Have you tried a commercial insecticidal soap? You'll probably have better results. Adding a little bit of vinegar to the solution can really boost its effectiveness by dropping the pH.

How many mums do you have?

If what you have is Aphids understanding something about them will help in controlling them. Most all of the pesticides people use in an attempt to control Aphids also kills off any beneficials that could be present to aid in control.
The simplest method of control is a sharp water spray that will knock the wee buggers off the plants. Many people, not knowing what they are talking about, will tell you that those Aphids just return to the plant which they do not. Aphids can produce young almost as soon as they are born, so what appears after you knock some off is the next generation, not those you knocked off that cannot find their way back to the plant they were knocked off.
Aphids are also attracted more to lush green growth so plants that are fed too much Nitrogen will be more susceptible to an Aphid incursion. An unbalanced diet is as bad for plants as it is for us.

Here is a link that might be useful: About Aphids

as with most of us.. you are moving from HOBBY .. to infatuation .. lol ... some might suggest the next steps is insanity .. lol ...

at which time.. you must move beyond cures found in your kitchen .. dish soap is not insecticidal soap .. and cooking oil.. is not horticultural oil ...

and then you jump to over-reaction ... sevin is just way over the top.. with a bug that can be set back with a burst of water ...

contemplate what they said.. and study an learn about your vermin.. and act accordingly ...

when i did mums in livonia MI .. i pinched back every 6 weeks.. for fall bloom and to create more bushy plants.. one might suggest.. at your first pinching.. that you will be removing 75 to 90% of the aphids.. and at that point would be a great time... to treat the remaining.. to break the reproductive cycle of the vermin ... and that treatment can be as simple as blasting them with water ....;

i always found it mildly amusing .. that mums are the base plant for pyrethrum-based bug killers.. and here they are.. eating/sucking your mum.. whats that all about.. i am sure there is a science based answer [like different mums].. but that is not as amusing as the former...

ken

Yep, natural pyrethrums come from a different species of chrysanthemum than garden mums.