Answer: Based on your description of the webbing and yellowing, the problem may be spider mites. These can be controlled by pruning out the most infected section of the plant (place this in the trash, not the compost pile)and spraying the remainder with insecticidal soap. Be sure to spray the bottoms of the leaves and along the stems. Read and follow the label instructions. (Coreopsis should regrow quickly, you could help it along with a little extra water.) Spider mites thrive in hot dry weather, so an occasional rain or strong spray of water from the garden hose can also help by knocking them down periodically.
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