The specialist baculovirus SeMNPV egt gene facilitates tree-top disease in their Spodoptera exigua caterpillar hosts

Many parasites enhance their transmission by manipulating host behaviour. One example concerns baculoviruses that induce hyperactivity and tree-top disease (i.e. climbing to elevated positions prior to death) in their caterpillar hosts. Little is known about the underlying mechanisms of such parasite-induced behavioural changes. Here, we studied tree-top disease in Spodoptera exigua caterpillars induced by the specialist baculovirus S. exigua multiple nucleopolyhedrovirus (SeMNPV). SeMNPV induces a light-dependent climbing behaviour prior to death in S. exigua larvae. Moreover, deletion of either the open reading frame or the ATG start codon of the viral ecdysteroid UDP-glucosyltransferase (egt) gene prevents this induced behaviour, indicating that the EGT protein is required for inducing tree-top disease. We hypothesize that EGT induces this behavioural change indirectly via prolonging the larval time to death.