Yellowjackets in Grapes
In late summer, outdoor meals are often disrupted by the appearance of yellowjacket wasps intent on sharing your food and drink. As you curse and swat at the hovering yellow and black wasps, always mindful of the painful sting if the invaders are faster than you, consider that these wasps are actually beneficial insects for the most part.
Yellowjackets are often some of the most important consumers of other insect pests -especially caterpillars in fruit crops - during the spring and early summer. At that time of year queens, and later their offspring, collect "meat" to feed their developing young or nest-mates. In late summer, yellowjackets become agricultural pests when their food preferences change from meat (caterpillars, scavenged flesh from dead animals, etc.) to sugar sources such as berries, grapes, or fruit. Of course, to picnickers they are always pests; likewise if they decide to nest in or around your home.
Yellowjackets feasting on grapes.
Although different species tend to nest in different places and may have somewhat different life histories, from a grape grower's perspective, all yellowjackets are problematic. They feed on ripe and damaged berries and are occasionally present in such large numbers as to seriously affect yield. In other cases they are just a nuisance when mechanically harvesting. However, for hand harvesting, yellowjackets can be a dangerous and disruptive pest for workers and grapes alike.
When yellowjackets are pests in grapes, they are very difficult to manage. There are no insecticides registered for commercial use on yellowjackets in grapes. Some landscapes provide excellent habitats for yellowjackets, and in these areas, yellowjackets may be a perennial problem. Early season trapping of foraging queens and destruction of new nests in the spring will have very limited (if any) impact on fall populations. Based on control attempts in other areas, in all likelihood, trapping queens in the spring is a waste of time. It would require a huge amount of effort to follow wasps to new nests, so trapping early season queens seems like a better strategy. Unfortunately, when tried in other countries, this approach did nothing to prevent later season migrants from entering and colonizing the area.
Trapping wasps later in the season may help lower the damage on grapes but the trapping needs to be started early and maintained through harvest. Trapping will not eliminate all yellowjackets in the area - it will only lessen the problem. Commercial traps are available at any hardware store or you can build some yourself. Any structure that acts as a funnel (like a minnow trap) to prevent the wasps from flying back out will work, but good fresh bait is essential (see below). You can also make a yellowjacket trap by suspending a fish over a pan of water with detergent in the water to lower the surface tension. Slit the sides of the fish to make it easy for the wasps to forage. They will fly in, chew off a chunk of fish, and when they try to fly away they will dip into the water and drown. The fish need to be changed every other day because yellowjackets prefer unspoiled meat. If raccoons are also a local problem, these traps should not be left out at night. Keep in mind that in years where yellowjackets are plentiful, you will need a lot of any kind of trap and a lot of time to service them every few days.
Early season bait should be fresh meat or fish (tuna works well) but later in August, sweet liquids are best. Commercial liquid baits do work, but recent research showed that Mountain Dew was the best attractant over orange soda or commercial bait (Wegner and Jordan, 2005). Jam, honey, molasses, yeast mixtures, and even beer (what a waste!) have also been used as wasp bait with varying degrees of success. When yellowjackets are plentiful, just about any sweet liquid will attract dozens to funnel traps each hour so the traps need to be serviced daily or they will lose effectiveness when full of dead wasps.
Necessity is the mother of invention, and I have heard of berry growers attempting to remove yellowjackets with portable vacuums just ahead of pickers! I'm not sure how well that worked, but grape growers in some areas have reported good results with extensive trapping in spite of some studies that have suggested trapping is ineffective. So be inventive and share your success stories with us if you have to battle an infestation of yellowjackets this fall.
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