Effects of viral rate and feeding time on mortality of early-, mid-, and late-stadium larvae of Helicoverpa zea (Lepidoptera : Noctuidae) fed Baculovirus heliothis

Early-, mid-, and late-stadium Helicoverpa zea larvae were fed on a diet wh ich was surface-treated with occlusion bodies (OB) at rates from 0.1 to 1,0 00 OB/mm(2) for periods ranging from 0.25 to 72 h. Fifty-percent of early-s tadium larvae fed 0.1, 1, 10, and 100 OB/mm(2) eventually died after they h ad fed for 7.5, 4.5, 2.3, and 0.3 h, respectively, and then were transferre d to virus-free diet. Greater than 98% mortality occurred, regardless of th e viral rate, after 24 h of feeding. Less than 0.5 h of feeding at the high est rate 11,000 OB/mm(2)) resulted in 100% mortality. Using mid-stadium lar vae, 50% mortality at 0.1, 1, or 10 OB/mm(2) occurred when larvae had fed f or 1 1.0, 3.5 or 0.3 h, respectively, and then were transferred to virus-fr ee diet. Viral rates of 100 or 1,000 OB/mm(2) resulted in >90% mortality af ter only 0.25 h of feeding. All mid-stadium larvae fed the highest rates (1 0, 100 or 1,000 OB/mm(2)) for at least 12 h and then transferred to virus-f ree diet, eventually died of a virus infection. One-hundred percent mortali ty, however, was never reached when larvae were fed for 72 h at the two low est rates of 0.1 and 1.0 OB/mm(2). Within each viral rate it did not seem t o matter whether late-stadium larvae had fed for 1 or 72 h on virus-treated diet. Mortality of late-stadium larvae was never greater than 75% regardle ss of the viral rate or feeding time. Mortality of early-, mid-, or late-st adium larvae not fed virus never exceeded 2%.