Watch out for yellowjacket nests. They can be underground, beneath landscape timber, in a rock wall or in the wall of a building.
Wasps die over the winter and only the pregnant queen survives, but by August and September, a colony can be huge. Yellowjackets alone account for one half of all human insect stings and their ability to sting repeatedly makes them even more of a threat.
With their definite yellow and black stripes and little hair, they are easily distinguished from honeybees which are more honey coloured and covered with fuzzy hair. Bees sting only once and do so in self-defence. The behaviour of wasps is more unpredictable.
Garden fruits are a great attraction to yellowjackets and can result in high populations. If you have wasps, avoid wearing perfumes, hair spray, or deodorant when working out of doors and don't go barefoot. Even bright or patterned clothing can attract their attention.
Don't swat or squish these wasps. Doing so releases a chemical that signals other wasps in the area to attack. The worst behaviour is to flail ones arms. Should they land on your skin to have a sip of sweat or inspect a smell, be patient, since they will fly away in due course. You can also brush them off with a piece of paper, but be slow and deliberate when doing so.
There are many useful wasps in the garden, most very small--almost too small to see. They do a wonderful job as parasites on undesirable insects and in pollination. Yellowjackets are one of the bad actors in a large family of beneficial insects.
All wasps are active during the day so control measures should be taken at dusk, preferably without a flashlight or cover the lens with red plastic since wasps can't see red. If nests are a distance from your property, you can leave them alone to die over the winter. If not, eliminate nests with GREAT CARE. FOLLOW THE INSTRUCTIONS FOR THE PRODUCT USED. Yellowjackets can be extremely aggressive when the nest is disturbed and a large nest can contain THOUSANDS OF WASPS. Don't hesitate to call a professional.