FACTORS AFFECTING BACULOVIRUS HELIOTHIS - INDUCED MORTALITY IN THE TOBACCO BUDWORM, HELIOTHIS VIRESCENS (F.).
AuthorPOTTER, MICHAEL FRED.
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PublisherThe University of Arizona.
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AbstractEfficacy of Baculovirus heliothis against larvae of the tobacco budworm was studied under laboratory, greenhouse, and field conditions. Dosage-mortality studies using a diet surface inoculation technique resulted in LC₅₀ values of less than 2 PIB/mm² for 1- to 5-day-old larvae. Onset of mortality was delayed in older larvae, and a greater quantity of inoculum was needed to produce the same mortality level as larvae matured. Length of the incubation period was shortened by increasing the dose. In laboratory and greenhouse studies, mortality of neonates was enhanced by the addition of commercial feeding stimulants. A cottonseed-base adjuvant was more effective than either virus alone or virus mixed with soybean flour. The value of the bait was particularly apparent when larvae were held for short durations on virus-treated terminals. Water extracts of fresh and dried garbanzo beans were shown to be highly attractive to tobacco budworm larvae, suggesting their potential for use as a feeding stimulant. Both bean treatments performed as well as the commercial cottonseed adjuvant. Virus-water extracts of garbanzo bean leaves and pods were no more effective in producing larval mortality than virus in water alone. Although addition of a feeding stimulant significantly extended activity of virus residues on cotton terminals bioassayed with H. virescens in the laboratory, the combination did not improve efficacy when larvae were allowed to feed on treated plants in the field. It may be that the effect of bait on young larvae was overridden by high temperatures or light intensities in the upper plant canopy. Time of application studies directed at the egg-stage showed that larvae are capable of ingesting lethal quantities of the pathogen while chewing out of treated eggs. Applications should coincide as closely as possible with egg hatch to maximize infection. Following hatching, there was a consistent decline in effectiveness as treatments were delayed. No significant effects on longevity or fecundity resulted from the feeding of virus to adults in a sucrose solution. Transovum transmission of virus to progeny was inefficient, regardless of the dose administered.