Efficient genetic transformation of okra (Abelmoschus esculentus (L.) Moench) and generation of insect-resistant transgenic plants expressing the cry1Ac gene.

KEY MESSAGE: Agrobacterium -mediated transformation system for okra using embryos was devised and the transgenic Bt plants showed resistance to the target pest, okra shoot, and fruit borer ( Earias vittella ). Okra is an important vegetable crop and progress in genetic improvement via genetic transformation has been impeded by its recalcitrant nature. In this paper, we describe a procedure using embryo explants for Agrobacterium-mediated transformation and tissue culture-based plant regeneration for efficient genetic transformation of okra. Twenty-one transgenic okra lines expressing the Bacillus thuringiensis gene cry1Ac were generated from five transformation experiments. Molecular analysis (PCR and Southern) confirmed the presence of the transgene and double-antibody sandwich ELISA analysis revealed Cry1Ac protein expression in the transgenic plants. All 21 transgenic plants were phenotypically normal and fertile. T1 generation plants from these lines were used in segregation analysis of the transgene. Ten transgenic lines were selected randomly for Southern hybridization and the results confirmed the presence of transgene integration into the genome. Normal Mendelian inheritance (3:1) of cry1Ac gene was observed in 12 lines out of the 21 T0 lines. We selected 11 transgenic lines segregating in a 3:1 ratio for the presence of one transgene for insect bioassays using larvae of fruit and shoot borer (Earias vittella). Fruit from seven transgenic lines caused 100 % larval mortality. We demonstrate an efficient transformation system for okra which will accelerate the development of transgenic okra with novel agronomically useful traits.

Insecticide use and competition shape the genetic diversity of the aphid Aphis gossypii in a cotton-growing landscape.

Field populations of the cotton aphid, Aphis gossypii Glover, are structured into geographically widespread host races. In the cotton-producing regions of West and Central Africa (WCA), two genotypes have been repeatedly detected within the cotton host race, one of which (Burk1) is prevalent (>90%) and resistant to several insecticides, as opposed to the second one (Ivo). Here, we conducted whole plant and field cage experiments to test hypotheses for such low genetic diversity, including selection from insecticide treatments, interclonal competition and adaptation to host plant, or climatic conditions. To assess the genetic diversity of immigrant aphids, alatae were trapped and collected on cotton and relay host plants (okra and roselle) in the early cropping season. Individuals were genotyped at eight specific microsatellite loci and characterized by a multilocus genotype (MLG). When independently transferred from cotton (Gossypium hirustum L.) leaf discs to whole plants (G. hirsutum and G. arboreum, roselle and okra), Ivo and Burk1 performed equally well. When concurrently transferred from cotton leaf discs to the same plant species, Ivo performed better than Burk1, indicating that competition favoured Ivo. This was also the case on G. hirsutum growing outdoors. Conversely, Burk1 prevailed when cotton plants were sprayed with insecticides. In experiments where aphids were allowed to move to neighbouring plants, Burk1 was better represented than Ivo on low-populated plants, suggesting that dispersal may be a way to avoid competition on crowded plants. Most cotton aphids collected on cotton or relay host plants in the early cropping season were Burk1 (>90%), indicating high dispersal ability and, probably reflecting high frequency on host plants from which they dispersed. In the agricultural landscape of WCA, the use of broad-range insecticides on both cotton and relay host plants has led to the prevalence of one genotype of A. gossypii resistant to different classes of insecticides. Deployment of widespread and integrated pest management strategies are needed to restore cotton aphid control.

Persistence of new insecticides and their efficacy against insect pests of okra.

Persistence and efficacy of bifenthrin (25 and 50 g ai ha(-1)), fipronil (50 and 100 g ai ha(-1)) and indoxacarb (70 and 140 g ai ha(-1)) has been studied in okra fruits. The initial deposits varied from 0.259-0.382 microg g(-1) at low and 0.461-0.688 microg g(-1) at high rate of application. The residues persisted upto 10 days with half-life of 1.32-1.58 days for bifenthrin, 0.65-1.12 days for fipronil and 0.58-1.02 days for indoxacarb. Based on ADI, the suggested waiting period was 1 day for bifenthrin and indoxacarb and 3 days for fipronil. All the insecticides were found effective against leafhopper and shoot and fruit borer.

Field efficacy of azadirachtin-A, tetrahydroazadirachtin-A, NeemAzal and endosulfan against key pests of okra (Abelmoschus esculentus).

UNLABELLED: BACKGROUND Unlike synthetic pesticides, azadirachtin-based neem pesticides are environmentally friendly and are well known for their diverse pest control properties. Their use is, however, limited by the instability of azadirachtin, necessitating application at short intervals. The efficacy of relatively stable tetrahydroazadirachtin-A, therefore, needed to be established under field conditions. RESULTS: Azadirachtin-A (Aza-A), its stable derivative tetrahydroazadirachtin-A (THA) and other neem pesticides have been evaluated for their field efficacy against major insect pests of okra, Abelmoschus esculentus (L.) Moench., during summer (kharif) 2003 and 2004. The optimum doses of Aza-A and THA against the fruit borer, Earias vittella F., were also established. Reductions in population of whitefly, Bemisia tabaci (Genn.), and leafhopper (jassid), Amrasca biguttulla biguttulla Ishida, after application of THA or endosulfan was evident up to 10 days after treatment (DAT), whereas with Aza-A and NeemAzal (NZ) the effect was observed up to 7 DAT only. Endosulfan and THA also caused higher reduction in the larvae of shoot and fruit borer E. vittella and E. insulana Boisd., and recorded the highest yields of 4600 and 4180 kg ha(-1). The efficacy of THA (0.05 g L(-1) emulsion) was comparable with that of 0.5 g L(-1) endosulfan emulsion in reducing fruit borer infestation, the reduction over the control being 86.0 and 87.3%, 84.9 and 94.1% and 90.2 and 92.6% at first, second and third picking. THA 0.02 g L(-1) and Aza-A 0.05 g L(-1) were on a par. Laboratory-made neem oil emulsifiable concentrate was the least effective, but was superior to untreated check. CONCLUSION: Three consecutive sprays of THA, a neem-based biopesticide, and endosulfan have been found to be superior in controlling field pests of okra to Aza-A and NZ, which were on a par. THA thus holds potential as a component of pest management strategies against okra pests.

Life history of Spodoptera exigua (Lepidoptera: Noctuidae) on various host plants.

The incubation period of Spodoptera exigua (Hübner) was not influenced by the host plant, whereas larval development time and pupal period were affected. Larval development time was longest on shallot and lady's finger, followed by cabbage and long bean. Larvae did not develop beyond the first instar when fed on chilli. The pupal period was longer on lady's finger than on cabbage, shallot and long bean. Overall, adult longevity was not influenced by the host plant but there was a difference between female and male longevity among the host plants. Survival of S. exigua was affected by the host plant at the larval stage. The number of larval instars varied between 5 and 8 within and between the studied host plants. Long bean was found to be the most suitable host plant and provide the best food quality for S. exigua compared to the other host plants, as it allowed faster development, fewer larval instars and a higher survival rate.

Assessment of Helicoverpa Armigera resistance to pyrethroid insecticides in northern Cameroon.

In Central Africa savannas, the noctuid Helicoverpa armigera (Hübner) is a major pest of cotton and vegetable crops (including tomato and okra) which directly affects production by attacking fruit-bearing organs. In order to prevent and anticipate problems, the national cotton company (SODECOTON), with the help of the agricultural research (IRAD-PRASAC), implemented in 1999 a monitoring network for the early detection of resistance of target insects to the main recommended insecticides. Over the last three years, from 1999 up to 2001, assessment of H. armigera susceptibility to pyrethroid insecticides was carried out through both vial tests and topical bioassays. Revealed in southern and western Africa, pyrethroid resistance now seems to affect Central Africa also. Although control failure due to resistance has not yet been reported from the field, these results indicate the presence of resistant genes within H. armigera populations in Northern Cameron. Further laboratory studies confirmed the loss of susceptibility to cypermethrin in wild populations. Priority actions to be developed must focus on the rational and concerted management of pesticide use and the implementation of a regional monitoring network for the early detection of the loss of sensibility of target insects to the main recommended insecticides. At the same time, more fundamental research should be undertaken on the epidemiological profile of resistance in order to define practical ways of reducing the selection pressure.