Pyrethroid resistance in Helicoverpa armigera (Hübner) field populations was investigated in Benin over several years by using third- and fourth-instar larval topicalbioassays. H. armigera was resistant to pyrethroids tested as cypermethrin, deltamethrin, bifenthrin, and fenvalerate. Pretreatment with piperonyl butoxide significantly decreased the LD50 value of cypermethrin and deltamethrin, and the resistance suppression by this synergist effect was observed. No significant decrease in the LD50 value was obtained when S,S,S-tributyl phosphoro-trithioate was applied before deltamethrin. In the field, cypermethrin's LD50 value varied, and the highest LD50 values were observed during the rainy season, the cotton, Gossypium hirsutum L., crop period, when pyrethroids are extensively used. In the dry season when there was no cotton cultivation, the lowest LD50 values were obtained. However, reversion was never total in the field; resistance did not revert to the level we observed in the susceptible strain. In the laboratory, when field populations were reared in insecticide-free conditions, resistance decreased and total reversion was observed. Results are discussed with regard to insecticide resistance fitness cost and resistance management strategies.
The aim of this survey was to assess the costs of cypermethrin resistance in Helicoverpa armigera strains by using variation in their biological parameters such as fecundity, number of larval slough, development time, weight and survival at each stage of insect development in comparison with susceptible strains. AGB01 and AGB03 are the resistant strains compared respectively with BK77 and OKP01 as susceptible strains. Fecundity, fertility and survival from egg to adult emergence of AGB03 strain were significantly (P < 0.001) lower than those observed in OKP01 strain. No difference (P > 0.05) was observed with time of pre-adult development, survival at each stage of insect development, fecundity and number of larval slough in comparison with AGB01 and BK77 strains. Larvae of AGB01 strain took significantly (P < 0.01) longer time to develop and were significantly (P < 0.01) lighter than BK77 at the same age, but the slower development of larvae of AGB01 strain was counteracted by the shorter times of egg and pupae stages. The lower fecundity, fertility and survival from egg to adult emergence should represent the main costs for the AGB03 strain resistant to cypermethrin.
Objectives: Insects are considered as the main vectors of Rice Yellow Mottle Virus (RYMV) in Africa. However, in Benin, little is known about the abundance and diversity of potential insect vectors of RYMV in rice fields to prevent and manage disease impact on rice production. Methodology and Results: The inventory of the potential insect vectors of this disease was carried out in the rice basin of the Southern and Central of Benin where three sites namely AfricaRice station, Koussin and Ouedeme, were prospected. The sweep net technique, visual observation and yellow plates trap were used to conduct the surveys. Eighty insect species belonging to 28 families and 8 orders were recorded at rice tillering, booting, heading and maturation stages. Variation of insect diversity was low among the sites. The shorthorned grasshopper Oxya hyla (Serville), the ladybird beetle Chnootriba similis (Mulsant), the rice white leafhopper Cofana spectra (Distant) and the stalk-eyed fly Diopsis thoracica (Westwood) were the most important species considering their relative abundance (10.62%, 5.19%, 7.99% and 7.01%, respectively) and frequency of occurrence (90%, 80%, 73.33% and 80%, respectively). These insects were mostly present at tillering and booting stages. Conclusion and application of results: Seventy-five (75 %) of the recorded species were not identified as RYMV vectors before. Special attention must be paid to the distribution and importance of these insects in the fields. Studies on their ability to transmit RYMV according ecological conditions should be done to know high-risk production areas and to prevent large epidemics of RYMV.
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