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They weren't paying much attention to me, so aside from keeping a mindful eye towards possible nest sites, I didn't think about them much, although it is early to be seeing so many.
But this morning, checking on things, and making mental notes of gardening & harvesting needs, I noticed a group of yellow jackets at the base of a tomato plant. Much to my surprise, they were all over a doomed tomato hornworm (the caterpillar of a sphinx moth), which was either still alive, and twitching post-stings, or was being flung about by their feeding.
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I hadn't much thought about what yellow jackets eat -- we normally worry about hitting a nest with a shovel or a mower and inciting stings or their late summer preference for sugary picnic foods -- but they're tremendously good predators of caterpillars, beetle larvae, flies, and other potentially troublesome garden insects. So these adults were carrying back bits of the protein-rich caterpillar to provision nest larvae, in addition to their nectar and fruit consumption.
There wasn't much left of the hornworm by the time I returned from my morning walk.