If you are planning to build a pergola, the short answer is no, you do not need pressure treated wood. However, there are some benefits to using pressure treated lumber when building outdoor structures like pergolas. Here are a few things to consider:
Pressure treated wood is more resistant to rot and decay than regular lumber. This is because it has been treated with chemicals that protect it from insects and fungal growth.
Pressure treated wood is also more resistant to water damage. This makes it a good choice for any structure that will be exposed to the elements, like a pergola.
If you choose to use pressure treated lumber for your pergola, be sure to select a type that is rated for ground contact. This will ensure that the lumber can withstand moisture and humidity without warping or rotting.