No abstract

Thirteen of the most common lepidopteran-specific Cry proteins of Bacillus thuringiensis have been tested for their efficacy against newly hatched larvae of two populations of the spiny bollworm, Earias insulana. At a concentration of 100 g of toxin per milliliter of artificial diet, six Cry toxins (Cry1Ca, Cry1Ea, Cry1Fa, Cry1Ja, Cry2Aa, and Cry2Ab) were not toxic at all. Cry1Aa, Cry1Ja, and Cry2Aa did not cause mortality but caused significant inhibition of growth. The other Cry toxins (Cry1Ab, Cry1Ac, Cry1Ba, Cry1Da, Cry1Ia, and Cry9Ca) were toxic to E. insulana larvae. The 50% lethal concentration values of these toxins ranged from 0.39 to 21.13 g/ml (for Cry9Ca and Cry1Ia, respectively) for an E. insulana laboratory colony originating from Egypt and from 0.20 to 4.25 g/ml (for Cry9Ca and Cry1Da, respectively) for a laboratory colony originating from Spain. The relative potencies of the toxins in the population from Egypt were highest for Cry9Ca and Cry1Ab, and they were both significantly more toxic than Cry1Ac and Cry1Ba, followed by Cry1Da and finally Cry1Ia. In the population from Spain, Cry9Ca was the most toxic, followed in decreasing order by Cry1Ac and Cry1Ba, and the least toxic was Cry1Da. Binding experiments were performed to test whether the toxic Cry proteins shared binding sites in this insect.125 I-labeled Cry1Ac and Cry1Ab and biotinylated Cry1Ba, Cry1Ia, and Cry9Ca showed specific binding to the brush border membrane vesicles from E. insulana. Competition binding experiments among these toxins showed that only Cry1Ab and Cry1Ac competed for the same binding sites, indicating a high possibility that this insect may develop cross-resistance to Cry1Ab upon exposure to Cry1Ac transgenic cotton but not to the other toxins tested.

No abstract

The insecticidal characteristics of two Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) strains produced on two different S. exigua colonies were measured using the same two host colonies. These strains constitute the active ingredients of the biological insecticides Vir-ex and Spexit and were produced on insect colonies from Spain and Switzerland. Demographic characteristics of insects from each colony were examined before infection. Larval developmental time, larval survival, and adult sex ratio did not differ between the colonies, whereas mean pupal weight was significantly higher in the Spanish colony insects. After infection, susceptibility to virus occlusion bodies (OBs), time to death, larval weight at death, and total production of OBs/larva varied significantly depending on virus strain and the colony used. Vir-ex OBs produced in Spanish colony larvae had improved insecticidal characteristics in terms in lethal dose and speed of kill metrics than other strain-colony combinations. OB production was significantly higher in Spanish colony insects infected with Spexit compared with Vir-ex infected insects from the Swiss colony, with intermediate values for the other two strain-colony combinations. Virus strain and host colony origin were highly influential in determining the insecticidal characteristics of OBs and should be considered as key parameters that require optimization during the production of SeMNPV-based insecticides.