Maurice Vaissayre

Pyrethroid resistance in the cotton bollworm, Helicoverpa armigera (Hübner), in West Africa.

The susceptibility of Helicoverpa armigera to pyrethroids has been investigated in West Africa by means of laboratory bioassays since 1985, the first year of widespread pyrethroid use. For some years, this survey has shown a tendency for the pest to become more tolerant to pyrethroids. During the 1996 growing season, farmers using calendar-based spraying programmes reported control failures in various countries. The strong efficacy of cypermethrin on small larvae was confirmed in experimental plots, but the effect decreased quickly in successive instars. Bioassays performed on resistant strains revealed an increase in LD50 that was related to different resistance mechanisms. Metabolic resistance (MFO) appears to be a possible primary mechanism of resistance to pyrethroids. Target modification (kdr) is involved to a small degree and esterases seem to appear only after additional selection pressure. n n n n© 2000 Society of Chemical Industry

Esterase-mediated resistance to pyrethroids in field populations of Helicoverpa armigera (Lepidoptera: Noctuidae) from Central Africa.

BACKGROUNDnEvolution of pyrethroid resistance in the cotton bollworm Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) threatens continued cotton production in Central Africa. Dose-response bioassays were conducted on area-wide collection of bollworm populations from major host plants, while biochemical techniques were used to evaluate basic mechanisms underlying resistance.nnnRESULTSnPyrethroid resistance is primarily associated with detoxification by enhanced esterase activity. High resistance to cypermethrin (RF = 67-1771), cross-resistance to deltamethrin (RF = 60-2972) and lack of cross-resistance to the non-ester pyrethroid etofenprox (RF = 2-10) were observed among H. armigera field populations and laboratory-selected strains. Enzymatic assays showed that esterase activity, but not oxidase content or glutathione-S-transferase activity, was positively correlated with resistance to cypermethrin. Pretreatment with piperonyl butoxide (PBO) resulted in significant synergism with cypermethrin in 6/10 field populations, but not in the laboratory-selected strain, indicating that additional mechanisms such as mixed-function oxidase (MFO) may be involved in field resistance. The absence of cross-resistance to DDT ruled out a possible target-site modification.nnnCONCLUSIONnKnowledge of the mechanisms involved in pyrethroid resistance and the lack of cross-resistance to spinosad and indoxacarb is a key to devising new resistance management strategies aimed at restoring the efficacy of pyrethroid-based programmes.

Oxidases responsible for resistance to pyrethroids sensitize Helicoverpa armigera (Hübner) to triazophos in West Africa

Helicoverpa armigera (Hübner) is the major insect pest of cotton in Africa, Turkey, Asia, India, Indonesia and Australia. Populations recently developed resistance to pyrethroids in West Africa via the overproduction of cytochrome P450 (oxidases) increasing pyrethroid metabolism. One way to overcome pyrethroid resistance is to use compounds that show negative cross-resistance to pyrethroids. Triazophos is one of these compounds: it is slightly more toxic against pyrethroid resistant larvae of H. armigera than against susceptible ones. Overproduced oxidases transform the non active triazophos into its active form, triazophos-oxon, since this form was significantly more often found in larvae from pyrethroid resistant strain (23%) than in susceptible strain (15%). This suggests that oxidases, which provide resistance by degradation of pyrethroids in the resistant individuals, also activate triazophos in its toxic oxon form resulting in a negative cross-resistance.

FACTORS LIMITING THE ADOPTION OF IPM PRACTICES BY COTTON FARMERS IN BENIN: A PARTICIPATORY APPROACH

Smallholders acceptance of innovations depends largely on the approach used to take their needs and constraints into account. The adoption of integrated pest management (IPM) strategies by smallholders can lead to a reduction in pesticide use in cotton, as soon as the recommended cropping practices are adapted to local conditions and associated with a threshold-based use of chemicals. To achieve this goal, farmers need to be trained on the biological basis of IPM. To ensure effective and rational implementation of IPM by farmers, it is essential to overcome constraints associated with pest scouting, identifying and preserving beneficial insects, and gaining access to the right inputs on time. In the current African context, where the extension system is sometimes in very poor shape, participatory methods fostered by the farmer field school concept could enable farmers to implement an integrated approach to pest management, while keeping researchers informed about farmers needs and constraints. Our paper is an attempt to use such a participatory method as a tool to explore farmers needs and constraints when smallholders are asked to adopt an integrated approach to cotton pest management. (Resume dauteur)

Evaluating pyrethroid alternatives for the management of cotton bollworms and resistance in Cameroon.

In sub-Saharan Africa, the bollworm complex, including Helicoverpa armigera, Diparopsis watersi and Earias spp., threatens the continued success of cotton production. Pyrethroid resistance in H. armigera led to serious crop losses while endosulfan, a suitable alternative to pyrethroids, was banned for cotton pest management. Five candidates with no cross-resistance to pyrethroids were evaluated in both on-station and on-farm trials from 2002 to 2006. Two applications were made at the early peak of H. armigera infestation in September, the period when pyrethroid use should be restricted for resistance management purposes. Results showed that, as expected, bollworm infestation consistently peaked from mid-September to mid-October. Spinosad, thiodicarb and emamectin-benzoate were the most suitable alternatives to reduce damage, regardless of the cotton bollworm species. Indoxacarb and lufenuron were less effective in controlling D. watersi. On-farm experiments confirmed the suitability of spinosad for control of pyrethroid-resistant H. armigera, particularly on late sown fields. These new chemistries offer control of bollworms which justify their relevance for pyrethroid resistance management in Cameroon and sub-Saharan Africa.