AbstractInfectivity tests revealed that sensitivity of larvae of Archips cerasivoranus Fitch to infection by a commercial preparation of Bacillus thuringiensis Berliner depends on the physiological condition of the insects (determined by age and nutrition), and temperature, but mostly on the presence or absence of infection by the microsporidia Plisliphora sp. and Nosema cerasivoranus Thomson.Mortality attributable to the bacillus was 90, 40, and 30% for healthy third-, fifth-, and seventh-instar larvae after 20 days, mortality reached 90% after 3, 9, and 19 days for larvae of the same ages naturally infected by microsporidia. Also, low temperatures decreased the effect of the pathogen and starvation reduced the resistance of larvae to infection.The results obtained suggest the action of B. thuringiensis in this tortricid is to provoke generalized septicemia rather than toxemia.Other infectivity tests made with cultures of B. thuringiensis and Bacillus cereus Frankland and Frankland confirmed these results. The action of the two bacteria was similar, both provoking septicemia in the insect. This effect was considerably increased by the presence of protozoan diseases.Analyses of unsaturated lipids by the iodine number method showed that infection by a microsporidian or B. thuringiensis lowered considerably the energy value index. The index reached 13.0, 25.5, and 27.0 in healthy larvae of third-, fifth-, and seventh-instar, was reduced to 5.2, 17.0, and 15.0 by a microsporidian infection, and to 7.5, 18.0, and 22.5 by a B. thuringiensis infection. When an infection with both pathogens was induced, the value decreased to 4.0, 8.0, and 7.5.