B.M. Diaz

Life Table and Population Parameters of Nasonovia ribisnigri (Homoptera: Aphididae) at Different Constant Temperatures

Abstract Age-specific life tables of the lettuce aphid, Nasonovia ribisnigri (Mosley), feeding on lettuce were determined at different constant temperatures under controlled conditions. Our results showed that the proportion of alates increased with increasing temperatures, remaining below 7% at 16°C and increasing to 40–57% at a temperature above 20°C. The longest developmental time of apterous aphids was obtained at 8°C (31.5 d), and the shortest was at 26°C (6.2 d), whereas the developmental time of alates was always 0.7–1.1 d longer than for apterous. Most aphids needed four instars to reach adult stage, but at 8, 26, and 28°C, many individuals passed through five or six molts. Age-specific survivorship (lx) was always above 90% at the temperature range of 16–24°C. Mortality rate (qx) was rather low but constant at 8°C. However, mortality was high at 28°C and occurred mainly in the last nymphal instars and adult stage. Unexpectedly, no nymphs were produced by the adult morphs at 28°C, but effective fecundity was high at 8°C. Fecundity for alates was always lower than for the apterous aphids at the same temperature. The largest intrinsic rate of natural increase (rm), and the mean relative growth rate (RGR) occurred at 24°C, for both apterous and alate morphs, and the lowest at 8°C. Our results show that lettuce aphid is better adapted to survive and reproduce at low (8°C) than at high (28°C) temperatures, and its best performance occurred at 20–24°C.

Temperature thresholds and thermal requirements for development of Nasonovia ribisnigri (Hemiptera: Aphididae).

Abstract Early detection of Nasonovia ribisnigri (Mosley) (Hemiptera: Aphididae) on lettuce is of primary importance for its effective control. Temperature thresholds for development of this pest were estimated using developmental rates [r(T)] at different constant temperatures (8, 12, 16, 20, 24, 26, and 28°C). Observed developmental rates data and temperature were fitted to two linear (Campbell and Muñiz and Gil) and a nonlinear (Lactin) models. Lower temperature threshold estimated by the Campbell model was 3.6°C for apterous, 4.1°C for alates, and 3.1°C for both aphid adult morphs together. Similar values of the lower temperature threshold were obtained with the Muñiz and Gil model, for apterous (4.0°C), alates (4.2°C), and both adult morphs together (3.7°C) of N. ribisnigri. Thermal requirements of N. ribisnigri to complete development were estimated by Campbell and Muñiz and Gil models for apterous in 125 and 129 DD and for both adult morphs together in 143 and 139 DD, respectively. For complete development from birth to adulthood, the alate morph needed 15–18 DD more than the apterous morph. The lower temperature threshold determined by the Lactin model was 5.3°C for alates, 2.3°C for apterous, and 1.9°C for both adult morphs together. The optimal and upper temperature thresholds were 25.2 and 33.6°C, respectively, for the alate morph, 27 and 35.9°C, respectively, for the apterous morph, and 26.1 and 35.3°C, respectively, for the two adult morphs together. The Campbell model provided the best fit to the observed developmental rates data of N. ribisnigri. This information could be incorporated in forecasting models of this pest.

Pathogenicity of hyphomycetous fungi against Cyclocephala signaticollis

Susceptibility of the white grub Cyclocephala signaticollis Burmeister (Coleoptera: Scarabaeidae: Dynastinae) larvae to seven isolates of Beauveria bassiana (Balsamo) Vuillemin, five of Metarhizium anisopliae (Metschnikoff) Sorokin and two of Paecilomyces lilacinus (Thom) Samson (Deuteromycotina: Hyphomycetes) was investigated. Among 14 fungal isolates screened the most virulent was a B. bassiana isolate (Bb 53) that caused 70% mortality of third instar larvae in 40 days after inoculation at 1 × 108 conida/ml. Strains of M. anisopliae and P. lilacinus showed low efficacy or no virulence to the target host.

Interplant movement and spatial distribution of alate and apterous morphs of Nasonovia ribisnigri (Homoptera: Aphididae) on lettuce

Knowledge on colonization modes and interplant movement of Nasonovia ribisnigri can contribute to the development of optimal control of this pest. The aim of this study was to determine the spatio-temporal distribution and the mode of spread between adult morphs of Nasonovia ribisnigri, comparing spring and autumn lettuce protected crops. The spatial and temporal pattern was analyzed using the spatial analysis by distance indices (SADIE) methodology and other related displacement indices. The population size of N. ribisnigri was greater in the autumn than in the spring growing seasons due to milder temperatures. The percentage of plants colonized by aphids was higher in spring than in autumn, showing the great dispersal potential of this aphid species independent of their population size. Differential propensity for initial displacement from the central plant was observed between adult morphs in spring, resulting in a greater ability of apterous than alate aphids to spread far away from the source plant. In autumn, both adult morphs showed an initial reduced displacement; however, the number of plants infested (≈20%) with at least one aphid at this initial time (seven days) was similar for both adult morphs and both growing seasons. Analysis of the spatial pattern of both adult morphs revealed a predominantly random distribution for both spring and autumn trials. This pattern was achieved by a prevalent random movement over the area (γ≈0.5). These results highlight the ability of the apterous N. ribisnigri to spread within greenhouse lettuce crops early in the spring, suggesting that detection of the pest by deep visual inspection is required after lettuce emergence.

Modelling Wild-Oat Density in Terms of Soil Factors: A Machine Learning Approach

Abstract.In crop fields, weed density varies spatially in non-random patterns. Initial knowledge of weed distribution would greatly improve weed management for Precision Agriculture operations. Site properties could be correlated to weed distribution, since the former vary among crop fields and also certain factors such as soil texture or nitrogen may condition the weed growth. This paper presents a method, based on artificial intelligence techniques, for inducing a model that appropriately predicts the heterogeneous distribution of wild-oat (Avena sterilis L.) in terms of some environmental variables. From several experiments, distinct rule sets have been found by applying a genetic algorithm to carry out the automatic learning process. The best rule set extracted was able to explain about 88% of weed variability.

The Development of an Economic Threshold for Nasonovia ribisnigri (Hemiptera: Aphididae) on Lettuce in Central Spain

ABSTRACT This study reports economic thresholds for the lettuce aphid Nasonovia ribisnigri (Mosley), based exclusively on cosmetic damage, that is, presence or absence of aphids at harvest time. Field trials were conducted in La Poveda Experimental Farm, Madrid (Spain) during autumn (2004 and 2005) and spring (2005 and 2006). Plants were arranged in plots and just before the formation of lettuce hearts they were infested with different densities of N. ribisnigri. Two days later, half of each plot was treated with tau-fluvalinate (Klartan24AF) and the other half remained as an untreated control. Economic thresholds were obtained from nonlinear regressions calculated between the percentage of commercial plants at the end of the crop cycle for both, treated and untreated semiplots, and the different initial densities of N. ribisnigri per plant. Two criteria were used to consider a commercial lettuce plant: a conservative estimate (0 aphids/plant) and a lax one (<5 aphids/plant). Thus, an economic threshold was established for each season and criterium. The economic thresholds that were obtained with the most and least conservative criteria were in spring 0.06 and 0.12 aphids per plant, and in autumn 0.07 and 0.13 aphids per plant, respectively. These results show that to avoid cosmetic damage, insecticide sprays are required when a very low aphid density is detected in lettuce seedlings soon after transplant.

Extracting fuzzy rules to describe weed infestations in terms of soil factors

Site-specific treatment of weeds in precision agriculture is essential to know which factors determine a high occurrence of weeds. Former statistical studies searching for relationships between individual soil variables and wild-oat occurrence found no clear linear relationship among the analyzed variables. However, farmers observations pointed to better wild oats grow in specific locations where the physical and chemical conditions are probably more conducive. Data mining is justified in applications where conventional analysis methods are not able to extract useful and task-oriented knowledge. This paper proposes a fuzzy rule-based model, generated from a supervised learning process, to describe the complex relationships among weed occurrence and some soil properties. A genetic algorithm is used to derive the set of fuzzy rules by driving a search in the space of possible solutions (or models). This linguistic model has the advantage of being both intuitive and directly interpretable by the operator involved in the field tasks.