Although stilbene fluorecent brighteners have been demonstrated to increase the susceptibilities of many lepidopterous insects to insect viruses, little information exists on the effects of a brightener on the susceptibility of the codling moth, Cydia pomonella (L.) (Lepidoptera: Tortricidae) and the false codling moth, Cryptophlebia leucotreta Meyrick (Lepidoptera: Tortricidae). In the present study, we tested the effects of the fluorescent brightener 28 (UNPA-GX) on increasing of the insects' susceptibility to a granuloviruses. The bioassay tests showed that brightener did not increase the susceptibility of codling moth neonate larvae and false codling moth larvae to the granulovirus (CpGV). The same result was obtained when the false codling moth GV was tested in presence of brightener against its homologous host. In addition, brightener failed to change the susceptibility of the codling moth to ClGV.

In histological sections from the codling moth's midgut, the peritrophic membrane (PM) was found only in the anterior part of the midgut of fifth instar larvae, while the middle and posterior part of the midgut was PM free. Thus, the food and/or virus is in direct contact with the microvilli. No membrane could be detected in the first and third instar larvae. Use of the electron microscope confirmed our findings by light microscope. In contrast, light and electron microscopic examination showed that the PM is much thicker in the turnip cutworm, Agrotis segetum (Denis & Schiff.), larvae and lines the entire midgut.

The present findings may explain why the tortricid species were highly susceptible and could not be enhanced by brightener but susceptibility of the turnip cutworm can be enhanced by brightener.