A study was made of the susceptibility of young larvae of the tobacco hornworm moth, Manduca sexta, to the action of the benzoylurea diflubenzuron (Dimilin). This insecticide was a very effective stomach poison but ED50 values were lower when insects were maintained in an atmosphere with high rather than low relative humidity. The susceptibility of the insect also depended on the stage (instar) of the insect and its diet. In contrast to its effects on many other insect species, Dimilin exhibited contact toxicity against Manduca and high stomach doses reduced faecal output. There being no previous reports of the isolation of a parasitic fungus from Manduca, a screen was performed using isolates of four species of entomopathogenic fungus, Metarhizium anisopliae (M.E.I.) proved to be very effective and a reproducible bioassay was developed using a dipping technique. Interference from antifungal compounds in the artificial diet was circumvented by feeding experimental insects on tomato leaves. Conidia which were presoaked (P) for 20 h in distilled water prior to inoculation on Manduca larvae germinated more quickly than fresh conidia (F); as a consequence pre-soaked conidia were more pathogenic than fresh. Dual applications of Dimilin (ED50 dose) and Metarhizium anisopliae (ED50 dose) were synergistic in their action against second instax Manduca larvae (using Benz's (1971) criteria). The greater efficiency of pre-soaked conidia (P) was still apparent when they were applied in combination with Dimilin (D). DP treatment killed significantly more second instar larvae than DF. It was hoped that the greater pathogenicity of P and DP treatments would be translated into a reduced period of high relative humidity necessary for the initiation of infection. However, the results were disappointing. Most deaths in the combined insecticide/fungus treatments were due to mycosis, adding support for the hypothesis that the action of Dimilin on the cuticle facilitated the entry of the fungus into the insect. An ultrastructural study was carried out to seek direct support for this conclusion. Treatment with Dimilin did not stop the growth of abdominal tergal cuticle. Post-ecdysial Dimilin-affected cuticle, however, was characterised by the absence of lamellae and presence of globules of melanin-like material. Lateral growth of Metarhizium between the lamellae of normal cuticle was a prelude to vertical penetration primarily via pore canals. Progress towards the epidermis apparently did not occur in a step-wise fashion. Cuticle of Dimilin-treated insects provided little resistance to penetration by hyphae of Metarhizium. Widespread histolysis of post-ecdysial cuticle occurred. In addition, although lamellate pre-ecdysial cuticle was not affected by Dimilin, the cuticle of the pore canals was similar in appearance to, and presumably laid down at the same time as, the post-ecdysial cuticle. Pore canals were, therefore, areas of weakness in the pre-ecdysial cuticle which as a consequence failed to provide a mechanical barrier to the penetrating fungus.
|Date of Award||1983|