Answer: Without seeing the pest it's hard to make an absolute identification, but your description matches that of the Mexican bean beetle larvae. They're yellow with black spines. If you can squish the larvae, you'll keep them from becoming egg-laying adults. Sevin is not the chemical of choice in this case. In fact, Sevin is highly toxic to bees and extreme care should be taken to use it only on plants that are not flowering. Since the Mexican bean beetle larvae are soft-bodied, you can use Insecticidal Soap to get rid of them. Next year plant your beans in a different part of the garden, and cover them with a floating row cover to exclude the egg-laying adults.
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