How to query NCBI (Nucleotide database) by a feature qualifier?
For this particular query, try host=Bemisia tabaci B[WORD].  However, you may also be interested in related queries like host=Bemisia tabaci B biotype[WORD].
But, I am not sure a general solution exists, as I do not believe all qualifiers are indexed.

Also, you can see how your query is interpreted on the right side under &quot;Search details&quot;.  For the original attempt ([host]&quot;Bemisia tabaci B&quot;), it is interpreted as:
host[All Fields] AND &quot;Bemisia tabaci B&quot;[All Fields]

while the goal is to produce
host=Bemisia tabaci B biotype[WORD]