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.
Synergy between chemical and biological control in the IPM of currant-lettuce aphid (Nasonovia ribisnigri) in Canterbury, New Zealand
Abstract Field trials were conducted at four Canterbury, New Zealand locations in 2005–06 to determine if the synergistic effects of biological control by natural enemies and standard drenching techniques controlled lettuce aphid populations throughout the entire growing season. Chemical usage significantly lowered aphid densities in the outer, wrapper and heart leaves compared to control plants at most times. However, in mid-summer, natural enemies, such as the brown lacewing (Micromus tasmaniae), 11-spotted ladybird beetle (Coccinella undecimpunctata) and small hoverfly larvae (Melanostoma fasciatum), were more than sufficient to control lettuce aphids without the use of insecticides. Drenching, in addition to natural enemy attack, appears to be required in early spring and late summer to maintain very low levels of lettuce aphid. Given the potential for imidacloprid resistance to develop, it may be advisable to restrict drenches to these key periods in order to allow populations of natural enemies to maintain control of prey populations. We recommend industry support the validation of action thresholds across different regions within New Zealand and focus on the seasonal biology of predators to assist growers with the sustainable long-term control of lettuce aphids. The inclusion of additional data into an economic model to compare pest damage with predator loading would be useful for growers in managing aphid problems. These results will assist in the continued improvement and development of a sustainable IPM strategy for lettuce aphids in New Zealand and elsewhere.
The impact of aphicide drenches on Micromus tasmaniae (Walker) (Neuroptera: Hemerobiidae) and the implications for pest control in lettuce crops
The recent arrival of lettuce aphid (Nasonovia ribis-nigri (Mosley) ) in Australia has resulted in a pesticide-based protection program based upon seedling drenches of imidacloprid being promoted by many advisory agencies and accepted by growers as the only option available. This has caused concern about potential for incompatibility with existing integrated pest management programs for other pests in lettuce. Two neonicotinoid insecticides, imidacloprid (Confidor 200SC) and thiamethoxam (Actara), were applied to lettuce seedlings by drenching. A model aphid (Macrosiphum euphorbiae (Thomas) ), used because N. ribis-nigri was not present in mainland Australia at that time, was periodically released onto the seedlings over 10 weeks. The effects of imidacloprid and thiamethoxam on larvae of predatory brown lacewings (Micromus tasmaniae (Walker) ) which fed on the aphids were measured over 10 weeks by bioassay. Imidacloprid applied at a rate of 11 mL active ingredient (ai) per 1000 seedlings and thiamethoxam applied at 0.5 g ai per 1000 seedlings were highly toxic to M. tasmaniae that consumed aphids from the seedlings for up to 4 weeks after application. A 1/10 rate of imidacloprid (1.1 mL ai per 1000 seedlings) caused moderate toxicity for 3 weeks, and was then harmless to M. tasmaniae. Thiamethoxam and the high rate of imidacloprid caused almost complete mortality of aphids for about 6 weeks after application, and the low rate of imidacloprid displayed similarly high activity for about 3 weeks.