Like all adult wasps, bees, and ants, adult paper wasps are limited to liquid diets – they have no chewing mouthparts, and the passageway between their head and abdomen, where food is digested, is so narrow that pieces of food wouldn’t fit through it. Wasp larvae (the white grub-like organisms in the upper third of the pictured wasp nest cells) are able to eat a wider range of food, due to mouthparts and their body structure. Adult paper wasps capture and feed caterpillars and other insects to their larvae. The larvae then digest their food and produce saliva rich in nutrients. The adult wasp proceeds to scrape her abdomen across the nest, producing a vibration that signals to the larvae to release some of their carbohydrate-rich saliva which the adult then drinks. (Cells covered with white paper nest material contain wasp pupae.)
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The European paper wasp (Polistes dominula) was introduced to the New Jersey Pine Barrens in 1968, and has since spread throughout most of the North America. Considered an invasive species by most entomologists, it looks more like a yellowjacket than the paper wasp that it is. The European paper wasp nests earlier in the spring, in a wider variety of nest sites, and feeds on a larger variety of insects than native species of wasps. In the photograph if you look closely you can see that it is scraping off a fine line of dead plant tissue on the underside of this leaf, assumedly for use in expanding its paper nest.