Insecticide resistance in Brazilian populations of the cotton leaf worm, Alabama argillacea

The cotton leaf worm, Alabama argillacea, is a key cotton pest in Brazil and is managed with repeated insecticide applications. Reports of insecticide control failures have recently increased, particularly with pyrethroids. The present work assessed the resistance status of A. argillacea to a number of different insecticides currently used in cotton crops. Bioassays were conducted to estimate the response of 2nd-instar A. argillacea populations to deltamethrin, chlorpyrifos, endosulfan, abamectin and spinosad. A leaf dip bioassay with diluted insecticide formulations was performed in the laboratory with five to nine populations depending on the insecticide. LC₅₀ values were estimated by probit analysis after correction for control mortality data and used to calculate the resistance ratios (RR). All assessed populations exhibited varied and significant levels of resistance to all insecticides tested, but only moderate levels of resistance to deltamethrin were observed (RR = 52.3). The LC₅₀ values for deltamethrin were higher than 30 mg/l for most populations, and above the field rate (12.5 mg/l). This suggests that the frequency of resistant individuals in these populations was likely above the critical frequency. There was low to moderate resistance to abamectin, chlorpyrifos, endosulfan and spinosad formulations (the highest RRs observed were 4.2, 8.4, 11.1 and 23.5, respectively). Despite the moderate levels of resistance to pyrethroids in A. argillacea, overall results indicate the presence of low to moderate resistance of A. argillacea to insecticides currently used against cotton pests in Brazil.     

Development of resistance to insecticides in the invasive mealybug Phenacoccus solenopsis (Hemiptera: Pseudococcidae) in Pakistan

The mealybug Phenacoccus solenopsis has been a serious pest of cotton, vegetables, ornamentals and other plants since its invasion into Pakistan in 2005. Its susceptibility to commonly-used insecticides was monitored during 2005–2013 by a nymphal dip bioassay. Initially, P. solenopsis was found susceptible to a range of insecticide classes. Lethal concentration values were particularly low for organophosphates and pyrethroids. After a year of its exposure to insecticides, P. solenopsis developed moderate to high levels of resistance to pyrethroids. After two years, resistance to organophosphates methidathion and chlorpyrifos also rose to moderate to high levels. After five years of use, resistance to pyrethroids, organophosphates, neonicotinoids, endosulfan, carbosulfan, and thiocyclam was generally high to very high. Acetamiprid resistance was slow to develop, as it reached moderate level of resistance after seven years of its extensive applications. Insecticide resistance in P. solenopsis could have been managed in Pakistan if the effective and diverse insecticides were used in rotation, along with other integrated pest management tactics, at the initial stages of resistance development.     

Monitoring of insecticide resistance in Spodoptera litura (Lepidoptera: Noctuidae) from four districts of Punjab,Pakistan to conventional and new chemistry insecticides

Studies were carried out to evaluate the resistance of Pakistani populations of the beet armyworm, Spodoptera litura (F) to several commonly used insecticides. Different field populations of S. litura from four districts of the Punjab were monitored from 2009 to 2011 for resistance to insecticides using a standard leaf dip bioassay method. For organophosphates and pyrethroids, resistance ratios compared with a susceptible Lab-Pk population were in the range of 8–109 fold for deltamethrin, 11–139 fold for cypermethrin, 19–143 fold for chlorpyrifos and 39–162 fold for profenofos. For new chemistry insecticides, resistance levels were 2–74 fold for spinosad, 4–216 fold for abamectin, 7–87 fold for indoxacarb, 2–77 fold for emamectin benzoate, 1.9–58 fold for lufenuron and 4–43 fold for methoxyfenozide. Pairwise correlation coefficients of LC₅₀ values showed a positive correlation with cross-resistance among deltamethrin, cypermethrin and chlorpyrifos, while resistance to profenofos showed correlations with resistances to other insecticides except chlorpyrifos. New chemistry insecticides showed no correlations between any of the tested insecticides. There were high to very high levels of resistance to organophosphates in most of the population, which suggested that the use of these should be avoided against this pest. Selective use of pyrethroids in several areas, including Bahawalpur and Lodhran, where the pest showed a low level of resistance, would appear to be acceptable, the new chemistry insecticides, lufenuron, methoxyfenozide, emamectin and indoxacarb had no, very low, low and moderate resistance levels against populations, respectively. These are considered to be safe to the environment and safer to natural enemies.     

Resistance in the mealybug Phenacoccus solenopsis Tinsley (Homoptera: Pseudococcidae) in Pakistan to selected organophosphate and pyrethroid insecticides

The mealybug Phenacoccus solenopsis is a destructive pest of cotton with the potential to develop resistance to most chemical classes of insecticides. Six populations of P. solenopsis from cotton crops at six different locations in Pakistan were evaluated for resistance to selected organophosphate and pyrethroid insecticides. Resistance ratios (RRs) at LC₅₀ were in the range of 2.7–13.3 fold for chlorpyrifos, 11.6–30.2 fold for profenofos and for the three pyrethroids tested were 10.6–46.4 for bifenthrin, 5.8–25.2 for deltamethrin and 4.1–25.0 for lambda-cyhalothrin. This is the first report of resistance to organophosphate and pyrethroid insecticides in Pakistani populations of P. solenopsis. Regular insecticide resistance monitoring programs are needed to prevent field control failures. Moreover, integrated approaches including the judicious use of insecticides and rotation of insecticides with different modes of action are needed to delay the development of insecticide resistance in P. solenopsis.     

Field resistance of Spodoptera litura (Fab.) to conventional insecticides in India

The present study was carried out to evaluate conventional insecticide resistance in populations of Spodoptera litura (Fab.) from seven different soybean-growing districts (Dharwad. Belgaum, Pune, Parbani, Adilabad, Hyderabad and Indore) of India. Experimental results revealed among the three chemical insecticides bioassayed, quinolphos 25 EC registered the highest LC₅₀ value (29.7 mg a.i./L) followed by chlorpyrifos 20 EC (18.3 mg a.i./L) while the lowest LC₅₀ value was found for lambda-cyhalothrin 5 EC (1.3 mg a.i./L) in a susceptible population of S. litura larvae. Evaluation of the seven different populations of S. litura from India showed that populations from Adilabad and Pune exhibited elevated LC₅₀ values for chlorpyrifos [(1622.0 mg a.i./L) and (1137.0 mg a.i./L)], quinolphos [(1892.0 mg a.i./L) and (1744.0 mg a.i./L)] and lambda-cyhalothrin [(56.4 mg a.i./L) and (41.6 mg a.i./L)], respectively. Seven different S. litura populations collected varied in their resistance ratio (RR) for three conventional insecticides used in this study. For chlorpyrifos RR values ranged from 3 to 88 fold, for quinolphos RR values ranged from 3 to 63 fold and for lambda-cyhalothrin RR values ranged from 2 to 42 fold in the seven different S. litura populations compared to the susceptible population. Based on the raised LC₅₀ values, the resistance is quite concerning for organophosphates (chlorpyrifos and quinolphos). The present study is a warning bell suggesting the cautious use of organophosphates and lambda-cyhalothrin in soybean.     

Monitoring resistance of Helicoverpa armigera to different insecticides used in cotton in Spain

Helicoverpa armigera is the key pest of cotton in Spain, resulting in many insecticide treatments against it. The resistance status of H. armigera to different insecticides currently used in cotton was evaluated in Spain in two different seasons, 1999 and 2004. Four populations were tested in total, two in each season. Toxicological bioassays were conducted in the laboratory, and performed on third instar larvae by topical application of the insecticides. LD₅₀'s were estimated by probit analysis and resistance factors (RF) were calculated at the LD₅₀ level. Four insecticides were evaluated, but only endosulfan reached a moderate resistance level (RF = 11.4), and the others (methomyl, chlorpyrifos and lambda-cyhalothrin) showed low resistance (RF between 1.9 and 6.0). Such results indicate the generally low resistance of H. armigera to most of the insecticides used against this pest in cotton in Spain. Possible explanations for this situation are discussed.     

防治水稻二化螟的高毒农药替代药剂的室内筛选

为了筛选防治水稻二化螟的高毒农药替代药剂,采用点滴法测定了7类32种药剂对不同水稻生态区二化螟种群的室内毒力。基于这些毒力数据,推荐对二化螟毒力较高的7种新型农药（氟虫腈、阿维菌素、甲氨基阿维菌素苯甲酸盐、虫酰肼、呋喃虫酰肼、氟铃脲和氟啶脲）和一些中低毒性的有机磷品种（如辛硫磷、喹硫磷、毒死蜱和哒嗪硫磷）作为大田药效试验的候选替代药剂。评估了田间二化螟对10种拟除虫菊酯类杀虫剂的敏感性,发现7种对鱼高毒的拟除虫菊酯类杀虫剂（高效氟氯氰菊酯、λ 高效氯氟氰菊酯、溴氰菊酯、高效氯氰菊酯、顺式氯氰菊酯、甲氰菊酯和S 氰戊菊酯）对二化螟的毒力高于大多数有机磷类农药。虽然我国禁止上述菊酯类杀虫剂在水稻上使用,但是浙江瑞安田间种群（RA05）已对高效氟氯氰菊酯、溴氰菊酯、高效氯氰菊酯和λ 高效氯氟氰菊酯等产生高水平抗性（抗性倍数分别为166.7、51.9、41.3和36.8倍）。对鱼低毒的3种菊酯类农药（乙氰菊酯、醚菊酯和氟硅菊酯）对抗性二化螟（RA05）的毒力高于三唑磷。因此,可考虑将这些对鱼低毒的菊酯类农药作为防治水稻二化螟的高毒农药的替代药剂和抗性治理的轮换品种。     

Field selection for esfenvalerate resistance by the Colorado potato beetle

Five insecticide application regimes were evaluated for two years to determine their influence on the development of esfenvalerate resistance by the Colorado potato beetle (CPB),Leptinotarsa decemlineata (Say). These included sequential applications of esfenvalerate; two alternated uses of esfenvalerate, azinphosmethyl, endosulfan, oxamyl, andBacillus thuringiensis or cryolite; an IPM program; and one early season esfenvalerate application. Both CPB adults and first instars were tested for esfenvalerate resistance before and after insecticide applications. The LC₅₀’s in the early 1992 season were correlated with values in the late 1991 season. However, over the winter, esfenvalerate resistance of CPB adults decreased, but was relatively stable for first instars. Esfenvalerate LC₅₀’s late in the season were generally higher than values early in the season. Increases in esfenvalerate LC₅₀’s were correlated with the number of esfenvalerate applications made in these fields during the growing season, with the greatest increase found in the sequential esfenvalerate treatment.     

High level of metaflumizone resistance and multiple insecticide resistance in field populations of Spodoptera exigua (Lepidoptera: Noctuidae) in Guangdong Province,China

Spodoptera exigua is a phytophagous pest that causes critical economic losses in vegetable crops, and insecticides are commonly used against it in vegetable growing areas. However, excessive and frequent applications of insecticides cause resistance in S. exigua. The current resistance in field populations of S. exigua collected from Huizhou, Guangdong Province, China to 12 insecticides was investigated. S. exiguahad developed very high resistance to lambda cyhalothrin (2925- to 3449-fold), chlorpyrifos (>1786-fold), emamectin benzoate (174- to 867-fold), and metaflumizone (60.3- to 942-fold). High resistance to tebufenozide (51.5- to 75.4-fold) and chlorfluazuron (60.4- to 63.0-fold) was also found. Synergism assays revealed that the resistance to metaflumizone and lambda cyhalothrin was associated with esterase and microsomal oxidases, respectively. The resistance to emamectin benzoate was not affected by detoxification enzymes inhibitors and might be conferred by other mechanisms. The selection of the field population by metaflumizone for 10generations in the laboratory resulted in a 6.1-fold increase in metaflumizone resistance but did not lead to increases in resistance to other insecticides. After metaflumizone selection, susceptibilities to spinosad and endosulfan did not change, and the susceptibilities to indoxacarb, methomyl, pyridalyl, tebufenozide, chlorfluazuron, emamectin benzoate and lambda cyhalothrin decreased slightly. However, no statistically significant differences in the resistance levels were observed among the selected population, its starting strain and the unselected strain. The resistance to chlorantraniliprole noticeably decreased in unselected strain and the strain subjected to selection for 10 generations compared with their starting strains. Lack of cross-resistance to tested insecticides suggested the involvement of multiple mechanisms of resistance and the need for wise application of these insecticides for the management of S. exigua.     

Insecticide resistance of Helicoverpa armigera to endosulfan, carbamates and organophosphates: the Spanish case

Helicoverpa armigera is a major pest on a wide range of crops in Europe, Africa, Asia and Australia. Insecticide treatments are currently indispensable for its control in almost all crops, which has resulted in insecticide resistance occuring in some situations. However, since information about insecticide resistance of H. armigera in Europe is very limited, the current resistance status of this pest was investigated in Spain from 1995 to 1999. Toxicological bioassays were conducted in the laboratory, LD₅₀'s estimated by probit analysis and resistance factors (RF) calculated at the LD₅₀ level. Eleven chemicals, including endosulfan, carbamates (carbaryl, methomyl, thiodicarb) and organophosphates (chlorpyrifos, fenitrothion, methamidophos, azinphos-methyl, trichlorphon, acephate, monocrotophos) were tested. Ninety-seven percent of insecticide–strain combinations showed susceptibility (RF=1) or low insecticide resistance (RF=2–10) (157 of 162, 97%). Moderate resistance (RF=11–18) was only recorded to carbamates (carbaryl, methomyl and/or thiodicarb) in five strains. Insecticide resistance of H. armigera in Spain was therefore not as high or widespread as in other areas of the world. Since insecticide pressure against the pest in Spain is not likely to be lower, nor IPM implementation better, than elsewhere, additional factors that potentially account for low levels of insecticide resistance of H. armigera, including migration and cropping structure leading to the existence of refugia are discussed from an agroecological perspective.     

Biological response of chilli thrips, Scirtothrips dorsalis Hood (Thysanoptera: Thripidae), to various regimes of chemical and biorational insecticides

The chilli thrips Scirtothrips dorsalis (Hood) (Thysanoptera: Thripidae), a new invasive pest in the USA, is an economically important pest of certain vegetable, ornamental and fruit crops in southern and eastern Asia, Oceania and parts of Africa. These crops cannot be protected from the pest without resorting to the use of chemical insecticides. In order to forestall or delay the development of insecticide resistance in S. dorsalis, we continued our focus on the discovery of insecticides with different modes of action for rotational use. In this study we evaluated candidate insecticides to control S. dorsalis on ‘Jalapeno’ pepper, Capsicum annuum L.; these materials belong to different IRAC mode of action classes as follows: (i) 4A – neonicotinoids, i.e., imidacloprid, thiamethoxam and dinotefuran, (ii) 5 – spinosyns, i.e., spinosad and spinetoram, (iii) 3A – pyrethroids, i.e., β-cyfluthrin, esfenvalerate, ζ-cypermethrin and λ-cyhalothrin and (iv) 8D – borax mixed together with orange oil and detergents in the TriCon^® formulation. In addition we evaluated the entomopathogenic fungus, Beauveria bassiana (Botanigard^®) alone and in combination with the borax formulation at ½ of their usual rates of application. Each of the 3 neonicotinoid insecticides when applied either as a single foliar spray or as a soil drench significantly suppressed both adults and larvae for at least 10 days; indeed imidacloprid did so for 15 days. Dinotefuran was more effective as a foliar spray than as a soil drench. Spinosyns applied as a single foliar spray, significantly suppressed both adults and larvae through 15 days after treatment (DAT). None of the 4 pyrethroids provided significant suppression of either adults or larvae. The borax formulation suppressed adults and larvae through 10 DAT. B. bassiana significantly suppressed only the larvae at 5 DAT and not at 10 DAT. This study brings the number of insecticides known to be effective against S. dorsalis to 10 and these belong to 7 different modes of action classes. The use of such insecticides in rotation belonging to different classes will help delay the development of insecticide resistance in S. dorsalis.     

Lethal and sublethal effects of buprofezin and imidacloprid on Bemisia tabaci (Hemiptera: Aleyrodidae)

The sweetpotato whitefly, Bemisia tabaci (Gennadius), has become a major threat to agriculture worldwide. The development of insecticide resistance in B. tabaci has necessitated the exploration of new management tactics. The toxicity of imidacloprid and buprofezin to various life stages of B. tabaci was determined in the laboratory. Also, the sublethal effects of both insecticides were studied on demographic and biological parameters of B. tabaci. Both insecticides were very toxic against first stage larvae of the pest with LC₅₀ values of 1.0 and 19.3 ppm for buprofezin and imidacloprid, respectively. Toxicities decreased between successive stages (LC₅₀ values ranging from 1.0 to 2854.0 ppm). The LC₅₀ values of imidacloprid for adult males, females and eggs were 11.8, 71.6 and 151.0 ppm, respectively. Buprofezin had no significant effect on adults and eggs. The sublethal concentration of imidacloprid had no significant effect on demographic and biological parameters of B. tabaci but the maximal value for the mean generation time (T) (18.8 day) was observed in imidacloprid treatment. Buprofezin significantly decreased stable population and biological parameters of B. tabaci except it did not decrease the rate of population increase or the sex ratio of offspring.     

Resistance monitoring and enzyme activity in three field populations of cowpea aphid (Aphis craccivora) from Egypt

The cowpea aphid (Aphis craccivora) is one of the most important sucking insect pests attacking certain legumes in Egypt particularly faba bean, cowpea and pea. In this study we monitored the resistance level of three field populations of A. craccivora to seven insecticides belonging to three different chemical classes (organophosphates, carbamates and neonicotinoids). The three populations were collected from three governorates in Egypt namely Dakahlia, Qalyobia and Beni Suef. Diagnostic concentrations (LC₉₀ values for susceptible strain) for each insecticide were established using a leaf dipping technique. Resistance monitoring showed that the field population from Dakahlia was highly resistant to all the tested insecticides. In a similar manner, the population from Qalyobia was also resistant to all insecticides except for fenitrothion to which only moderate resistance was observed. The field population from Beni Suef exhibited a lower level of resistance to all the seven tested insecticides.Biochemical assay showed that esterase activity in these three field populations was generally higher as the enzyme activity ratio ranged from 4.3 to 7.8 fold more than that for the susceptible strain. The activity of the other measured detoxifying enzymes (glutathione -S- transferase and mixed function oxidases) was moderate in the populations from Qalyobia and Dakahlia. Nevertheless, the enzyme activity in A. craccivora collected from Beni Suef was variable and differed slightly from the activity measured in the susceptible strain. Monitoring insecticide resistance among the three aphid populations was a proactive approach to detect any shift in insecticide efficiency. The possible occurrence of resistance in the cowpea aphid to the tested insecticides may be due to the higher activity of carboxylesterases. Further studies on the resistance mechanism to these insecticides are needed to provide insights in how to manage and delay the onset of the resistance and thus prolong the performance of insecticides against A. craccivora.     

Compatibility of entomopathogenic nematodes (Nematoda: Rhabditida) with registered insecticides for Spodoptera frugiperda (Smith, 1797) (Lepidoptera: Noctuidae) under laboratory conditions

The fall armyworm, Spodoptera frugiperda (Smith, 1797) (Lepidoptera: Noctuidae) is considered the main key pest of corn crops in Brazil. Entomopathogenic nematodes (EPNs) may be used to control this pest, applied together with other different entomopathogen agents or phytosanity products in the spraying mixture. Thus, the objective of work was to evaluate the compatibility of EPNs with different insecticides used of S. frugiperda control in laboratory conditions. Three species of EPNs (Heterorhabditis indica, Steinernema carpocapsae and Steinernema glaseri) and 18 insecticides registered to control of S. frugiperda in corn crops were tested. Compatibility of the insecticides with EPNs was evaluated by observing mortality and infectivity of infecting juveniles (IJs) 48 h after immersion in solution of the insecticide formulations. Among all insecticides tested, Lorsban™ (chlorpyrifos), Decis™ (deltamethrin), Match™ (lufenuron), Deltaphos™ (deltramethrin + triazophos), Dimilin™ (diflubenzuron), Stallion™ (gamacyhalothrin), Karate Zeon™ (lambdacyhalothrin) Tracer™ (spinosad), Vexter™ (chlorpyrifos), Galgotrin™ (cypermethrin), Certero™ (triflumuron), and Talcord™ (permethrin) were compatible (class 1) with the three nematode species tested under laboratory conditions.     

Insecticide resistance, mixture potentiation and fitness in populations of the maize weevil (Sitophilus zeamais)

High levels of pyrethroid resistance and emerging organophosphate resistance in Brazilian populations of the maize weevil Sitophilus zeamais (Coleoptera: Curculionidae) led to the registration of esfenvalerate + fenitrothion against them. Thus, a survey of physiological and behavioural resistance was carried out in 27 insect populations for two pyrethroids, esfenvalerate and permethrin, and the esfenvalerate + fenitrothion mixture. Physiological resistance to fenitrothion was also assessed, as was the potentiation of the mixture. The potential fitness cost associated with insecticide resistance was also investigated. The resistance levels were low to fenitrothion (<14.1-fold), low to moderate to the pyrethroids (1.6–70.0-fold) and low to the pyrethroid-organophosphate mixture (<5-fold) with a high heterogeneity of response among populations. The potentiation of insecticidal activity achieved with the insecticide mixture was very high (>350-fold) reinforcing its usefulness for managing weevils. There was little variation in walking behaviour (and insecticide avoidance) among populations; there was no significant variation in fitness, body mass and respiration rate among the populations of the insect. These however, displayed variable rates of grain consumption and activity of amylase and lipase. A decrease in insecticide resistance in maize weevil populations was observed relative to previous studies, but with an initial development of resistance to the insecticide mixture. Behavioural and physiological resistance were not correlated and not associated with fitness cost. Our results support the use of esfenvalerate + fenitrothion against the maize weevil, which is likely to have reduced the levels of pyrethroid resistance in field populations. However, resistance to this mixture seems to be evolving, justifying concerns regarding its use.     

Monitoring resistance of field populations of diamondback moth Plutella xylostella L. (Lepidoptera: Yponomeutidae) to five insecticides in South China: A ten-year case study

Diamondback moth Plutella xylostella (L.) is a cosmopolitan pest of crucifers and it is particularly notorious for its resistance to commonly used insecticides. To provide a basis for future resistance management strategies, this study evaluated the resistance status of this pest to five insecticides, namely abamectin, β-cypermethrin, fipronil, monosultap and phoxim, in South China from 1999 to 2009 with 4-6 populations tested each year. Laboratory bioassays using a standard leaf-dip method were conducted on 3rd instar larvae which were the progeny of field collected insects. LC₅₀ values were estimated by probit analysis and resistance factors were calculated by comparing the field populations with a fully insecticide-susceptible lab population. The results showed that the diamondback moth populations generally had low resistance to abamectin from 1999 to 2005, but that resistance increased significantly during 2007-2009 with resistance factors as high as 122.4. Resistance factors for β-cypermethrin in 35 populations tested were high to very high in all 10 years with values ranging from 32.2 to 683.6. Resistance to fipronil had a progressive increase and increased markedly from 2007 to 2009 with one resistance factor of 56.1. Resistance to monosultap was low in 2004, but increased progressively from 2007 to 2009 with one resistance factor of 129.1. Resistance to phoxim was low to moderate from 2004 to 2008, but was moderate and high in 2009 with resistance factors ranging from 58.7 to 129.1. These results indicate that resistance of diamondback moth populations to most commonly used insecticides in South China has increased and is now generally high.     

Season-long programs for control of Drosophila suzukii in southeastern U.S. blueberries

Drosophila suzukii is an invasive insect pest which has major impacts on small fruit production throughout North America. Current control programs use frequent applications of broad-spectrum insecticides. To prevent resistance, modes of action must be rotated. This study examines the acute and residual efficacy of rotational treatment programs designed to meet the needs of commercial growers in the southeastern United States, a region which experiences significant precipitation throughout the growing season. All insecticide applications had greater adult acute mortality than controls, and some had residual efficacy for three or more days after treatment. Programs had no residual effects at seven days after treatment. Within programs, organophosphates (phosmet and malathion) and pyrethroids (zeta-cypermethrin and fenpropathrin) were the most effective. Some programs also resulted in less offspring production by D. suzukii in bioassays and reduced field infestation compared to controls. Results show that effective, season-long rotational chemical treatment programs can be designed to minimize crop damage, meet exportation requirements and manage for resistance. Understanding the effect of regional climate on the performance of individual insecticide treatments and their efficacy will be critical for widespread implementation of effective control programs for D. suzukii.     

Comparative assessment of the field-susceptibility of Sesamia nonagrioides to the Cry1Ab toxin in areas with different adoption rates of Bt maize and in Bt-free areas

A three year (2003-2005) field study compared the susceptibility to the Cry1Ab toxin, expressed in Bt maize, of Mediterranean corn borer (MCB) Sesamia nonagrioides populations collected from areas with different adoption rates of Bt maize in Spain with Bt-free areas in Greece. Spain is the only European country where the cultivar Compa CB derived from the event Bt176 was commercially grown, from 1998 to 2005. The large decrease of the titer of the toxin in this cultivar at later growth stages represented the worst-case scenario for resistance development of MCB, since larvae of the second and third generations were exposed to sublethal concentrations of Cry1Ab toxin. Our data revealed that the variation in susceptibility to Cry1Ab for the MCB Spanish field populations analyzed in the three years was very low, with LC₅₀ values fluctuating between 12 and 30 ng Cry1Ab/cm², regardless of the region of origin, the type of maize (Bt or non-Bt) and the year. Furthermore, no significant differences were found when comparisons were made with a laboratory population (LC₅₀ values: 18-26 ng Cry1Ab/cm²) or with field populations from Greece (Bt-free areas), which displayed LC₅₀ values ranging between 22 and 27 ng Cry1Ab/cm². Standardizing bioassay protocols proved to be essential for obtaining comparable results. These findings suggest that resistant MCB populations did not evolve in those Spanish maize areas where Compa CB was largely cultivated for eight years, contradicting the expected rapid development of resistance under these unfavourable conditions. Additionally, our results can be used as baseline indices in post-market resistance monitoring programs if Bt maize is introduced in Greece. Further studies should continue, since the insights gained from a resistance monitoring program may help to enhance the durability of Bt maize.     

Toxicity of quinones against two-spotted spider mite and three species of aphids in laboratory and greenhouse conditions

Toxicities of the eight quinones were evaluated through leaf dip bioassays conducted against Tetranychus urticae, Myzus persicae, Myzocallis walshii, and Illinoia liriodendri. Based on LC₅₀ values, plumbagin (LC₅₀ = 0.001%) was the most active compound against T. urticae and ubiquinone Q₀ (LC₅₀ = 0.005%), plumbagin (LC₅₀ = 0.010%), and dibromothymoquinone (LC₅₀ = 0.012%) were the most active compounds against M. persicae. The most active compounds against M. walshii were juglone (LC₅₀ = 0.011%) and ubiquinone Q₀ (LC₅₀ = 0.019%), whereas dibromothymoquinone (LC₅₀ = 0.030%), plumbagin (LC₅₀ = 0.033%) and ubiquinone Q₀ (LC₅₀ = 0.058%) were the most toxic to I. liriodendri. Ecotrol (positive control) was the least toxic compound (LC₅₀ = 0.39%) against T. urticae and M. persicae (LC₅₀ = 0.447%). Although the majority of the compounds tested were toxic to all four test species in residual bioassays, there was little overlap among the test species in terms of susceptibility to the compounds and interspecific differences were observed. Regarding structure-activity relationships for quinones, the addition of a hydroxyl group resulted in a significant increase in the toxicity of the 1,4-naphthoquinones, and those possessing a methyl group exhibited the highest levels of activity in T. urticae. The bromine atom at the 2- and 5-positions of the benzoquinone ring is crucial to the toxicity of the compounds against I. liriodendri. Toxicity was greatly affected not only by the number of hydroxyl groups, but also by their positions in the ring in the case of M. walshii. Juglone and plumbagin as residual toxins in the laboratory also reduced the population of two-spotted spider mites compared to EcoTrol™ (positive control) and the negative control in the greenhouse experiment. Some quinones tested may have potential as commercial insecticides and miticides, or alternatively, could serve as lead compounds for the development of more potent crop protection agents.     

Insecticide selectivity and behavioral response of the earwig Doru luteipes

Insecticide lethal and sub-lethal effects on non-target species are a focus in pest management programs. However, such studies are usually centered in relatively few groups of natural enemies of insect pests. Earwigs, although insect pest predators of key importance in Neotropical maize fields, have received very little attention. The earwig Doru luteipes (Sccuder) (Dermaptera: Forficulidae) is one of the main predators of the fall armyworm Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae). Both species were subjected to toxicity and selectivity studies with the insecticides chlorantraniliprole, chlorfenapyr, chlorpyrifos, λ-cyhalothrin, deltamethrin, etofenprox, methomyl and spinosad. The behavioral locomotory response of D. luteipes to these compounds was also assessed. Concentration-response bioassays indicated very low potency of chlorantraniliprole (>550,000× less toxic), followed by spinosad (>3,500× less toxic) and etofenprox (>1,100× less toxic) as compared to chlorpyrifos, the most toxic insecticide studied against this earwig species. These same three compounds exhibited the highest selectivity when comparing the earwig with its prey, the fall armyworm. Time-response bioassays using the insecticide label rates recommended against the fall armyworm confirmed the high selectivity of chlorantraniliprole and etofenprox, in addition to deltamethrin and methomyl. Again chlorpyrifos exhibited the lowest levels of selectivity. Exposure of the earwig to insecticide-treated surfaces indicated that spinosad reduced the locomotory activity of the adults probably increasing their insecticide exposure, while they avoided chlorfenapyr-, etofenprox-, and chlorpyrifos-treated surfaces. Chlorantraniliprole and etofenprox seem the most promising compounds for use against S. frugiperda whilst preserving populations of D. luteipes.