A number of species of yellowjacket wasps are pests of grape, the most numerous and troublesome being the western yellowjacket (Vespula pennsylvanica) and the German yellowjacket (V. germanica).
Two uncommon species of paper wasp (Polistes sp.) are native to our area. The European paper wasp (P. dominula) was found in B.C. in 2003 and has since become very common, particularly around buildings.
Yellowjackets, hornets and paper wasps are important predators of grape pests throughout most of the year, but in fall they often feed in large numbers on damaged and intact fruit.
Wasps are also an annoyance and potential health hazard to vineyard workers and to pickers during harvest operations. Paper wasps are less aggressive than yellowjackets or hornets and are not as prone to stinging.
Honey bees occasionally feed in large numbers on ripe fruit. The extent of the damage is variable, mostly occurring on the edges of only certain vineyards and in association with wasp damage. Honey bees will be unintentionally damaged by sprays directed at other grape pests, and it is important to try to preserve these beneficial pollinators.
Yellowjacket workers are about 1.5 cm in length, and their abdomen appears largely yellow with thin black bands.
Hornets are slightly larger and appear largely black with yellow or white bands.
Paper wasps have thinner bodies with a more pronounced constriction (stalk) between the thorax and abdomen. The body is dark with some yellow markings, but they appear reddish-brown due to the colour of the wings and distal parts of the legs. In flight, the legs of paper wasps dangle visibly below the body.
All of these wasps produce paper nests, but the brood cells of European paper wasps are not enclosed within an outer covering.
Yellowjackets and paper wasps overwinter as fertilized females in sheltered locations. These queens seek suitable nest sites in spring and establish a new colony.
Nests are initially small, consisting of only a few paper cells, but as workers develop, they assist in building up colony numbers and the size of the nest. Depending on the species, nests can be in the ground, under the eaves of houses, in hollow trees and the walls of buildings, in milk carton vine shelters or exposed in trees.
In fall, in addition to workers, colonies produce both males (drones) and queens. When freezing weather arrives, all the drones, workers and the undeveloped larvae and pupae are killed.
Wasps prey on other insects, scavenge food and feed on nectar and the juice of fruit. It has been reported that wasps do not damage fruit themselves but take advantage of injury caused by other insects or birds, diseases such as sour rot or splitting of fruit. There are contrary reports, however, particularly relating to the European paper wasp, saying that they are able to directly feed on intact fruit.
As noted, wasps are also a major annoyance, and their stings result in swelling and a painful itch and can trigger severe allergic reactions in some individuals.
There are no established thresholds for wasps, and the need to spray is determined by the variety of grape, past history of damage and tolerance level of individual vineyard managers.
Trapping of wasps is most effective early in the season before the colonies have increased in number. The arrival of the European paper wasp has made it necessary to control wasps more frequently.
For more information about insecticides, see the Pesticides page.
Wasp colonies can be attacked by other wasps or hornets. Eastern and western kingbirds feed on foraging adults, and larvae are subject to certain diseases. The onset of winter eliminates established colonies. The overwintering queens may also succumb to cold or be eaten by other insects or even by mice.
Elimination of nests early in the season can be an effective management strategy, but they are often located outside the vineyard or are well hidden.
Aerial nests in trees or under eaves can be removed and placed in a plastic bag at night when wasps are inactive. Nests in walls or natural cavities can be treated by spraying or puffing commercial wasp control products into the entrance holes at night. Sealing of entrances can help prevent future nests.
Trapping can be used to reduce numbers of foraging wasps in localized areas. Commercial wasp traps that use an attractant dissolved in water have proven effective, as have homemade versions containing a sugary solution placed in recycled 1 litre plastic pop bottles. Wasps enter through small holes and eventually fall into the liquid solution and drown.
Trapping early in the season when populations are low and queens are establishing colonies is generally more effective than attempting to remove large numbers of wasps later in the season.
Baiting traps with fish or other meats in spring when wasps are seeking a protein source can be an effective alternative to other baits, but care should be taken to avoid attracting unwanted wildlife.
