Big spiders are not spider mites. Spider mites can be found on the bottom of the leaves and can hardly be seen without a magnifying glass. In fact, one of the ways to determine if your plant is infested with these critters is to put a piece of white paper under a damaged leaf and tap the leaf. If there are a lot of little black dots on the paper, you probably have a spider mite problem. As to whether or not the big spiders are a problem, I suspect it depends on the type of spider you have in your garden. Generally, spiders are predators and eat other insects which may cause damage to plants and really are not considered to be a problem unless they are poisoness or you hate spiders.

Smiles,
Lyn

[From A Year of Roses, by Stephen Scanniello, p. 108:] As if overnight, spider mites will explode into action if conditions are right. In July, the potential is there: hot and humid weather with very little rainfall... There are many different types of these tiny pests, the most common one in the garden being the Two-spotted variety. Often called red spider mites, or simply red spiders, they have the appearance of tiny specks of paprika on the underside of the foliage.
An infestation is evident when the lower foliage of your roses starts to turn dusty yellow or even gray. If you're not sure, hold a leaf under a hand lens and you'll see the mites... [Scanniello provides a lot of information about these pests, including ways to defeat them, so please refer to the text for more information.]
[From Green Thumb Wisdom: Garden Myths Revealed!, by Doc & Katy Abraham, p. 120:] The hotter it gets, the hotter an insect gets. When hot weather hits, insect populations explode... As temperatures increase, the time for insects to complete their life cycle decreases... With spider mites, the effect of temperature is more dramatic. Mites take 30 days to complete the egg-to-adult life cycle at 60 degrees F (16 degrees C), 21 days at 64 degrees F (18 degrees C), 14.5 days at 70 degrees F (21 degrees C), and only 3.5 days at 86 degrees F (30 degrees C)...
[From Roses, by Susan Bales, p. 86:] A weekly spray of strong water aimed at the underside of leaves ought to prevent them.