Field experiments were conducted during 1999 and 2000 cotton seasons, at Kalubia governorate to study the relation between the percentage of small bollworm larvae on the efficacy of insecticidal treatment against cotton bollworms. Two bio-insecucides, MVPII and Xentari at three rates (rec., double rec. & Y2 rec.), Dursban (rec. rate) alone and in combination with Xentari or MVPII (Y2 rec. rate) were tested. Four applications were started on July 24th to 14th August. Results indicated that the efficacy of tested insecticides against PBW and/or SBW infestation increased as the percentage of small larvae decreased before starting of spraying. Also, results revealed that, MVPII (Y2 ree. rate) was effective against SBW and not effective against PBW in the field. On the other hand, Xentari at rec. rate was effective against PBW in occurrence of high population density of young PBW larvae, while the double rate ofXentari was effective against SBW when the percent of young SBW larvae was 7.6 % of
. the total number of SBW larvae. In addition, results proved that the combination Of Dursban with either Xentari or MVPII was more potent against PBW or SBW than each alone.
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