Answer: Based on your description it sounds like your plants might have a spider mite infestation. These insects are especially active in hot and dry weather. You can try knocking them down with a sharp spray from the hose or use insecticidal soap according to the label instructions. In my experience these are not usually a problem on foxgloves, but every now and then they do crop up. These plants are biennials as a rule, so if they are blooming this year they will in all probability die out rather than come back next year. You may want to allow a few to set seed each year to replenish your foxglove patch for coming years. If you have the "perennial" foxglove Digitalis mertonensis, the infestation might weaken the plant but I wouldn't expect it to cause long term harm. Just be sure the plant receives plenty of water to help it recover, especially if the summer is hot and dry.
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