Activity spectrum of spinosad and indoxacarb : rationale for an innovative pyrethroid resistance management strategy in West Africa

To face pyrethroid resistance in the cotton ballworm Helicoverpa armigera (Hübner), endosulfan (700 g/ha) has been used in a resistance management strategy for four years in Côte d'Ivoire, West Africa. Lately the recommendation is being questioned with regard to its acute mammalian toxicity and environmental issues. Earlier works revealed that insecticides such as spinosad (48 g/ ha) and indoxacarb (25 g/ha) proved as effective as endosulfan in controlling H. armigera. In contrast to endosulfan, the activity spectrum of these non-pyrethroids insecticides appears to be restricted to a few bollworm and leaf-feeding pests. The present study pointed out the strength and weakness of these new insecticides with respect to major insect pests and beneficial species. On the basis of their activity spectrum and in the light of cotton crop and main pest pheriology, new IRM was designed. Indoxacarb is more appropriate in the fruiting stage (101 - 115 DAE (Days After Emergence)) as it appeared very effective against the cotton stainer Dysdercus voëlkeri (Schmidt) while showing lower performance against Earias spp. and the mite Polyphagotarsonemus latus (Bank). In contrast, spinosad is preferred at the vegetative stage (45-66 DAE) as it proved safer to coccinellids, more effective against Earias spp., while its lower effectiveness against D. voëlkeri suggests avoiding its positioning at the later stages of cotton growth. Various benefits related to these new insecticides strongly advise their use as alternatives to pyrethroids. However, to be more attractive, their activity needs to be reinforced by other insecticides in such a way as to control the whole arthropod pest complex. (Résumé d'auteur

An innovative approach to reduce chemicals in mite control

The efficacy of an innovative technique using acaricide treated nets was recently demonstrated to control phytophagous mites on vegetables. The net impregnated with the acaricide dicofol was efficient against the broad mite Polyphagotarsonemus latus (Banks) and spider mites (Tetranychus spp.) when used temporarily (once every three nights) covering eggplant. The technique was evaluated with chlorpyriphos ethyl impregnating nets in southern Benin. Results showed that the populations of Tetranychus urticae, and T. ludeni in the first trials and the invasive specie T. evansi in a last trial were significantly controlled by this technique. Very few mites were observed on plants covered with the acaricide treated net compared with very high densities on leaves in the unprotected control plots. This new concept of mite control using an acaricide-treated net temporarily covering vegetable crop appears to be an efficient tool which is easy to use by small-scale farmers. With this technique the pesticide remains on the material reducing the risk of plant contamination and environmental pollution. Thus the same treatment can be applied many times reducing chemicals for controlling mites. In greenhouses, particularly where populations of mites can increase rapidly on vegetables or ornamental flowers acaricide-treated nets could be used to control outbreaks just before the prompt release of useful insects as is permitted by absence of pesticide residues on plants with this innovative technique. In Sub-Saharian countries where mite outbreaks are observed mainly in the dry season the application of acaricide treated net poses a very low risk of pesticide workout by rain reducing the risk of pesticide residues on plants and their negative impact on useful insects. (Résumé d'auteur

How protecting fruit and vegetable in organic farm in sub-Saharan Africa

Under the tropics fruits and vegetables can be damaged by a lot of pest species throughout the year. Pest outbreaks depend generally of the rains and plant hosts. Healthy plants are more or less able to protect themselves against pests. For this reason, a crop properly watering or fertilized would be less damaged by pest than stressed plants. In the same way, different plant species in good rotation and in association would be less susceptible or attractive to pests than a single crop in a big plot. The major pest of fruit and vegetable are mainly the caterpillars and the worms from Lepidoptera and fly species respectively. Indeed, their larva damage directly the production eating leaves, flowers and/or fruits. Natural enemies such as birds, insect predator or parasitoids could regulate part of these pests but they have to be attracted and maintained in the field as they are negatively impacted by heavy chemical use in agriculture. Moreover, alien species such as the red spider mite Tetranychus evansi or the tomato leafminer Tuta absoluta are unknown by the local natural enemies and outbreaks can be very high, with 100% of crop losses. Conversely, indigenous vegetables more tolerant to local pest than exotic species can be highly damaged by alien species. Then, as one larva can damage one fruit, sometimes more, its impact on production can be significant for the farmer. To protect high value crops producing leaves such as cabbage or fruits such as tomato, netting technology has been showed to be very effective against worms and caterpillars. As such, the use of netting technology has generated significant interest among smallholder growers particularly organic farmers. Cost benefice analysis with net on nurseries or cabbages in Benin has been showed to be significantly higher as compared with open field and the return on investment of a nethouse in Kenya was 1.5 year with a rotation of tomato, green beans and cabbage. Depending of the crop, netting technology can be used on low or high tunnel with wood or metal frame. Ventilation can be managed with the size of mesh. Higher is the temperature and relative humidity, larger would be the mesh size. Against the small pest such as whiteflies, aphids, thrips or mites, biological control methods can be used as these piercing and sucking insect do not damaging directly the production. Sprays of black soap or plant extracts based on garlic, onion or pepper may reduce sucking pest outbreaks. Even the association of companion plants such as thyme, African marigold or lemongrass could help at a low level by repelling some pest species. Biopesticides based on Bacillus thuringiensis or entomopathogenic fungi can also be used alone or in association with other biological control methods like insect traps to reduce pest populations. All these biological methods have to be use all together and mainly in a preventive way

Impact of human activities on the dynamics of insecticide resistance in Anopheles gambiae: Case of Benin (West Africa)

Vector control for malaria prevention remains mainly based on the use of Long Lasting Insecticide Nets (LLINs) and Indoor Residual Spraying (IRS) in Sub-Saharan Africa. The effectiveness of these tools is threatened by insecticide resistance of An. gambiae s.s., one of the main African vectors of human Plasmodium. Historically, resistance against DDT in natural populations of African malaria vector since the 60's has been selected by insecticide use in public health. Nowadays, it is also clear that most cases of resistance against others classes of insecticides in An gambiae s.s. are the result of massive use of these products against crop pests in vegetable and cotton growing areas in the whole West African region. In addition, some of the pesticides used in agriculture are used against other pest insects (cockroaches, fleas, termites ...), increasing insecticide pressure on Anopheles mosquitoes and representing also a risk to the health of humans exposed to these products. This phenomenon remains poorly studied. To develop food resources, extension of cultivated areas and the need of modern agricultural practices with high yield have to be implemented in sub-Saharan region of Africa. This could further increase the amount of pesticides and selection pressure for resistance in mosquitoes knowing that much more insecticides are used for agriculture purpose than public health. In this situation, what can we do to ensure the efficacy of current or news resistance management strategies and therefore prevent the failure of vector control tools used ? (Texte intégral

Oviposition behavior of Plutella xylostella onto cabbages covered with nets

Plusieurs expérimentations de terrains menées en zone tropicale en Afrique et en Asie ont montré l'efficacité des filets anti-insectes pour protéger les cultures de choux contre les Lépidoptères en particulier Plutella xylostella. La taille de maille recommandée varie de 0.8 mm à 1.2 mm selon les auteurs. Le comportement de P. xylostella a été étudié au laboratoire en présence de différents types de filets protégeant des feuilles ou des plants de choux en situation de choix et de non choix. Les résultats ont montrés que les filets de 0.85, 0.93, 1.33 et 1.80 mm de maille ont laissé passer respectivement 0%, 10%, 50% et 85% d'adultes en contact forcé. Les femelles ont toujours préfèré pondre directement sur les feuilles de choux en situation de choix mais en l'absence de choix elles ont pondu à travers le filet en contact avec une feuille. Dans le cas contraire, quand le filet n'était pas en contact avec une feuille ou un plant de choux, les femelles ont pondu sur un filet multifilament à maille complexe mais pas sur un filet monofilament. Nous avons montré également que les adultes du parasitoïde Cotesia plutellae sont passés à travers le filet de 0.93 mm. Ces résultats ont permis de mieux comprendre l'efficacité des filets anti-insectes généralement utilisés sur le terrain en situation de choix dans des dispositifs expérimentaux. Les caractéristiques des filets pour la protection des cultures de choux ont été discutés selon leur mode d'utilisation au champ et le système de culture. (Texte intégral

Monitoring insecticide resistance in the bollworm Helicoverpa armigera (Hübner) from 1998 to 2002 in Cote d'Ivoire, West Africa

Helicoverpa armigera (Hübner) is the major insect pest of the cotton crop in West Africa. Populations recently developed resistance to pyrethroids via the overproduction of oxidases. To control this pest, a resistance management strategy was applied in the major West African cotton growing countries from 1999 onwards. In Côte d'Ivoire insecticide resistance of H. armigera was monitored in field strains from 7998 to 2002 using insecticide coated vial tests and topical applications of insecticides with third-instar larvae. Vial tests with discriminating doses of cypermethrin were used directly on field-collected larvae at the end of the cotton season. The percentage of resistant larvae varied around a mean of 67% with 6 [mu]g cypermethrin/vial and around 13% with 30 [mu]g/ vial in different years and places. Topical applications with various insecticides were used in the laboratory on the first generation of populations collected from cotton (Gossypium hirsutum), tomato (Lycopersicum esculentum) or a strongly infested ornamental flower (Antirrhinum majus). The resistance factors calculated from dose-mortality regressions varied from 5 to 38 with deltomethrin. In the Bouaké area they were always higher for strains collected from cotton at the end of the season. Concerning the pyrethroid alternatives currently used in Côte d'Ivoire, no reduction in susceptibility in the cotton field strains was detected for endosulfan or profenofos showing their potential for use in a resistance management strategy. These results suggest a relative stability of the pyrethroid resistance in H. armigera from 1998 to 2002 and confirm the success of the resistance management strategy. (Résumé d'auteur

A repellent treated net for protecting cabbages against aphids

Insect-proof nets with mesh size higher than 0.5 mm cannot efficiently protect cabbages against aphid infestation and insect-proof nets with very fine mesh reduce the ventilation. The objective of this study was to show the biological efficacy of a polyethylene knitted net with 0.9 mm mesh size impregnated with 1% alphacypermethrin, an excito-repellent insecticide, using the Olyset® process recommended for public health use by WHO. A cabbage crop was used to study the treated net efficacy in a statistical bloc design, compared with non-treated nets (with 0.9 or 0.4 mm mesh size). A non-protected control was included in the experiment. All cabbage pests where observed periodically and particularly the aphids: Lipaphis erysimi, Brevicoryne brassicae and Myzus persicae . In the same time, laboratory tests were used to show the insecticide susceptibility and the behavior of M. persicae stimulated to cross treated and non-treated nets. The major result was total protection of cabbages under the treated net against all aphid species during 10 weeks of monitoring until harvest. In the contrary M. persicae and L. erysimi outbreaks were both observed under non-treated nets. In the laboratory, the treated net did not show a toxic effect on M. Persicae and on the parasitoid Aphidius colemani. The bioassay with dipping leaves showed a highly resistance of M. persicae to alphacypermethrin. Therefore a repellent effect with the treated net was observed against M. persicae and against the parasitoid A. colemani. Thus that repellent treated net could be a solution for protecting vegetables against aphids with a low impact on beneficials. (Texte intégral