Stings from yellow jackets and hornets are -becoming an increasing problem for orchard -workers who pick fruit and for -customers who patronize outdoor farm markets. These wasps, like people, are drawn to ripe fruit and sugary drinks like apple cider.
Dr. David Biddinger, an entomologist at the Pennsylvania State Fruit Research and Extension Center in Biglerville, attributes larger -numbers of wasps to a change in insecticides. The newer pesticides that are less dangerous to people are also less lethal to wasps, so wasps are more -numerous at harvest time than before.
The old broad-spectrum products like Guthion (azinphos-methyl), Lorsban (chlorpyrifos), and Penncap-M, which used to be the mainstays of pest control in orchards, have largely been replaced.
“I believe the softer pesticide programs we are using now are what is driving the increase in yellow jackets and hornets we see now,” Biddinger said.
“Softer, thin-skinned varieties are more -susceptible,” he said. “But earlier varieties like Honeycrisp and Gala miss a lot of the yellow jacket injury, as the insects have not developed their ‘sweet tooth’ phase yet and are still mostly predators.”
Interestingly, the appearance of brown -marmorated stinkbug in eastern orchards has probably reduced the incidence of wasp stings in the last three years. The reason? Growers have had to resort to older, “harder” insecticides to kill stinkbugs, and wasps have been hit as well.
“The use of pyrethroids and neonicotinoids in the fall for brown marmorated stinkbug has reduced the sting problem greatly, but comes at the cost of disruption of aphid and mite biocontrol and with potential impacts on bees,” he said.
The shift to more selective pesticides has complicated orchard pest -control in many ways for fruit growers, Biddinger said. “While there are greater-opportunities for biological control of some apple pests, and we are seeing a greater diversity of generalist predators, there has also been a resurgence of some pests such as plum curculio, woolly apple aphid, San Jose scale, and European apple sawfly that were incidentally controlled with applications of broad-spectrum insecticides for primary pests such as codling moth.
“We have also noticed increases of nuisance pests such as ticks, deerflies and mosquitoes in orchards as broad-spectrum insecticide use declined.”
And, he added, “The most dangerous resurgence directly affecting humans has to be that of increased numbers of hornets and yellow jackets.”
Not only do these critters sting humans, they damage ripe fruit, gouging out chunks varying in size from small holes to nothing-left-but-the-core. In u-pick orchards or farm stands, the consequences of biting into an apple or pear—or drinking a cup of juice—that has attracted a wasp can be life-threatening. A sting in the mouth or throat for some can lead to deadly swelling.
About one in a hundred people—including orchard workers—are allergic to wasp or bee stings, where health consequences can be deadly.
Is there evidence to show that more workers are being stung now than before?
“Growers I talk to are having more problems, especially with bald-faced -hornets, which are nesting more in the apple trees now that Guthion and other toxic insecticides are gone,” Biddinger said. “Pickers rushing to place ladders in trees for picking or just reaching through foliage on small trees are encountering these hornets more.”
Sweet tooth phase
The life cycle of hornets and yellow jackets is intriguing. Of the 5,000 or so that may live in a wasp nest by late fall, only a few specially cared-for females will survive to be fertilized and overwinter to become queens, creating new colonies the following season. All the others die, including the males that fertilize the new queens. (In warmer climates, nests may survive for more than one year.)
The new queens build a small paper nest and lay 50 or so eggs the next spring, from which workers will develop to care for more brood and build more nest. The queen quits working and becomes an egg-laying machine.
Developing larvae need protein to grow, so early in the season the worker wasps are predators bringing in meat to feed the brood.
“The adults of all species are voracious predators on all types of insects, including leafrollers, adult flies, and moths, in the summer months until they switch to their sweet tooth phase in the fall,” Biddinger said.
The sweet tooth is there all the time, but is satisfied in a different way early in the season. When tending brood, the adult wasps feed insects to the young.
And the developing larvae secrete a sugary substance, in a process called -trophallaxis, that provides energy to the workers who feed them.
When the brood becomes very large, this sugar source becomes inadequate and the adults become fruit eaters to get the sugar they need. That’s when -contact with humans escalates to problem levels.
“Roadside markets are having the most problems since the ripe fruit and cider samples are usually outside next to an orchard,” Biddinger said. “The problem comes with the public freaking out and swatting at them and getting stung.”
There are yellow jacket traps, he said, but it’s hard to lure them away from cider, which is more attractive.
There are five types of yellow jackets in Pennsylvania fruit orchards, -Biddinger said. They are the eastern yellow jacket (Vespula maculifrons), the German -yellow jacket (Vespula germanica), the southern yellow jacket -(Vespula -squamosa), the common yellow jacket (Vespula vulgaris), and the bald-faced hornet -(Dolichovespula maculata). The hornet is actually an unusually large, black and white yellow jacket. The Pennsylvania yellow jacket (Vespula -pennsylvanica) is not found in Pennsylvania, but only in the Midwest and -western states.
“For all of the species except the bald-faced hornet, the names matter little, since the biology is very similar amongst most species, and the only significant difference between species is nest size,” Biddinger said.
Yellow jackets, which are the most aggressive of all the wasps in the East, also build the largest populations, with over 5,000 individuals in a single nest by fall.
Hornets build nests in trees. The hornets are normally more docile than -yellow jackets, Biddinger said. “They will, however, vigorously defend their nest when it is disturbed by ladders or hands reaching for fruit. The more aggressive nature of the ground-nesting yellow jacket still causes problems for fruit pickers who disturb the insects as they feed on injured or fallen fruit.”
Since yellow jackets release an alarm pheromone when aggravated or squashed that attracts other yellow jackets, attacks near nests can easily lead to attacks in high numbers. Unlike the honeybee, which can only sting once and dies soon after, yellow jackets and hornets have smooth stingers that can sting repeatedly.
Yellow jackets mostly fly into orchards from nests in nearby buildings or in the ground.
To date, control measures have not been developed—other than a return to harsh, potentially IPM-disruptive insecticides, Biddinger said, and these probably would not affect the yellow jacket source nests. Yellow jacket damage is likely to be greater at orchard edges next to wooded areas or other nesting sites. Border sprays or -poisoned baits might have some effect.
Homeowners can buy control products for both -species, which generally consist of pyrethroids and -penetrants for quick knockdown. These are expensive and impractical for control in orchards except on a -limited basis, he said.
“The use of even a single pyrethroid spray after bloom would also most likely kill predatory mites and flare populations of pest mites,” he said. Biddinger has spent much of his career developing biocontrol -methods, only to see them disrupted by the invasion of brown -marmorated stinkbug.
Biddinger thinks a potential method of controlling hornets and yellow jackets would be to focus on the new queens in the spring, when they are just starting to build nests. “That would be much easier than killing them in the fall when the nests are much bigger, better protected, and when they consist of several thousand individuals.
“Timing of a single broad-spectrum organophosphate insecticide spray at a critical time may be key to reducing populations,” he said. The timing and effectiveness have not yet been studied.
The use of traps baited with fish or other meat early in the season may also be effective. These also have not been evaluated for use on a commercial scale.