To solve the problem, we follow the steps closely related to the given solution:

1. **Substitute $x=1$ into the equation:**
Given that when $x=1$, $ax^{4}+bx^{2}+2=-3$, we substitute $x=1$ to find the relationship between $a$ and $b$.
$$a(1)^4 + b(1)^2 + 2 = -3$$
$$a + b + 2 = -3$$

2. **Solve for $a + b$:**
From the equation above, we can solve for $a + b$ by subtracting $2$ from both sides.
$$a + b = -3 - 2$$
$$a + b = -5$$

3. **Substitute $x=-1$ into the equation:**
Now, we need to find the value of the expression when $x=-1$. Given the original expression $ax^{4}+bx^{2}-2$, we substitute $x=-1$.
Since $x^4$ and $x^2$ are even powers, $(-1)^4 = 1$ and $(-1)^2 = 1$, the expression simplifies to:
$$a(-1)^4 + b(-1)^2 - 2$$
$$a + b - 2$$

4. **Substitute the value of $a + b$:**
Knowing that $a + b = -5$, we substitute this value into the expression from step 3.
$$a + b - 2 = -5 - 2$$
$$-5 - 2 = -7$$

Therefore, the value of the expression when $x=-1$ is $-7$, which corresponds to choice D. So, the final answer, encapsulated as requested, is $\boxed{D}$.