[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.