Insufficient evidence of infection-induced phototactic behaviour in Spodoptera exigua: a comment on van Houte et al. (2014).

Some of the most spectacular interactions between hosts and parasites occur when parasites manipulate their hosts’ behaviour. Acanthocephalan worms that infect gammarid shrimps induce host behaviours that elevate predation vulnerability when they need to transmit to their final vertebrate host [1]. Ophiocordyceps fungi similarly increase transmission by forcing ants to clamp their jaws around leaves in elevated positions before killing them [2]. However, the mechanisms underlying such manipulations remain relatively obscure. A recent paper by van Houte et al. [3] claims to demonstrate that (i) infection with the baculovirus Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) causes S. exigua larvae to die in an elevated position; and (ii) this is achieved by the virus triggering a positive phototactic response in its larval host. Their study is grounded in knowledge that baculoviruses manipulate climbing behaviour in some lepidopteran species [4]. Here, we argue van Houte et al.’s study has significant limitations: the experimental design cannot test the authors’ hypotheses, and the data presented are open to other interpretations that do not support the authors’ conclusions.