> I'm confused. DWV is a picornavirus, which is a positive, ssRNA virus, is it not?

Yes, the viruses are single-stranded, but the RNA sequences used to knock them down are double-stranded.

> Honey bees are commonly infected by positive sense single-stranded RNA (+ssRNA) viruses, which replicate via a dsRNA intermediate. RNAi mediated by virus-specific dsRNA is an important antiviral defense mechanism in honey bees ...

But note:

> ... nonspecific dsRNA-triggered antiviral response pathway(s) also play a role in honey bee antiviral defense. Honey bees treated with sp-dsRNA or ns-dsRNA had reduced virus abundance. 

In other words, the defense mechanism kicks in regardless of the sequence of the interfering RNA. Continuing:

> Many viruses generate long dsRNA molecules during their replication cycle. Long dsRNA molecules are not a typical product of eukaryotic gene expression, so they serve as triggers of eukaryotic antiviral immune responses

source of info:

Brutscher, Laura M., Katie F. Daughenbaugh, and Michelle L. Flenniken. "Virus and dsRNA-triggered transcriptional responses reveal key components of honey bee antiviral defense." Scientific reports 7.1 (2017): 6448.

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