After being fed on a diet of antibiotics, hawkmoth (Manduca sexta) caterpillars were infected by non-pathogenic bacteria (E. coli), followed by exposure to a second, but lethal insect pathogen (Photorhabdus). Investigation of their blood then showed that antibacterial peptides were being produced by the so-called 'fat-body', an organ specialised in protein production. These proteins appear to be able to persist from the initial benign E. coli infection and then confer resistance against the second, usually lethal, infection by the pathogen. Using RNAi techniques workers at the University of Bath have shown that several different proteins can confer this protective effect against subsequent infections.
Micro-organisms such as bacteria, fungi and nematodes are often used as biocontrol agents against insects. It had always been assumed that insects in the field would be naïve to such control agents but these results raise the possibility that control with one pathogen may confer resistance to another. Most experiments on insect immunity are conducted in the laboratory on insects often fed on antibiotic containing diet so these results suggest that the immune response of insect constantly exposed to pathogens in the field may be very different from all the work described in the laboratory. This raises new challenges for the field and places into ques tion the relevance of laboratory based studies on immunity.