Implicitly declared destructor

If I'm correct, a default destructor is always implicitly declared, unless the user declares it. According to cppreference:

Deleting an object through pointer to base invokes undefined behavior unless the destructor in the base class is virtual

Now, consider this example:

struct B {};
struct D : B {};

Is the implicitly declared destructor B::~B() virtual? If not, should I always declare a virtual destructor when using inheritance?