Application of Essential Oils of Ocimum gratissimum and Cymbopogon citratus as Bioinsecticides for the Management of Two Major Biting-Sucking Insects (Bemisia tabaci Gennadius and Jacobiella fascialis Jacobi) and the Improvement of Seed and Fiber Quality of Cotton Plants in Ivory Coast.

In recent years, in all cotton production areas in Ivory Coast, whitefly (Bemisia tabaci) and jassid (Jacobiella facialis), two biting sucker insects were causing depreciation of quality of the cottonseed and fiber obtained at harvest. However, the excessive use of synthetic chemical insecticides against these pests presents a danger for the sustainability of the production system. This study aims to evaluate the insecticidal efficacy of essential oils of Ocimum gratissimum and Cymbopogon citratus on B. tabaci and J. fascialis and on the technological parameters of the cottonseed and fiber. A field experiment was conducted in which essential oils of O. gratissimum and C. citratus were applied to cotton plants every two weeks from day 45 to 115 after emergence using a backpack sprayer. The effects of these extracts were compared to those of a synthetic chemical insecticide composed of acetamiprid and alphacypermethrin known under the trade name IBIS A 52 EC (positive control). After 160 days of cultivation where entomological data were collected, the seed cotton samples were harvested, ginned and submitted for analysis. O. gratissimum essential oil at concentrations of 1 and 5 % gave to the cotton plants better protection against whiteflies and jassids. In addition to the synthetic chemical, these cotton plants received a better proportion of healthy cottonseed, fat and a fiber with very good reflectivity. The essential oil of O. gratissimum could therefore be integrated into the phytosanitary protection programs of the cotton plant.

Chemical Composition and Spectrum of Insecticidal Activity of the Essential Oils of Ocimum gratissimum L. and Cymbopogon citratus Stapf on the Main Insects of the Cotton Entomofauna in Côte d'Ivoire.

Among the alternatives to environmentally toxic and socio-economically unacceptable chemical pesticides, essential oils from Ocimum gratissimum and Cymbopogon citratus were tested on the main pests and beneficial insects of the cotton plant in Côte d'Ivoire. After extraction and chemical analysis of the essential oils, field trials were carried out using a Fisher block system with three treatment repetitions where their effects compared with those of a registered synthetic insecticide (IBIS A 52 EC). Foliar applications of the products were carried out in accordance with the cotton plant protection extension programme in Côte d'Ivoire from the 45th to the 115th day after plant emergence, with one application every fortnight. Twenty-three and forty compounds representing about 96 and 99 % of the oil composition of O. gratissimum and C. citratus respectively were elucidated. The most abundant compounds were p-cymene and thymol (O. gratissimum) and myrcene, neral and geranial (C. citratus). The essential oil of O. gratissimum at concentrations of 2 and 5 % showed insecticidal activity on all pests (biting-sucking and carpophagous), except the phyllophagous Syllepte derogata. C. citratus, at a low concentration (1 %), was particularly toxic to whiteflies (Bemisia tabaci), however, it favoured the action of beneficial insects, specifically black ants and ladybirds in the cotton plots, unlike the chemical product. EO of O. gratissimum (1.60 and 4.62 mg GALAE/g, respectively) and C. citratus (2.26 and 2.78 mg GALAE, respectively) exhibited also significant acetyl and butyryl cholinesterase inhibitors. Insecticide formulations based on the essential oils of O. gratissimum and C. citratus offer favourable prospects for their use in cotton cultivation as an alternative to chemical pesticides.

Landscape effects on the abundance and larval diet of the polyphagous pest Helicoverpa armigera in cotton fields in North Benin.

BACKGROUND: The noctuid Helicoverpa armigera is one of the key cotton pests in the Old World. One possible pest regulation method may be the management of host crop in the landscapes. For polyphagous pests such as H. armigera, crop diversity and rotations can offer sequential and alternate resources that may enhance abundance. We explore the impact of landscape composition and host crop diversity on the abundance and natal host plant use of H. armigera in northern Benin. RESULTS: Host plant diversity at the largest scale examined (500 m diameter) was positively correlated with H. armigera abundance. Host plant diversity and the cover of tomato crops were the most important variables in relation to high abundance of H. armigera. Host plant (cotton, maize, tomato, sorghum) proportions and C3 versus C4 plants did not consistently correlate positively with H. armigera abundance. Moth proportion derived from cotton-fed larvae was low, 15% in 2011 and 11% in 2012, and not significantly related to H. armigera abundance. CONCLUSION: Cotton crop cover was not significantly related to H. armigera abundance and may be considered as a sink crop. Landscape composition and sequential availability of host plants should be considered as keys factors for further studies on H. armigera regulation. © 2015 Society of Chemical Industry.

Genetic structure of the polyphagous pest Helicoverpa armigera (Lepidoptera: Noctuidae) across the Sub-Saharan cotton belt.

The cotton bollworm Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) is a key pest of various cropping systems in West and Central Africa, and developed insecticide resistance recently. To understand how such insecticide resistance expands across the region, the genetic structure of bollworm populations was studied using microsatellite markers. At first, the study was performed within several populations from Northern Cameroon: during one year, 19 populations (504 larvae) were sampled in different locations, dates and host plants (6 villages, 6 dates, 5 host plants). Their genetic relationship was analysed using 10 polymorphic microsatellite markers. Despite the high polymorphism (5 to 50 alleles per locus), results reveal low level of genetic distances among locations, collection dates and host plants. The estimated values of F(ST) were very low across all populations and reveal a high level of gene flow. Moreover, all the loci presented heterozygote deficiency. This may arise either from inbreeding (sampling methodology) or from the presence of null alleles. Subsequently, larval sampling was performed at a higher scale, in five locations from Africa (Senegal, Mali, Burkina-Faso, Togo and Cameroon), to detect population differentiations according to geographic distance/isolation. Two other samples, from Madagascar and from Australia, were added to this experiment. F(ST) values and heterozygote frequency data were identical to the first one, indicating a high level of gene flow between these locations and the high migration capacity of the pest. Samples from Thailand, China, Pakistan and France were added to this study but it has been impossible to infer the presence of distinct populations. The opportunity to use neutral markers as microsatellites to understand population dynamics of H. armigera is discussed.

Mutations of acetylcholinesterase which confer insecticide resistance in Drosophila melanogaster populations.

BACKGROUND: Organophosphate and carbamate insecticides irreversibly inhibit acetylcholinesterase causing death of insects. Resistance-modified acetylcholinesterases(AChEs) have been described in many insect species and sequencing of their genes allowed several point mutations to be described. However, their relative frequency and their cartography had not yet been addressed. RESULTS: To analyze the most frequent mutations providing insecticide resistance in Drosophila melanogaster acetylcholinesterase, the Ace gene was cloned and sequenced in several strains harvested from different parts of the world. Sequence comparison revealed four widespread mutations, I161V, G265A, F330Y and G368A. We confirm here that mutations are found either isolated or in combination in the same protein and we show that most natural populations are heterogeneous, composed of a mixture of different alleles. In vitro expression of mutated proteins showed that combining mutations in the same protein has two consequences: it increases resistance level and provides a wide spectrum of resistance. CONCLUSION: The presence of several alleles in natural populations, offering various resistance to carbamate and organophosphate compounds will complicate the establishment of resistance management programs.