|aerial wasp nest|
Most wasp species forage for live prey, mostly other insects, as well as carrion and plant sap. Maturing grapes are a source of plant sap and wasps will tear the outer skin to get at the grape juice inside causing yield and quality losses. Since wasp nests reach their maximum size about the time grapes mature, this damage can be significant in some varieties, in some years.
Worker Productivity and Safety
The social wasps build large communal colonies (nests) consisting of hundreds to thousands of individual wasps each capable of delivering a painful sting to an intruder. These colonies are built both above ground (aerial nests, see photo), and below ground in abandoned rodent burrows or other cavities (ground nests). Nests are aggressively defended against intruders and even a slight disturbance can incite swarming of a perceived enemy. These large, aggressive colonies can be a hazard to vineyard workers and can slow or even stop a work crew. Multiple stings from swarming wasps can be a medical emergency as well, especially if the victim is allergic to wasp venom.
For these reasons vineyards have tried to manage wasp nests in a variety of ways. Some managers depend on early season capture of queens prior to nest establishment while other vineyards use a kind of seek and destroy strategy for existing nests during the summer.
Poison Baits to the Rescue
Until about 10 years ago some growers used poison baits to control yellowjacket wasps in vineyards and orchards. At the time there was an insecticide that could legally be used to prepare a poison bait that when deployed in vineyards and orchards significantly reduced yellowjacket activity, sometimes for several years following a successful baiting program. The baiting technology was inexpensive and had virtually no effect on non-target organisms or the environment***. About ten years ago this pesticide was removed from the market, for reasons unrelated to its use in wasp baits, and there has been no replacement until now.
Onslaught Microencapsulated Insecticide is now labelled for use in preparing poison bait for yellowjacket control (read and follow label instructions). See Using Poison Bait To Control Yellowjacket Wasp Nests for details regarding bait preparation and deployment.
One or two bait stations per acre (see page cited above) should be sufficient to maintain control in most situations. Start baiting about mid-summer, replace baits about every three days until the level of control you want is achieved. Don't start much earlier than mid-summer since the low number of foraging yellowjackets prior to this time will limit the effectiveness of the bait.
***Because all wasps on predators they are considered beneficial insects, especially in agricultural systems. This benefit, however, must be weight against the potential for losses as outlined above.