You did not mention what Sevin product you are referring to but the Sevin Concentrate product must make direct contact on the fleas for product to be effective per the product label so you would be making applications anytime fleas are present outdoors.
If you are looking to treat inside and outside for fleas, you should definitely get our Indoor/Outdoor Flea Kit. It comes with an excellent product called Onslaught, which is great for both outdoors and indoors. It is also very safe around pets and has a long residual. Also in the kit is an IGR called Martins IGR. It is photo-stable up to 14 days which is about as long as you will find for an IGR that will be outside. Of course you should mix both together in the same tank and apply at the same time both outside and inside.
Sevin Concentrate can be applied on plum trees. The product label does not list plum maggots. We recommend that you have the insect properly identified. Please click on the link below to receive the contact information to your local cooperative extension office.