modelos de población áfido/predator utilizando un valorK de 15.2 predijeron que la siguiente colonizatión de la población del áfido verde quedaría estable por 20 días en Russet Norkotah (resistente), mientras que en Red La Soda (susceptible), las poblaciones serían más de 54,000. Con insecticidas foliares no persistentes como sólo método de control, los modelos de población indicaron que serían necesarias tres aplicaciones para mantener el áfido verde por debajo del umbral de actión recomendado en Minnesota para 21 días sobre Red La Soda, mientras que una sola aplicación sería necesaria para el áfido verde sobre Russet Norkotah. En combinatión con control biológico o uso de insecticida, esta resistencia podría proporcionar control substancial reduciendo la dependencia sobre pesticidas.

Systemic effects of thiamethoxam and chlorantraniliprole seed treatments on adult Lissorhoptrus oryzophilus (Coleoptera: Curculionidae) in rice.

BACKGROUND Feeding assays using adult rice water weevils and foliage of plants treated as seeds with chlorantraniliprole and thiamethoxam at different rates were conducted to evaluate the systemic adulticidal and feeding effects. Dose-mortality relationships were determined for thiamethoxam seed treatments by combining leaf area lost due to feeding and insecticide residues analyzed by LC/MS/MS. Changes in adulticidal activity of thiamethoxam were also investigated by contrasting adult mortalities at the 5-6-leaf and tillering stages of rice. RESULTS Adult weevil mortalities and leaf consumption rates on foliage were affected in thiamethoxam but not in chlorantraniliprole treatments when rice was at the 6-7-leaf stage. The LD(50) for weevils feeding on thiamethoxam-treated rice at the 2-3-leaf stage was 447 pg insecticide weevil(-1) (95% CL: 25-830 pg weevil(-1)) but was lower (142 pg weevil(-1); 95% CL: 102-180 pg weevil(-1)) in experiments with 3-4-leaf-stage plants. Mortalities on leaves from 5-6-leaf-stage plants were consistently higher than on leaves from tillering plants. Thiamethoxam residues measured by ELISA increased with seed treatment rate and differed between plant stages. CONCLUSION The LD(50) values developed in this study are the first values for leaf-feeding insects on foliage of plants treated as seeds with thiamethoxam. The attrition of adulticidal activity of thiamethoxam in foliage of older plants may help to explain the reduced effectiveness of seed treatments against rice water larvae that is seen at later stages of rice growth in field studies. The differential activity of these two seed treatments on adults suggests that adult mortality contributes to the field efficacy of thiamethoxam but not to that of chlorantraniliprole.

Reproduction and Feeding Behavior of Myzus persicae on Four Cereals

Abstract Green peach aphid, Myzus persicae (Sulzer), does not overwinter outdoors in Minnesota; it arrives each spring on low-level jet streams from the south. After arrival, anholocylic reproduction occurs on numerous herbaceous species, including many common weeds, before movement to potato, Solanum tuberosum L. In investigating aphid feeding behavior on barrier crops, we observed winter wheat, Triticum aestivum L., colonized by green peach aphid. The Northern Great Plains grows 94,000 ha of potatoes and 20.5 million ha of small grain cereals each year, the latter potentially providing an early emerging and widely distributed green peach aphid host to influence early season potato colonization. Life tables statistics indicated green peach aphid had its highest reproductive potential among cereals on winter wheat, with rye (Secale cereale L.) > barley (Hordeum vulgare L.) > oats (Avena sativa L.). Green peach aphid was found to colonize barley, rye, and winter wheat, but not oats. Mean generation time, net reproductive rate, doubling time, and finite rate of increase were significantly different between host plants. Electrical penetration graph technique indicated mean nonpenetration duration by green peach aphid was significantly different among plant species, and significantly longer on winter wheat than on the other cereals. Mean xylem phase duration was not significantly different among plant species but sieve element salivation was of longest duration on potato. Phloem sap ingestion (E2) was also significantly different among plant species with longest E2 duration on winter wheat. This study demonstrates that this aphid can effectively use key cereals at the vegetative stage.

Effect of Host Plant, Aphid Species, and Virus Infection Status on Transmission of Sweetpotato feathery mottle virus

Sweetpotato feathery mottle virus (SPFMV) is a nonpersistently transmitted virus known to infect sweetpotato (Ipomoea batatas) and wild morning glory plants. SPFMV is vectored by various aphid species, among them the green peach aphid, Myzus persicae, and the cotton aphid, Aphis gossypii. Our objective was to determine whether differences in acquisition hosts (sweetpotato and morning glory), aphid species (M. persicae and A. gossypii), and infection status (single versus mixed infection) influenced transmission of SPFMV. SPFMV transmission from I. hederacea with a natural mixed infection by A. gossypii (39%) was significantly greater than in other host-virus combinations. Successful transmissions by A. gossypii were significantly greater compared with M. persicae in all host-virus combinations. Virus titers in source leaves were significantly greater in single- and mixed-infected I. hederacea and single-infected I. cordatotriloba compared with other host-virus combinations. There was a significant positive correlation between virus titer and transmission by both aphid species. These results suggest that, under controlled conditions, SPFMV is more readily transmitted from infected morning glory plants than from sweetpotato. Additionally, mixed-infected plants are better virus sources for transmission than single-infected, and A. gossypii is a more efficient vector than M. persicae under laboratory conditions.

Keys to the Increased Use of Host Plant Resistance in Integrated Pest Management

Abstract Host-plant resistance as a management tactic involves both the exploitation of intraspecific variation in genetically based plant resistance to breed crop varieties that support lower populations of herbivores or that better tolerate injury by herbivores and the integration of said varieties with other management tactics such as insecticide applications and biological control. There are several barriers to the increased development and use of resistant cultivars in IPM. Many of these barriers arise from the complex genetic and phenotypic nature of plant resistance. In addition, insufficient attention has been given to the integration of plant resistance with other IPM tactics, and to quantifying the benefits of plant resistance in multi-tactic IPM programs. Three keys to overcoming these barriers are described: increased understanding of the causal bases of plant resistance, increased application of modern genetic tools, and a more quantitative approach to implementing host-plant resistance.

Susceptibility of Southern Green Stink Bug and Redbanded Stink Bug to Insecticides in Soybean Field Experiments and Laboratory Bioassays

Abstract Recently the redbanded stink bug, Piezodorus guildinii (Westwood), has become a major pest of soybean, Glycine max (L.) Merrill, in Louisiana and has begun to infest soybeans throughout the Mid-South. Soybean industry and cooperative extension personnel have reported differential susceptibility between the southern green stink bug, Nezara viridula L., and redbanded stink bug treated with currently labeled insecticides in commercial production fields. To address their concern, we evaluated susceptibility of southern green stink bug and redbanded stink bug to insecticides in field and laboratory experiments. Insecticide field efficacy experiments during 5 years indicated control with pyrethroids was 94.4 ± 1.3% for southern green stink bug and 75.1 ± 1.9% for redbanded stink bug, with organophosphates, 89.8 ± 2.7% for southern green stink bug, and 84.8 ± 1.7% for redbanded stink bug, and with neonicotinoid insecticides, 78.0 ± 5.6 and 63.2 ± 6.0%, respectively. Insecticide pre-mixtures or product combinations provided 98.6 ± 0.6 and 83.8 ± 2.4% control, respectively. Bioassays of adults in vials validated results in the field. Pyrethroid and organophosphate LC50 values ranged from 0.02 to 2.36 µg per vial for southern green stink bug and 0.21 to 4.86 µg per vial for redbanded stink bug. The redbanded stink bug was four to eight-fold less susceptible to pyrethroids and two to eight-fold less susceptible to organophosphates than was the southern green stink bug.