DOSAGE RESPONSES OF THE LARVAL INSTARS OF THE BERTHA ARMYWORM, MAMESTRA CONFIGURATA (LEPIDOPTERA: NOCTUIDAE), TO A NATIVE NUCLEAR POLYHEDROSIS

Abstract

The dosage–mortality relationship of a multi-embedded nuclear polyhedrosis virus was determined for each instar of the bertha armyworm by probit analysis of mortality following the ingestion of polyhedral inclusion bodies (PIBs) placed on the surface of an artificial diet. Susceptibility to infection decreased as larvae aged; the median exposure doses (LE₅₀) increased from 18 PIBs/larva for the first larval instar (L₁) to 21 × 10⁶ PIBs/larva for half-grown L₆. When given equivalent doses, the time for infected insects to die increased with the age of the larvae. Within any given instar the length of time to death shortened with increasing doses. Larvae that survived to become adults showed no signs of infection. There was no evidence that any infected individual could recover from infection. From the data presented, calculations show that maximum production, 2.90 × 10⁹ PIBs/treated insect, is achieved by feeding 10,000–20,000 PIBs/larva to young L₅. Larvae dying as L₆ have about 40% of their normal dry weight of essential tissue converted to PIBs. Although this MNPV currently would not be cost effective as a spray for protecting crops from bertha armyworm damage, its potential as a biocontrol agent, applied in small amounts at an early stage in bertha armyworm outbreaks, should be evaluated.