Answer: The hard, white to yellow spots under the skin of a tomato are caused by insects with piercing, sucking mouthparts feeding on the fruit as it nears maturity. This would include stinkbugs and their "cousins" the leaf-footed plant bug. When these same pests feed on small, newly formed fruit, it causes dimples in the fruit known as "catfacing".
Products with pyrethrin and rotenone (several brands are available) are a natural control option. Products with methoxychlor (Marlate is one brand) are one of the lowest toxicity synthetic options. If you want to avoid spraying entirely, you can protect the plants with a fabric row cover placed over hoops to keep the insects out. Since tomatoes are self-pollinating, you don't have to worry about leaving the cover in place all season.
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