Zombies without puppeteers: Morgan's Canon and the passive manipulation hypothesis in Xenos vesparum/Polistes dominula.

Parasite-associated changes in host behavior are often framed as manipulation, especially when altered behavior benefits parasite transmission. In the Xenos vesparum/Polistes dominula system, infected wasps abandon nests, aggregate before hibernation, overwinter, and later join established nests; a sequence often described through zombie or puppet-master metaphors. Recent studies have tied this phenotype to caste-related plasticity, gene expression, neuroanatomy, and ecological context, but the causal distinction between active parasite control and passive activation of host responses remains unresolved. Here I argue that passive manipulation should be treated as a useful null model and possible evolutionary starting point: infection may trigger pre-existing host responses that the parasite subsequently exploits. Archived pseudo-parasite observations are presented as an illustration: a sterile nylon implant, with no capacity for signaling, was associated with reduced energy use and nest desertion, whereas encapsulation intensity did not predict metabolic change. These observations do not prove passive manipulation by X. vesparum, but they show that early components of the infected phenotype can be induced without a living parasite. More broadly, parasite-benefiting behavior should not be treated as sufficient evidence for parasite-designed control. Distinguishing active from passive manipulation requires experiments that compare true infection, mechanical insult, parasite secretions, and naturally occurring host alternative phenotypes.