If you see a caterpillar near a cluster of aphids, don't squash it. It could very well be the larva of a syrphid or hover fly (family Syrphidae) and it's eating aphids.
What do they look like? I happened to capture an image of a tiny syrphid larva on a rose leaf, and sure enough, it was eating aphids.
Community ecologist Louie Yang, who joined the UC Davis Department of Entomology faculty last year, has also photographed syrphid larvae. He recognized this one right away.
If you want to learn more about syrphid flies, be sure to read Flower Flies (Syrphidae) and Other Biological Control Agents for Aphids in Vegetable Crops, Publication 8285 (May 2008), UC Division of Agriculture and Natural Resources. It's primarily the work of UC Davis entomologist Robert Bugg; with expertise offered by Ramy Colfer, chief organic agricultural researcher, Earthbound Farms, Salinas; William Chaney, farm advisor, UC Cooperative Extension (UCCE) Monterey County; Hugh Smith, farm advisior, UCCE Santa Barbara and San Luis Obispo counties; and James Cannon, UC Davis computer resource specialist.
In the publicaiton overivew, Bugg writes that "Flower fly development involves complete metamorphosis, including egg, three larval stages, puparium, and adult. Adults of many flower fly species resemble stinging bees and wasps. This phenomenon is called Batesian mimicry, indicating that palatable organisms resemble or 'mimic' unpalatable models. Worldwide, there are many aphidophagous syrphid speices."
"Adult hover flies require honeydew or nectar and pollen to ensure reproduction, whereas larvae usually require aphid feeding to complete thir development."
Below, you'll see a syrphid larva doing what it does best: eating aphids.