George Heimpel is a little like a general who sent his troops out to attack the enemy and is now awaiting news from the front.
The University of Minnesota entomologist is leading the war against soybean aphids, a foreign invader that costs soybean growers millions of dollars in treatment costs and lost yields. His warriors: tiny parasitic wasps that kill soybean aphids.
The stingless wasp, Binodoxys communis, is an important natural enemy of soybean aphids in their Asian homelands. When soybean aphids arrived in the U.S. in 2000, their natural enemies — which keep them in check back home — were left behind.
So Heimpel and other Midwest entomologists went to China and brought back Binodoxys communis and several other strains of wasps that attack soybean aphids. After four years of laboratory safety testing at the University of Minnesota Insect Quarantine Facility and two more years of federal and state permitting, Binodoxys communis was approved for release last summer in seven Midwestern states.
Now, scientists are waiting to see if the wasps they let loose in soybean fields last summer survived.
A successful introduction would be a big advance in long-term soybean aphid control, says Iowa State University Entomologist Matt O'Neal. He's heading up the soybean aphid research in Iowa, where a record number of acres were sprayed for aphids in 2007. Binodoxys communis may become part of an integrated aphid management strategy that includes insecticides and — before long — aphid-resistant soybean varieties, O'Neal says.
“Right now, insecticides work well against soybean aphids, and we recommend their use,” he says. But “we want to reduce the need to use one method over and over because that leads to insecticide resistance.”
IT'S A BUG-EAT-BUG world out there. The wasps were released last June through August at three-dozen sites in Minnesota, Iowa, South Dakota, Wisconsin, Illinois, Indiana and Michigan. Heimpel supplied the insects to researchers in participating states.
Small cages were set up over soybean plants in aphid-infested fields. The cages allowed aphid numbers to build up inside and kept out predators. Adult Binodoxys communis and pupae were placed in the cages.
The wasps lay their eggs inside soybean aphids. When the tiny larvae hatch, they eat the aphid's inner organs, and pupate inside the aphid's hollowed-out body, forming what is called a “mummy.” The wasps were allowed to reproduce for one or two generations in the cages, and then they were liberated to prey on aphids in the field.
During the summer, researchers monitored the release sites, documenting wasp survival, numbers and movement. In the late fall, they searched for the wasps in the aphids' winter habitat, buckthorn.
Because these wasps are no bigger than a pinhead, researchers monitor their movements in the field by looking for telltale mummies. This is pretty much like “looking for a needle in a haystack,” Heimpel says.
Entomologists were able to confirm that Binodoxys communis reproduced in soybean fields last summer, Heimpel says. In Minnesota, where most of the 2007 fieldwork was done, more than 12,000 wasps were released at 17 sites. “In most cases, we found mummies outside the cages,” he says.
What happened during the fall migration was less clear. Researchers looked for evidence that winged soybean aphids carried the parasite to their winter host plant. However, “We were not able to find them in buckthorn,” Heimpel says.
Establishment may have been hampered by an abrupt crash in soybean aphid populations last August, following heavy rains across the Corn Belt, Heimpel says.
Iowa, for example, had the third-wettest August on record. “Fungal pathogens wiped out the aphids,” O'Neal says, but only after “one of the worst aphid outbreaks since they arrived.”
Half of Iowa soybean acres were sprayed for aphids in July, O'Neal says, “and in some parts of the state, up to 90% of acres.”
Other states released fewer wasps than Minnesota, because of permit timing, but reported similar results.
IN ILLINOIS, wasp cages were opened Aug. 6 in an organic soybean field in Mercer County that had over 1,000 soybean aphids/plant, reports Kevin Steffey, a University of Illinois entomologist. By Aug. 22, after heavy rains, the aphid population had plummeted to 50/plant, he says. At a second release site in DeKalb County, aphid numbers were over 400/plant on Aug. 15 when the wasps were freed, but dropped to 5/plant a week later.
At both Illinois release sites, researchers found aphid mummies parasitized by native wasps, but no Binodoxys communis mummies, Steffey says. “That doesn't mean they weren't there,” he adds. “These are very tiny things.”
In Indiana, Purdue University entomologist Thelma Heidel performed 30 wasp releases in three locations across the northern half of the state. Later, she found Binodoxys communis mummies at one site, mainly within 5 ft. of the cages.
In Wisconsin, wasps were released at two sites that had soybean aphid populations of 100-200/plant. The wasps reproduced within the field cages, says Dave Hogg, a University of Wisconsin entomologist, but researchers found no signs of them later in soybean fields or in buckthorn.
“But we did see a new lady beetle in Wisconsin this year,” Hogg reports: Hippodamia variegata, which appeared in eastern Canada in the 1980s and has been moving westward. “It's possible we didn't have such a severe aphid outbreak because of this new lady beetle,” he says. In addition, a native parasitic wasp that attacks aphids on grain, and sometimes on soybeans, “was unusually active this year.”
Native predators may have affected Binodoxys communis in Michigan, too, says University of Michigan entomologist Chris DiFonzo, who released wasps at nine sites. For the first time, DiFonzo saw large numbers of a North American wasp that's common in Ontario. In fact, those wasps got into some of her field cages and killed the aphids, complicating research results, she says.
In South Dakota, lady beetles got into Kelley Tilmon's three field cages and ate the mummies. “We had problems getting good numbers of parasitoids in our cages,” says Tilmon, a South Dakota State University entomologist. “The year 2007 was a dry run for us.”
This spring, researchers will try to determine if any wasps survived the winter. But “even with ultimately successful parasitoid releases, it's not uncommon to have no parasitoid detection in the first year or two, or even more, after release,” Tilmon says. “Only time will tell.”
More Binodoxys communis will be released in cages in 2008. Several other strains of parasitic wasps from Asia are also being evaluated for release, if Binodoxys communis fails to take off, Heimpel says.
Meanwhile, soybean aphid populations are predicted to be low in 2008. That's based on aphid counts from the North Central Regional Soybean Aphid Suction Trap Network, says Dave Voegtlin, entomologist at the Illinois Natural History Survey who oversees the network. “They'll be here,” Voegtlin says, “scattered all over the Midwest, but the population will be very much reduced, and fewer control measures should be needed this year.”