Late summer is the time of year when wasps become most noticeable in Iowa. Unfortunately, one of the places they are noticed is in the orchard and vineyard where they feed on the ripe fruit. This creates two problems: wasps may be an annoyance and a danger to pickers and damaged fruit will be unsalable. The two types of wasps most commonly seen attacking fruit are paper wasps and yellowjackets.
Both are social insects that build paper nests in a sheltered location –– under building eaves for paper wasps and in the ground, or building wall or attic for yellowjackets. The workers from the colony travel up to a few hundred yards from the nest while looking for food. In the early summer the wasps forage for caterpillars and other “meat” items and are beneficial predators. However, in the fall they prefer sweets such as soda pop, candy and the sweet juices of fruits and vegetables.
Yellowjacket wasps are the size and shape of honey bees (about 1/2 inch long). Because of the similarity in size and in behavior they are commonly called “bees.” Even from a distance you can tell the difference. Yellowjackets are shiny black with bright yellow markings. Honey bees are golden brown and fuzzy.
Paper wasps build the familiar umbrella shaped nests found hanging by a short stalk on the undersides of building eaves. Only a single tier of cells is constructed and there is no external covering over the nest. Each colony normally contains fewer than 25 wasps, but late in the season, the number may swell to over 100. Paper wasps are slightly longer and more slender than yellowjackets, and color is variable among the many species.
Yellowjackets and paper wasps are ecologically beneficial (they eat other insects) and they would be tolerated if the nest were in an out of the way location and if they did not feed on the fruit. Control options are limited and effectiveness varies, but in general the sooner wasp control measures are taken, the better. Insecticide sprays are not usually practical because spraying kills very few of the foraging workers and sprays are difficult to use because of harvest waiting intervals.
Discourage wasps and yellowjackets from feeding on ripe fruit by harvesting frequently and thoroughly. Pick as soon as fruit begins to ripen and remove all over–ripe and damaged fruit, beverages, candy, lunches and other items that might be attractive to wasps.
Several yellowjacket traps are on the market that can be used outside the perimeter of the planting to discourage yellowjackets from feeding on fruit. Place traps before the fruit begins to ripen. Different baits and traps may have to be tried to determine if any traps/baits will work in a particular planting or orchard. Place traps early (July or early August) to improve chances of success. Once the yellowjackets have found the ripened fruit, the traps will be of little benefit.
Finally, nests can be destroyed by applying an insecticide into the nest during the night. This provides control within a few days for those nests that can be located. Do not plug a nest opening in a house wall until you are sure all activity within the nest has stopped.
Paper wasps and hornets build above–ground nests on trees and buildings. Use a “wasp and hornet” aerosol spray specifically made for treating a distance of several feet. Retreat in 2 or 3 days if necessary. Yellowjacket nests in walls or in the ground can be destroyed by placing an insecticide dust (e.g., Sevin or permethrin) in the nest entrance during the night. Dusts are preferred to sprays that will soak into the soil. Dusts will remain on the tunnel surface to be picked up by passing wasps.