Biological factors hypothesized to affect parasitization by Cotesia marginiventris (Cresson) (Hymenoptera: Braconidae), an endoparasitoid of larvae of the beet armyworm, Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae), as well as its sex ratio, were examined in the laboratory. Highest parasitization occurred when (1) adult female parasitoids were closely associated with hosts, (2) adult female parasitoids were 1 day old, (3) a host:female parasitoid ratio of between 10:1 and 30:1 was maintained, (4) second-instar beet armyworms were used as hosts, and (5) adult female parasitoids were exposed to hosts for 24 h. Percentage of female progeny was higher when females were closely associated with second instars.

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