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.
Temperature-dependent life table parameters of Aphidius matricariae ( Hym . : Braconidae ) , an important parasitoid of the currant lettuce dae ) i ( Hem . : Aphid Nasonovia ribisnigri aphid
The effects of temperature on demographic parameters of the aphid parasitoid, Aphidius matricariae (Haliday), on lettuce aphid, Nasonovia ribisnigri Mosely, were studied at five constant temperatures, 15, 20, 22, 25 and 27 ± 1°C, 65 ± 5% RH and a photoperiod of 16L: 8D h. The life table parameters were estimated using age-stage, two-sex life table procedure. Moreover, the bootstrap method was used for estimating variance, mean and standard error of the population growth parameters, at studied temperatures. All estimated parameters were considerably affected by temperature. Accordingly, nymphs of N. ribisnigri could not settle and survive on the leaves at 27°C. Thus, mummies were not formed. Developmental time of the parasitoid wasp decreased with increasing temperature from 21.21 days at 15°C to 11.19 days at 25°C. The oviposition periods were 15.21±0.12, 9.47±0.10, 5.91±0.05 and 5.6±0.09 days at the temperatures of 15, 20, 22 and 25°C. Similarly, average number of mummies was 249.74±12.6, 192.72±12.8, 105.15±6.2 and 102.3±7.0, at the mentioned temperatures, respectively. The highest intrinsic rate of increase (r) and finite rate of increase (λ) were estimated to be 0.251 and 1.285 (d) at 20°C, respectively. However, there was no significant difference among the estimated values regarding intrinsic rate of increase and finite rate of increase at the temperatures of 20, 22 and 25°C. The highest value of net reproductive rate (R0) was 123.06 (offspring/individual) at 15°C and the shortest mean generation time (T) was 13.98 (day) at 25°C. According to the obtained results, the temperatures 20-25°C, can be considered as optimal temperature range for the biological control of the currant lettuce aphid by using A. matricariae.
TESTING FOR PESTICIDE RESISTANCE IN LETTUCE APHID
Lettuce aphid, Nasonovia ribisnigri Mosely (Hemiptera: Aphidoidae), was first detected in New Zealand on lettuce (Lactuca sativa) in Christchurch in March 2002. Lettuce growers immediately found the new pest difficult to control using insecticides. Failure of insecticides to provide adequate control could have been due to inadequate spray coverage because lettuce aphid prefers to feed in the interior of the lettuce or to insecticide resistance, which has been recorded in lettuce aphid in Europe. To determine the efficacy of topically applied insecticides on lettuce aphid in New Zealand, 12 pesticides were applied at recommended field rates with a Potter tower to apterous lettuce aphids on lettuce discs imbedded in agar. Only acephate (80.6%) and methomyl (90.7%) failed to give complete mortality after 48 hours at 18˚C. The concentration-mortality relationships for acephate, methomyl and pirimicarb were estimated and compared with data from Europe.