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.
Efficacy of pest and pathogen control, yield and quality of winter lettuce crops managed with reduced pesticide applications
Abstract In conventional agriculture, lettuce crops receive large amounts of pesticides to meet stringent industrial specifications and market requirements. Pesticides are used on lettuce to ensure high-yielding, attractive products free from foreign bodies and damage. Pesticide reduction is a major challenge for lettuce growers in this context. The objective of this study was to assess the risk arising from a reduction in pesticide applications by using a combination of alternative techniques for the management of pests and diseases in winter lettuce crops. Two alternative crop protection strategies (called low-input and intermediate) were designed by prototyping and then compared to a conventional lettuce protection strategy in independent trials carried out in three locations over two successive winters. The efficacy of each strategy for pest and pathogen control, as well as lettuce yield and quality, were assessed and compared. Pesticides were reduced by 32% in the intermediate crop protection strategy, and by 48% in the low-input crop protection strategy. At least 15% of lettuces were affected by pest and pathogen damage, whatever the strategy. Among possible pests or diseases, aphids were the only biotic stress which differed significantly between strategies (9.25% of lettuces infested and 2.7% of commercial losses under the low-input strategy, as compared to 0.83% of lettuces infested and 0% of commercial losses under the conventional strategy). Globally, biotic damage was less important than abiotic damage (frost and tip burn), and resulted in low commercial losses under all strategies. Similar yields and lettuce quality were recorded under the three systems. Therefore the performances of intermediate and low-input crop protection strategies were consistent with market expectations, and possible improvements are discussed. This work provides a methodology and landmarks for the design and dissemination of cropping systems targeted to leafy vegetables and less dependent on chemical control of pests and pathogens.
Investigations on the rôle of organic matter in plant nutrition
1. As the result of decomposition of green manure in a swamp soil, the coarse fractions (including coarse and fine sand) of the surface layer (first 3 inches) decreased by 11·2 per cent. while the fine ones (silt+clay) increased by 9·8 per cent. in the course of 43 days. The effect was less pronounced in the manured soil maintained at 60 per cent. saturation, while no alteration was to be noticed in the case of the unmanured specimens. 2. Treatment with dilute hydrochloric acid prior to dispersion had no appreciable effect on the estimates of mechanical composition obtained in the different cases. Loss in solution was also comparatively small. 3. The silica-sesquioxide ratio increased slightly as the result of decomposition of green manure in swamp soil. The composition of the fine fractions was also altered, the total percentages of the oxides of iron, aluminium and silicon increasing from 90·5 to 99·3 in the course of 71 days. 4. The practical significance of the above and related observations has been discussed.