Martin’s advice (always with humor) for picnics is the chapter for “Outdoor Eating.” Here it is:
It is true that some rules for eating outdoors are different from those that apply indoors. For example, it is permissible to execute extraneous wildlife found crawling across the picnic table, while any such creature making an appearance at a private indoor dinner table must be ignored by the guests. At picnics, one may kill ants, but not complain of their presence. Accepting discomfort cheerfully is the basic rule of picnic behavior. If one is unalterably opposed to being bitten, sunburned, and having sand mixed with one’s food, one should not go picnicking. The exception is that a small child drowning in a creek may call out to the adults at the picnic table, even if it means interrupting their conversation.
Nevertheless, it is important not to introduce the discomforts of civilization into a picnic to compete with nature’s own discomforts. Radios, plastic forks and knives, paper plates and napkins, and tin cans are among the abominations that one has no right to bring to the countryside. The well-supplied picnic basket must include implements with which food may be served and eaten with dignity, and no one can eat decently from a paper plate with a plastic fork, since when the side of the latter is applied to the center of the former, they both buckle, with disastrous results.
Food should be chosen that can be served at its proper temperature and should be repackaged so that it may be served with no commercial containers appearing on the table or grass. I food is cooked at the picnic area, no more allowances are made for the chef’s ruing it than would be made at a dinner party indoors.
The difference between indoor and outdoor manners are: One may never spill food accidentally indoors, but it is permissible to spill outdoors onto the grass, although not the blanket, tablecloth or dog. Children may be served first at picnics in hope that they will then go play in the poison ivy. One may perform such normally unacceptable acts as reaching across the table, reclining at one’s place and licking one’s fingers, provided that they are done with grace. Everyone gets to help clean up, this not being indoors, the sole privilege of the hosts. There is no seating plan, so that people may sit where they wish, although it is customary to ask permission before putting one’s head on someone else’s lap.
See Judith Martin. Miss Manner’s Guide to Excruciatingly Correct Behavior New York: Norton, 2005.