Besouro vs. escaravelho
While both besouro and escaravelho describe insects of the order Coleoptera, the choice between them is largely geographical. Generally, besouro is the standard term used in Brazil, whereas escaravelho is the most common generic term used in European Portuguese.
Besouro
B1In Brazil, besouro is the everyday, generic word for any type of beetle. In Portugal, this word is also understood but is often used specifically to describe large beetles that make a loud buzzing sound while flying.
O besouro pousou na flor amarela.
(The beetle landed on the yellow flower.)
Tem um besouro enorme voando perto da luz.
(There is a huge beetle flying near the light.)
As crianças brincavam de caçar besouros no quintal.
(The children were playing at hunting beetles in the backyard.)
Escaravelho
B2In Portugal, escaravelho is the standard word for any beetle. In Brazil, this word is quite specific and usually refers only to the dung beetle or the sacred insects depicted in Ancient Egyptian history.
O escaravelho é uma praga comum na agricultura portuguesa.
(The beetle is a common pest in Portuguese agriculture.)
Os antigos egípcios adoravam o escaravelho como um símbolo divino.
(The ancient Egyptians worshipped the beetle as a divine symbol.)
Encontrei um pequeno escaravelho caminhando na terra.
(I found a small beetle walking on the dirt.)
Summary
To sound natural, use besouro for general beetles when speaking Brazilian Portuguese. However, specific types like the Egyptian dung beetle are always called escaravelho in Brazil. If you are speaking European Portuguese, escaravelho is the safer, generic choice for the insect.