AbstractParasites have evolved different virulence strategies to manipulate host physiological functions. The parasitoid waspCotesia congregatainduces developmental arrest and immune suppression of its Lepidopteran hostManduca sexta. In this interaction, a symbiotic virus (C. congregataBracovirus, CcBV) associated with the wasp is essential for parasitism success. The virus is injected into the host with wasp eggs and virus genes are expressed in host tissues. Among potential CcBV virulence genes, cystatins, which are tight binding inhibitors of C1A cysteine proteases, are suspected to play an important role in the interaction owing to their high level of expression. So far, however, potentialin vivotargets inM. sextaare unknown. Here, we characterized for the first time fourM. sextaC1A cysteine proteases corresponding to cathepsin L and cathepsin B and two different ‘26–29 kDa’ cysteine proteases (MsCath1 and MsCath2). Our analyses revealed that MsCath1 and MsCath2 are transcriptionally downregulated in the course of parasitism. Moreover, viral Cystatin1 and MsCath1 co-localize in the plasma following parasitism, strongly suggesting that they interact. We also show that parasitism induces a general increase of cysteine protease activity which is later controlled. The potential involvement of cysteine proteases in defense against parasitoids is discussed.