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The efficiency of Bembidion lampros (Herbst) (Coleoptera:Carabidae) as a predator of Hylemya brassicae (Bouché) (Diptera:Anthomyiidae) eggs and the effects of several insecticides on the beetle Obadofin, Adegboyega Adekunle

Abstract

The efficiency of the carabid beetle, Bembidion lampros (Herbst) as a predator of the eggs of Hylemya brassicae (Bouché) and the effects of the insecticides Dipel, methomyl and chlorfenvinphos on the beetle were studied by introducing some B. lampros into experimental plots of Brussels sprouts and restricting their movements by surrounding the plots with polythene barriers. More eggs were laid in the first than in the second generation of the cabbage root fly. There was progressive decrease in the number of root fly eggs and the number of B. lampros as the plants matured. During the first generation the untreated control had significantly more eggs than the other treatments. Egg predation by B. lampros resulted in a 45% reduction. In plots containing B. lampros and treated with methomyl, Dipel or chlorfenvinphos, the numbers of eggs were reduced by 35, 44 and 66% respectively. Laboratory toxicity studies showed that methomyl at 1 g/ litre produced 100% mortality of B. lampros one day after treatment. When the rate was reduced to 1/2, 1/4 and 1/8, the mortality of B. lampros dropped to 70, 40 and 20% respectively. Dipel [Bacillus thurinqiensis Berliner (16000 IU/mg)] at 1 g/litre and 5 g/litre and chlorfenvinphos at 10 ppm and 40 ppm; produced no mortality three days after treatment. Foliar application of methomyl for aphid control in the field significantly reduced the B. lampros population. There was no significant effect on B. lampros when Dipel was applied as a foliar spray to control lepidopterous larvae. Chlorfenvinphos granules applied once early in the season as a subsurface treatment prevented damage by cabbage maggot and was not toxic to B. lampros. Cabbage maggot damage was not severe enough to cause significant reduction in yield at harvest but examination of roots showed that untreated plots had significantly more maggot damage than other treatments. The damage index ranged from 2.5 for untreated plants to 0.0 in plants from plots treated with chlorfenvinphos and containing B. lampros. Although differences were not significant, the numbers of overwintering root fly puparia were highest in untreated plots. Significantly more empty puparia, indicating second generation fly emergence, were also found in the untreated plots. Besides B. lampros, other carabids removed from the experimental plots included: Harpalus affinis Schr., Amara spp., Calathus fuscipes Goeze, Pterostichus melanarius 111. and other Bembidion spp., in decreasing order of abundance. B. lampros alone does not give complete protection against root maggot, especially if fly oviposition is very heavy during the first generation when the beetle is most effective. But the beetle will go a long way to suppress part of the population. The use of non-selective insecticides for control of pests of Brassica may lead to reduction of B. lampros populations and a consequent increase in cabbage maggot attack.

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