SUSCEPTIBILITY OF SOME FOREST INSECTS TO MIXTURES OF COMMERCIAL BACILLUS THURINGIENSIS AND CHEMICAL INSECTICIDES, AND SENSITIVITY OF THE PATHOGEN TO THE INSECTICIDES

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Abstract
Experiments were designed to determine the effects of low doses of some insecticides on the viability and physiological characteristics of commercial Bacillus thuringiensis (Berliner) as a prelude to studies on the susceptibility of lepidopterous forest defoliators to mixtures of bacteria and insecticides.The results show that the insecticides tested had no effect on viability or physiological integrity of the bacteria.Bacteria–insecticide combination effects varied from strongly antagonistic to strongly supplemental depending on insecticide or Thuricide® formulations and concentrations and on the relative susceptibility of the test species to either agent applied separately. Pyrethrum at 10−3 was highly potentiating to BT at 10−1 against third instar Halisidota argentata. Among Hyphantria cunea, a supplemental effect always resulted from combining Thuricide® at 10−2 with low concentrations of malathion, phosphamidon, or pyrethrum.There was generally a drastic reduction in feeding activity among BT-treated insects and a further reduction among BT-chemical-insecticide-treated ones compared with controls.