Etymology (dawa)

From Arabic دَوَاء ‎(dawāʾ).

Noun

dawa (n class, plural dawa) or dawa (ma class, plural madawa)

1) medicine

Usage notes

In its strictest sense, this word means "medicine", but is used generally for anything that improves or protects the condition of another thing; for example dawa ya viatu ‎(“dawa of the shoes”) means "shoe polish".

From Wiktionary: https://en.wiktionary.org/wiki/dawa

Note: So, for insects the meaning of "medicine" or "improving protection" is not applied. ;)

I think the Arabic etimology can explain better this meaning:

Etymology (دواء)

From the root د و ي ‎(d-w-y); compare دَوًى ‎(dawan, “disease”), دَاوَى ‎(dāwā, “to treat a disease”).

Noun

دَوَاء • (dawāʾ) m (plural أَدْوِيَة ‎(ʾadwiya))

1) (medicine) remedy, medicine, medication, drug

From Wiktionary: https://en.wiktionary.org/wiki/%D8%AF%D9%88%D8%A7%D8%A1#Arabic

(So, perhaps the meaning for this phrase would be "Drug of insects" or "Drug for insects) =)

bug spray is synonymous with insecticide