Your treatment will absolutely work in the rain. In fact, rainfall actually spreads the pesticide into more cracks and crevices around the outside of your house and allows for even better penetration in the top layer of the soil. This is why we frequently use granular pesticides in the rain. This method of application is much like spreading fertilizer. The pesticide granule is activated by rain in the same way a fertilizer granule is activated. These materials are in no way ill-affected by the rainfall. The only exception to this is when a pesticide is applied next to an outside foundation wall and heavy rains and poor drainage allow for rushing water to carry the material away. These occurrences are very rare and our professionals are trained not to apply materials in such circumstances.
Also, the materials we use in liquid applications are residuals that separate from the water in which they are mixed. These residuals are similar to micro-droplets that firmly adhere to treated surfaces and aren't easily washed away by rain. Most structures have eaves that also protect the outside foundation wall to soil juncture and further keep rains from affecting our treatment.
In the case of liquid termite treatments, termiticides are applied in high volume to trenches dug by our service professionals. Once the trench has been treated, it is backfilled with the excavated soil protecting the treatment. Of course any treatments in a crawlspace or a sub-slab concrete area aren't exposed and are not of concern during rainfall.
So as you can see, except in the case of heavy, flooding rains general pest control and termite control can be performed in the rain while still ensuring you of a quality treatment.