Answer: There are many caterpillar-type pests in the garden and they have a penchant for sweet flowers. I'd suspect caterpillars (or budworms) are doing the damage, especially if they are leaving the bitter tasting Nasturtiums alone. Caterpillars are the larvae of moths and butterflies, both considered beneficial pollinators. If you feel you must control them, the botanical Bt (Bacillus Thuringiensis) is registered for control of caterpillars.
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