Sudanese cotton and the whitefly: a case study of the emergence of a new primary pest

In the late 1970s the whitefly Bemisia tabaci (Gennadius) became the primary cotton pest in the Sudan, superseding the American bollworm Heliothis armigera (Hübner). DDT and a DDT/dimethoate combination were used to control the bollworm and, simultaneously, jassids and whiteflies. B. tabaci, a secondary pest at first, became resistant to dimethoate by frequent selections from 1964 onwards. At the same time, fertility stimulation occurred due to DDT residues on cotton plants. Finally, resistance reached a level so that the whitefly were not controlled by dimethoate, monocrotophos or other organophosphorus insecticides, and stimulation by DDT could exert its full effect. The consequence of this was a tremendous flare-up of the whitefly by 1980/81. This train of events was concluded from laboratory and field studies of the resistance patterns, as well as the acceleration effects from DDT residues on plants to the whitefly. A current hypothesis claiming that the problems arose from the elimination of beneficial insects through insecticide applications is reviewed in the light of experimental evidence and practical experience.     

Daily application of ultra-low-volume (ULV) insecticides at low dosages to control insect pests of cotton in Malawi

ULV carbaryl and DDT were applied to cotton (Gossypium hirsutum) at two sites, five times a week, using five swath widths from 4·5 to 18 m, representing high to low dosages. Sprays were applied between 0830 and 1400 h during the day or between 1830 and 2100 h during the evening. Subsequently, Bemisia spp. (whitefly) and Paurocephala gossypii Russell (psyllid) populations were significantly larger on plants treated with lower dosages applied over wide swaths. Yields of seed cotton were significantly lower from plants treated with low dosages (< 0·5 kg a.i./ha weekly) but the results of spraying in the morning and in the evening were comparable, at both sites.     

Resistance to whitefly (Bemisia tabaci) in cotton (Gossypium hirsutum) in the Sudan

Over the past 10–15 years, the cotton whitefly (Bemisia tabaci Genn.) has caused severe damage to cotton in many tropical countries. Effects of three leaf-hair densities (LHD) (high, moderate and low) and three leaf shapes (LS) (normal, okra and super okra) on whitefly numbers were examined in the Sudan in 1982. Resistance was conferred by low LHD and okra or super okra LS, with significant reductions in numbers of whitefly adults per leaf and adults, scales and pupae per unit area. Adults per leaf were lowest on the most resistant line, ORS-13 (exotic Gossypium hirsutum cv., low LHD/okra LS) throughout the season. Pupal parasitism was significantly higher on this line than on a hairy plant of normal leaf shape. Honeydew causing stickiness of lint was reduced to acceptable levels on resistant lines. Leaf shape influenced the microclimate, in that relative humidity was lower and temperature was slightly higher in the canopy of Sudac-K than in Barac (67)B. Fewer whiteflies on Sudac-K may be due to a more adverse microclimate for whitefly and improved host-searching conditions for adult parasites. A breeding programme in the Sudan should be maintained for selection of field-resistant varieties.     

Effects of chlorfenvinphos on Bemisia tabaci Genn. and its parasites on cotton in the Sudan Gezira

Two large-scale field trials were carried out in consecutive cotton seasons to evaluate the effects of a chlorfenvinphos-based spray regime and a standard commercial spray regime on whiteflies (B. tabaci) and their parasites. During both trials B. tabaci was the dominant pest species and occurred in generally increasing numbers throughout October to January. The only abundant natural enemy of B. tabaci was an aphelinid parasite, Eretmocerus sp. This parasite occurred in very large numbers towards the end of the first cotton season but was less abundant in the following year. In both seasons, cotton fields treated with chlorfenvinphos had significantly lower populations of whiteflies than those treated with the standard commercial spray regime. In contrast, the percentage of parasitized whitefly scales and numbers of parasite adults was significantly higher in the chlorfenvinphos-treated fields. It is suggested that the better control of whiteflies achieved in fields sprayed with chlorfenvinphos is at least in part due to this greater conservation of natural enemies.     

Cotton leaf trichomes and populations of Empoasca lybica and Bemisia tabaci

Breeding cotton, Gossypium spp., for resistance to leafhoppers of the genus Empoasca and to sweetpotato whitefly, Bemisia tabaci (Gennadius), poses a dilemma. Glabrous (smooth-leaved) cotton plants have higher leafhopper populations but fewer whiteflies and less leaf and bract trash in mechanically harvested seedcotton than plants with pubescent leaves. In this study, both leafhopper and whitefly populations were observed in 31 cotton entries planted at Bet She'an, Israel, in 1987. Leafhopper populations decreased but the whitefly populations increased as the number of trichomes increased. At the end of June, plants with 40 trichomes per 13.7 mm² supported relatively low populations of both leafhoppers and whiteflies. However, the whitefly populations increased later in the season to 43 times the earlier population levels. Whitefly populations increased as the number of trichomes increased up to 70 trichomes per 13.7 mm² and then decreased as the trichomes became more dense. Various control measures and breeding schemes that might interact to provide an answer to the control of these pests are discussed.     

Combining reflective mulch and host plant resistance for sweetpotato whitefly (Hemiptera: Aleyrodidae) management in watermelon

A study was conducted to evaluate the use of reflective mulch and host plant resistance for the management of the sweetpotato whitefly, Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae), in watermelon [Citrullus lanatus var. lanatus (Thunberg) Matsum & Nakai]. Whitefly abundance data were collected under both greenhouse (caged and uncaged) and field conditions. Consistently, a reflective mulch (also called silver or metallic) treatment resulted in a lower incidence of adult whiteflies as compared with a standard black mulch treatment. Moreover, two whitefly resistant Citrullus colocynthis (L.) Schrad genotypes, which are wild relatives of cultivated watermelon, reduced whitefly populations as compared with standard watermelon. There was generally no interaction between the mulch and genotype variables. No effect of mulch color was observed on sticky card capture of Delphastus catalinae (Horn), a whitefly predator, or on capture of an Eretmocerus sp. whitefly parasitoid in caged trials, which suggests no adverse effect on these natural enemies when using reflective mulch. Overall whitefly populations were relatively low during four seasons of field trials (2006–2009). Results from this study suggest that a combination of using reflective mulch and host plant resistance can additively suppress whitefly infestations, which have particular importance in the fast-growing organic vegetable production industry.     

Cross-resistance, inheritance and stability of resistance to acetamiprid in cotton whitefly, Bemisia tabaci Genn (Hemiptera: Aleyrodidae)

The cotton whitefly Bemisia tabaci, (Genn.) is an important pest of field crops, vegetables and ornamentals worldwide. Neonicotinoids are considered an important group of insecticides being used against B. tabaci for several years. B. tabaci has developed resistance to some of the compounds of the group. This study was designed to investigate if the selection of B. tabaci with acetamiprid would give a broad-spectrum of cross-resistance and to genetically classify the resistance. At G₁ a low level of resistance to acetamiprid, imidacloprid, thiamethoxam, thiacloprid and nitenpyram was observed with resistance ratios of 3-fold, 8-, 9-, 6- and 5-fold, respectively, compared with a laboratory susceptible population. After selection for eight generations with acetamiprid, resistance to acetamiprid increased to 118-fold compared with the laboratory susceptible population. Selection also increased resistance to imidacloprid, thiamethoxam, thiacloprid, nitenpyram, endosulfan and bifenthrin but no change in susceptibility to fipronil was observed. Furthermore resistance in a field population was stable in the absence of acetamiprid selection pressure. Genetic crosses between resistant and susceptible populations indicated autosomal and incompletely recessive resistance. Further genetic analysis suggested that resistance could be controlled by a single factor. The high level of cross-resistance and stability of incomplete resistance in the field population is of some concern. However, lack of cross-resistance between acetamiprid and fipronil or unstable resistance in the resistant population could provide options to use alternative products which could reduce acetamiprid selection pressure.     

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.     

Sampling and management of Bemisia tabaci (Genn.) biotype B in Australian cotton

Data on seasonal population abundance of Bemisia tabaci biotype B (silverleaf whitefly (SLW)) in Australian cotton fields collected over four consecutive growing seasons (2002/2003–2005/2006) were used to develop and validate a multiple-threshold-based management and sampling plan. Non-linear growth trajectories estimated from the field sampling data were used as benchmarks to classify adult SLW field populations into six density-based management zones with associated control recommendations in the context of peak flowering and open boll crop growth stages. Control options based on application of insect growth regulators (IGRs) are recommended for high-density populations (>2 adults/leaf) whereas conventional (non-IGR) products are recommended for the control of low to moderate population densities. A computerised re-sampling program was used to develop and test a binomial sampling plan. Binomial models with thresholds of T=1, 2 and 3 adults/leaf were tested using the field abundance data. A binomial plan based on a tally threshold of T=2 adults/leaf and a minimum sample of 20 leaves at nodes 3, 4 or 5 below the terminal is recommended as the most parsimonious and practical sampling protocol for Australian cotton fields. A decision support guide with management zone boundaries expressed as binomial counts and control options appropriate for various SLW density situations is presented. Appropriate use of chemical insecticides and tactics for successful field control of whiteflies are discussed.     

Root-knot and reniform nematode reproduction on kenaf and sunn hemp compared with that on nematode resistant and susceptible cotton

Kenaf (Hibiscus cannabinus L.) and sunn hemp (Crotalaria juncea L.) are industrial fiber crops that are being grown increasingly in areas where cotton (Gossypium hirsutum L.) is the major crop. The nematodes causing most cotton yield losses in the USA are the southern root-knot nematode, Meloidogyne incognita (Kofoid and White) Chitwood, and the reniform nematode, Rotylenchulus reniformis Linford and Oliveira. For nematode management, knowledge about the comparative effects of kenaf, sunn hemp, and cotton on nematode population densities in the soil is important. The objective of this study was to clarify the host status of kenaf and sunn hemp to M. incognita and R. reniformis by directly comparing reproduction of each nematode on each crop with the same nematode's reproduction on nematode-resistant and nematode-susceptible cotton. Comparisons were made in six experiments under growth chamber, microplot, and field conditions. The results showed that sunn hemp is resistant, but not equally resistant to M. incognita and R. reniformis. Reproduction of R. reniformis on sunn hemp was nearly undetectable, whereas reproduction of M. incognita was greater than on resistant cotton (Auburn 623, Auburn 634) and up to 20% of that on susceptible cotton (Deltapine 16, Deltapine 50, or Deltapine 5409). Kenaf was confirmed to elevate populations of M. incognita to levels that devestate cotton, but found to support substantially less reproduction by R. reniformis than cotton does. Two populations of R. reniformis differed substantially in reproduction on kenaf even though the same populations reproduced similarly on cotton. In conclusion, distinguishing M. incognita from R. reniformis is essential to the management of nematodes in rotations involving kenaf, sunn hemp and cotton.     

Manual topping decreases bollworm infestations in cotton cultivation in Mali

In sub-Saharan areas of Africa, cotton growers no longer cut the shoot tips from plants (topping), although manual topping was promoted at the start of the 20th century to improve yield and, surprisingly, to reduce pest incidence. In these areas, the bollworms Helicoverpa armigera Hübner, Earias spp., and Diparopsis watersi Rothschild are responsible for the majority of cotton yield losses, and the use of pyrethroids has resulted in resistance in field populations of H. armigera. In the face of these problems and given the scarcity of literature on the effects of topping on pest control, we assessed bollworm infestation levels in 12 trials comparing manual topping and non-topping cotton plots in Mali over a six-year period (2002, 2003, and 2005 to 2008). Topping was performed at the emergence of the 15th sympodial branch or at 10 days after the first flower opening. Our results showed no significant difference in seed cotton yields between topped and non-topped cotton. Bollworm infestations (all species) were always lower on topped cotton and 7 out of 12 trials showed significantly lower infestations on topped cotton. In plots of topped cotton, we recorded an average of 56% fewer H. armigera larvae, 68% fewer Earias spp. larvae, and 71% fewer D. watersi larvae with respectively 5, 4, and 3 out of 12 trials with significant differences in favor of topping and no significant difference in favor of non-topping. To our knowledge, our study is the first to report decreases in D. watersi larval infestation with cotton topping. Further studies are required to understand the mechanisms involved in these effects and to ensure that topping is economically attractive for farmers.     

Cotton-insect control in the Sudan Gezira: analysis of a crisis

In the Sudan Gezira, cotton growing is in a state of crisis. Important among the underlying factors are escalating costs of crop spraying in conjunction with increasing infestations of the cotton whitefly, Bemisia tabaci Genn. It is argued that the whitefly has risen to the status of a major pest as a result of certain features of current control strategies, based on intensive aerial spraying. The significance is discussed of the widespread adoption during the period 1972–1980 of so-called ‘package deal’ arrangements. Under this scheme, the Gezira management delegated responsibility for all aspects of insect control to commercial companies. The crop-protection predicament is considered in the larger context of the entire complex of factors which limit cotton productivity, relating not only to crop husbandry in general, but also to socio-economic and public health problems. Development of a more economic and ecologically sound insect-control strategy by itself would not solve the crisis. Nevertheless, without relief from the current crushing financial burden of crop spraying, the means for urgently needed remedial action in the various spheres involved are severely curtailed.     

Cotton water-deficit stress,age, and cultivars as moderating factors of cotton fleahopper abundance and yield loss

Field experiments were conducted in 2012 and 2013 during drought conditions in South Texas and the Texas High Plains to test whether cotton water-deficit stress, age, and cultivars are moderating and interacting factors that affect cotton fleahopper, Pseudatomoscelis seriatus (Reuter) (Hemiptera: Miridae), abundance and yield loss. Irrigation and sequential plantings of several cultivars were used to simulate a range of water stress, plant ages, and cultivar variability. Cotton grown under these experimental conditions were exposed to cotton fleahopper using natural and artificial infestation. Cotton cultivars had a strong influence on cotton fleahopper abundance, with higher densities on Stoneville cultivar 5458 B2RF, which is relatively pubescent, than on the Phytogen cultivar 367 WRF, which is relatively glabrous, in South Texas (p < 0.04). But the strong cultivar effects on cotton fleahopper abundance did not correspond to yield reduction. No water stress effects on cotton fleahopper densities were observed in 2012 (p > 0.05), whereas cotton fleahopper densities increased on older cotton grown under no water stress in 2013 in South Texas (p < 0.05). In contrast, yield response was primarily sensitive to soil moisture conditions (up to 50% yield reduction when grown in dryland mimic conditions below 75% crop ET replacement, p < 0.0009). Water and cotton fleahopper stress synergies were detected but variable, with greatest lint yield loss attributable to cotton fleahopper seen in cotton grown in high water stress conditions in the High Plains (p < 0.05). Yield trends were consistent across cultivars (no interaction with cultivar), even though cotton fleahopper populations varied significantly across cultivars and exceeded regional economic thresholds beginning the second week of squaring (p < 0.05).     

Functional and numerical responses of Eretmocerus mundus Mercet (Hymenoptera: Aphelinidae) parasitizing cotton whitefly, Bemisia tabaci Gennadius (Homoptera: Aleyrodidae)

Laboratory experiments were conducted to measure the functional and numerical responses of Eretmocerus mundus Mercet parasitizing cotton whitefly, Bemisia tabaci Gennadius. Parasitoids were isolated singly for 24 h in 5 cm petri dishes with either 5, 10, 20, 40, 60 or 100 second nymphal stages of B. tabaci. Results showed a typical type II functional response, with up to 43.2 hosts attacked when 100 hosts were provided. The rate of attack and handling time were 0.0465 and 0.3341 days, respectively. At above mentioned densities, the parasitoids lived an average of approximately 4 days. At host densities of < or =20 host/day, the total number of eggs laid was limited by the number of hosts available. Total fecundity was highest with an average of 196 eggs/female at density 100.     

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.     

N-halamine-bonded cotton fabric with antimicrobial and easy-care properties

N-halamine precursor 2,2,6,6-tetramethyl piperidinol (TMP), a hindered amine light stabilizer, was bonded onto cotton fabric by using 1,2,3,4-butanetetracarboxylic acid (BTCA) as a crosslinking agent. A variety of treating conditions including TMP concentration, curing temperature and time, and catalyst were studied. The treated fabrics were characterized using FTIR spectra and scanning electron microscope (SEM). The cotton fabric treated with TMP precursor could be rendered biocidal upon exposure to dilute household bleach. The chlorinated cotton swatches showed great efficacy and inactivated 100 % of Staphylococcus aureus with 7.1 log reduction with 5 min of contact and 83.25 % of E. coli O157:H7 at 10 min of contact. In addition, the wrinkle recovery angle of the treated cotton fabrics increased from 229 ° of untreated cotton fabrics to 253 °. This study provided a practical finishing process to produce cotton fabrics with easy care and antibacterial functionalities at the same time.     

Antibacterial finishing of cotton fabrics using biologically active natural compounds

The paper discusses a method to functionalize cotton fabrics using biologically active natural compounds to achieve the antibacterial characteristics required for medical application. The biologically active natural compounds include propolis, beeswax, and chitosan. Three 100 % cotton knitted fabrics with different degrees of compactness were impregnated in the emulsions containing the active ingredients and fabric variant G3 with the highest degree of impregnation was considered for the evaluation of the antibacterial properties and comfort characteristics. The results show that the treated cotton fabric had high antibacterial activity against both gram positive bacteria Staphylococcus aureus and Streptococcus β haemolytic, and gram negative bacteria Escherichia coli and Pseudomonas aeruginosa. The presence of the biologically active natural compounds on the cotton substrates modified the surface of the textile fibers as seen in the SEM images. The treatment also improved fabric comfort properties, the cotton substrates became less air permissive and more hygroscopic after the treatment. The experimental results indicated that propolis, beeswax and chitosan can be applied as an emulsion to functionalize cotton textile materials. The antibacterial performance of the functionalized fabrics suggested that the cotton fabrics treated with those biologically active natural compounds have the potentials to be used in medical fields.     

Threshold-based interventions for cotton pest control in West Africa: What's up 10 years later?

In the late 1980s, after a long period during which insecticides were sprayed at preset dates to control cotton (Gossypium hirsutum L.) pests and their damage, some French-speaking countries in sub-Saharan areas decided to disseminate a special form of crop protection approach among smallholders, i.e. targeted staggered control (LEC, for Lutte étagée ciblée). According to this approach, decisions on some insecticide applications were made on the basis of infestation levels or the extent of crop injury caused by major pests: Aphis gossypii Glover aphids, Haritalodes (=Syllepte) derogata F., leaf-eating caterpillars, and more generally Helicoverpa armigera (Hübner), Diparopsis watersi Rothschild, Earias insulana (Boisduval) and Earias biplaga (Walker) bollworms. Polyphagotarsonemus latus (Banks) mites were sometimes included on this list. Today, the calendar-based or conventional program is still widely implemented with some changes in insecticides applied due to the development of pyrethroid resistance in H. armigera. Depending on the country, protection programs based on pest monitoring have been preserved or replaced by programs still using thresholds (staggered or true). In Benin, there are two forms of LEC tailored to two regions delineated according to the extent of damage caused by of bollworms that live inside cotton bolls, i.e. Thaumatotibia (=Cryptophlebia) leucotreta (Meyrick). The logical follow-up to LEC, involving true thresholds, was developed in Mali, Cameroon and, recently, Senegal. Cameroon opted for a sequential plan for individual decision program or LOIC (for Lutte après observation individuelle des chenilles), based on control after sequential sampling of bollworms. A calendar program with additional applications of insecticides based on a particular scouting of H. armigera was developed in Togo. In Ivory Coast, the use of true thresholds is limited to the beginning of the cotton crop cycle whereas in Burkina Faso true thresholds are used after the first two calendar sprayings. The present article describes the diversity of these cotton protection programs, sampling methods and associated intervention thresholds based on pests or injury levels in addition to the advantages and constraints associated with their adoption.     

Frequency of Bt resistance alleles in Helicoverpa armigera in 2007-2009 in the Henan cotton growing region of China

Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) is a key insect pest of cotton in the Henan cotton growing region of China. In this region, cotton is grown on small acreages in rich agricultural landscapes, contrary to cropping systems in the United States or Australia. Under such cropping regimes, naturally occurring refuges (with non-Bt plants) may be sufficient to prevent H. armigera resistance development to Bt toxins. In order to gain a timely understanding of the evolution of resistance of H. armigera to Bt toxin after continuous cultivation of Bt cotton for c. 10 years, we assessed the frequency of alleles conferring resistance to Cry1Ac toxin in field populations of H. armigera sampled from Xinxiang County in Henan province during 2007-2009. Screening F₁ and F₂ generations from isofemale lines, derived from female moths trapped in the field, were used with a discriminating dose of Cry1Ac diet to estimate the frequency of resistance alleles. Totals of 625, 516 and 488 isofemale lines were screened for the F₁ generation in 2007, 2008 and 2009, respectively. Resistance gene frequency in Xinxiang fluctuated between 0.0000 and 0.0005, and it did not increase significantly from 2007 to 2009. Based on the relative average development rates (RADR) of H. armigera larvae in F₁ tests, no substantial increase in Cry1Ac tolerance was found in the Xinxiang region over the 3-yr period.     

The behaviour-modifying effect of chlordimeform and endosulfan on the adult whitefly Bemisia tabaci (Genn.) which attacks cotton in the Sudan

Chlordimeform and endosulfan were used as experimental chemicals to demonstrate the presence of behaviour-modifying properties in conventional insecticides. Their modifying effect on the adult whitely Bemisia tabaci (Genn.) was observed and quantified under field conditions. Dosages of 500–2500 g/ha of chlordimeform and 960 g/ha of endosulfan caused irritation and mass emigration of the adults from the treated cotton foliage without producing detectable direct mortality. Laboratory studies using choice-chambers confirmed the repellent effect of these two compounds. The potential usefulness of behaviour-modifying properties in controlling a prolific and intractable pest such as the whitefly is discussed on the basis of a simplified model. An ideal type of behaviour modifier should be of simple chemistry, non-lethal to the pest, and environmentally safe. It should induce only irritation and continuous mobility of the pest in order to prevent it from feeding, mating and ovipositing. This is expected to reduce the population below the economic threshold without risking rapid selection for resistance which invariably results from the direct killing action of conventional insecticides.