Good Morning Paige, 

Thank you for contacting the U of M Extension Service. 

Here is a link from the U of M Extension Yard and Garden website about scale insects: https://extension.umn.edu/yard-and-garden-insects/scale-insects

This information states: 

• There are a variety of low impact and residual insecticides available to treat scales; correct timing is critical for successful management.

As you scroll through the information the information expands. There are several remedies to deal with the scales. 

Good Luck!

Paige, 

First it is important to identify which scale insect you are dealing with. The article states: 

It is important to properly identify the scale species present. This will tell you when scale crawlers are most likely to be active. The crawler stage is when scales are most vulnerable to management.

It also states: 

Horticultural oils can be applied in late winter or early spring as a dormant treatment.

• This is only effective against species that overwinter as crawlers or adults.
• It is not effective against scales overwintering as eggs

Residual contact insecticides

There are a variety of residual insecticides available to treat scales.

• Time their application to coincide when crawlers are active.
• These insecticides include pyrethroids (such as permethrin, bifenthrin, lambda cyhalothrin) and carbaryl.
• These products generally last one to two weeks after they have been applied.

I hope this answers your question. Timing depends on the activity of the scales. If you are not happy with this information, I suggest you contact a certified arborist: https://extension.umn.edu/planting-and-growing-guides/how-hire-tree-care-professional

Good Luck!