Susceptibility of Helicoverpa armigera (Lepidoptera: Noctuidae) to Cry1Ac and Cry2Ab2 insecticidal proteins in four countries of the West African cotton belt.

Transgenic cotton, Gossypium hirsutum L., producing Cry1Ac and Cry2Ab2 insecticidal proteins from Bacillus thuringiensis (Bt) was first planted in Burkina Faso (West Africa) in 2008. Here, we provide the first baseline data on susceptibility of Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) populations collected in West Africa (Benin, Cameroon, Nigeria, and Chad) to Cry1Ac and Cry2Ab2 toxins incorporated separately or in combination in a semisynthetic diet. For populations collected in 2006 and 2008, dose-response curves were used to estimate mortality (LC50), growth inhibition (IC50), and stunting (EC50) of larvae. For each of these parameters, susceptibility respectively varied 44-, 23-, and 37-fold for Cry1Ac; 10-, 40-, and 25-fold for Cry2Ab2; and 37-, 11-, and nine-fold for the mixture. Country or laboratory of testing did not significantly affect susceptibility to Cry1Ac and Cry2Ab2. LC50 was significantly higher in 2008 than in 2006 for Cry1Ac, possibly due to variation in experimental conditions. LC50 and IC50 (but not EC50) were positively correlated and presented similar precision and variability across regions. However, IC could provide a more practical indicator of resistance than LC, because measuring LC and IC was equally labor intensive but estimating IC required lower amounts of toxins. Cry1Ac was two- to three-fold more toxic than Cry2Ab2 and no cross-resistance occurred among populations. Incorporation of both toxins in diet had an additive effect on mortality and growth inhibition. Our results provide a basis to establish resistance-monitoring for H. armigera before the widespread use of Bt cotton in West Africa.

Tracking pyrethroid resistance in the polyphagous bollworm, Helicoverpa armigera (Lepidoptera: Noctuidae), in the shifting landscape of a cotton-growing area.

In cotton-growing areas of Central Africa, timing of host crops and pest management practices in annual rainfed cropping systems result in a shifting mosaic of habitats that influence the dynamics and resistance of Helicoverpa armigera (Hübner) populations on spatial scales, both within and across seasons. From 2002 to 2006, regional and local resistance was monitored among cotton fields and among the major host plants of the bollworm. From 2002, pyrethroid resistance increased within and across cotton-growing seasons to reach a worrying situation at the end of the 2005 growing season. Cotton crops played a fundamental role in the increase in seasonal resistance, even if the intensive use of insecticides on local tomato crops strongly concentrated resistance alleles in residual populations throughout the off-season. Due to the relative stability of resistance in H. armigera populations despite a long off-season, we believe that after the dispersal of the moths southwards at the end of the growing season, reverse migration mainly accounts for the reconstitution of populations at the onset of the following growing season. In addition, local resistance monitoring in 2005 and 2006 showed that it was possible to control the increase in resistance by temporarily stopping the use of pyrethroids during the period of peak infestation of cotton by H. armigera. On the other hand, the similar resistance frequency of populations sampled from sprayed and unsprayed synchronous hosts confirmed the absence of reproductive isolation between adults. As a result, diversity in cropping systems should be encouraged by planting alternative host plants to provide a mosaic of habitats, which in return would provide insecticide-free refuges. The implications for insecticide resistance management in annual cropping systems are discussed.

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.