Art Agnello adjusts five potted apple trees in the middle of a hardwood forest that borders an apple orchard near Huron, New York, in early July. The young trees are part of a pest control trial to combat the spread of black stem borers. This location has 20 sets of five stressed trees each; 10 sets use ethanol lures, 10 use ethanol and the repellent verbenone. (TJ Mullinax/Good Fruit Grower)
Control is so difficult for the black stem borer, researchers have been recommending that growers remove infested trees and burn them. Now, efforts are afoot in New York — again — to find a way to stop the borer.
Packets of commercially available verbenone, left, and ethanol, right, hang from stressed apple trees several hundred feet into a wooded area bordering an apple orchard near Huron, New York. Verbenone is an organic insect repellent used against pine beetles in the western U.S., and the ethanol packets are used as a lure for black stem borers. (TJ Mullinax/Good Fruit Grower)
Researchers are making strides to better understand the beetle, and several biopesticides under review are showing promise.
The bad news: Field trials run last year using Lorsban (chlorpyrifos) and a number of trunk sprays produced disappointing results.
“None of the treatments were tremendously effective,” said Art Agnello, a Cornell University Extension entomologist.
Agnello is continuing to research effective controls, baiting borers with ethanol and testing, among other things, verbenone — currently used as an organic insect repellent in pine forests — in wooded areas just outside orchards.
Meanwhile, Ithaca, New York-based USDA-ARS research entomologist John Vandenberg is investigating organic controls, and Kerik Cox, an associate professor in Cornell University’s plant pathology and plant-microbe biology section, examined a possible connection to fire blight and blister bark.
The insect, which hails from Asia, has been in this country since the 1930s. In the not-too-distant past, the black stem borer — a type of ambrosia beetle — limited its activities to forest hardwood and ornamental species like ash, black cherry, black walnut, cedar, dogwood, oak, pine, poplar, rhododendron and willow.
Recently, it developed a taste for apple trees. “We’re seeing them in all varieties of apples, although we see them more in young plantings, which tend to be newer varieties,” Agnello said.
The insects tend to attack smaller caliper trees suffering from flood or cold injury, severe pruning or disease, which may be why they moved into orchards and nurseries. “There were enough stress-producing events to get them moving into nurseries and orchards,” he said.
Black stem borer females excavate galleries in apple tree trunks to lay eggs and hatch larvae. Later in the season, they move deeper into the trunk to overwinter. (Courtesy of USDA-ARS)
Vandenberg suggests nearly all modern apple trees may be stressed to some extent, raising the stakes against the pest. “Pruning, the cutting, the shaping, it all causes trees some stress and may make them more vulnerable to insect attack,” he said.
Last month, the research was part of a scheduled field day held at the New York State Agricultural Experiment Station in Geneva, New York, which coincided with the International Fruit Tree Association’s summer tour.
Vandenberg and Louela Castrillo, a research scientist in Cornell University’s entomology department, are evaluating biocontrols that may provide an alternative to uprooting and destroying infested trees.
Working in concert with scientists in Tennessee, they have identified three biopesticides capable of either killing the insects or limiting their ability to reproduce. (Female borers hollow out galleries in the sapwood or heartwood of the tree in which to lay eggs.)
The active ingredients in each of the products — Botanigard, Met52 and RootShield — are fungi.
Beauveria bassiana is Botanigard’s active ingredient. It is a fungal pathogen of insects occurring naturally in soils throughout the world and is not harmful to humans.
The fungus requires contact to kill beetles, though it will not kill them when they ingest it. B. bassiana attacks both adults and larvae, producing spores called conidia, which produce enzymes that attack the beetle’s cuticle, then later its organs, killing it within three to seven days.
Met52’s active ingredient is Metarhizium brunneum, and its action is similar to that of B. bassiana. “We found it killed about half the females and significantly reduced the remaining females’ abilities to reproduce,” Vandenberg said.
Adults soon become infected, which, in turn, leads to infected larvae. “It creates an epidemic in the galleries,” he said.
The downside to this is that the female walks through the compound sprayed onto the tree but it doesn’t kill her on contact. “She will still invade, and still create her gallery, which is both a good and bad outcome for the tree.”
RootShield’s active ingredient is a mycoparasite (Trichoderma harzianum). Mycoparasites kill other fungi; in this case, T. harzianum targets the fungus that female borers produce and deposit onto the walls of the galleries.
The borers somehow recognize that their own fungus (Ambrosiella grosmanniae) has taken hold in the gallery and begin to lay eggs, but T. harzianum will stop the production of A. grosmanniae before it gets started.
“If we can interrupt the symbiont (the ambrosia fungus), then it can’t get established in the gallery walls,” Vandenberg said.
However, that’s not a final solution. When females don’t detect their own fungus, they keep drilling, expanding the wound inside the tree as they continue to excavate. “This product will require more testing to determine if it can be a useful tool for apple growers,” he said.
Fire blight, no … blister bark, maybe
The good news: Where researchers once believed there was a connection between black stem borers and fire blight, now they don’t. “We originally thought there was one,” Cox said, “but we have no solid link between the two.”
In 2013, when Cornell Cooperative Extension personnel found black stem borer in western New York orchards, they found trees showing oozing holes in scions and rootstock. Six of the sites featured borers and fire blight.
So they took some borers from the site, put them into petri dishes, examined some and put others in petri dishes with bleach and ethanol and ground them up to see what was inside.
They found no evidence of fire blight. What they did find were a number of fungi and two in large supply — A. grosmanniae and Nectria haematococca. “Both can be cultivated by BSB; they farm it and eat it,” Cox said.
According to information filed in the U.S. Department of Energy’s Joint Genome Institute, N. haematococca is actually the reproductive stage of Fusarium solani, the fungus identified as the causal agent for a number of vegetable diseases as well as sudden death syndrome in soybeans.
N. haematococca occurs in several forms, among them as a fungus that feeds on soil organic matter and as a plant pathogen.
It has an extensive range of metabolic capabilities, including the ability to degrade lignin — the organic polymer that makes many plants rigid and woody — and pesticides.
It is also tolerant to more than a few compounds that are toxic to other fungi, such as antibiotics, heavy metals and metabolic poisons.
One of more curious symptoms not explained by the beetle or the fungi is bark blistering around the sites where the bugs tunnel.
When researchers scraped the bark away, they found a white, chalky material, something they later identified as callus cells that appear in injured wood.
Curiously, however, they did not find any fungi usually associated with black stem borers in blistered bark samples they examined. Instead, they found lots of fluorescent Pseudomonas bacteria, which have been associated with symptoms like this in apples in Europe.
“The overall wilting symptoms observed on infested apples trees are most likely the result of tunneling,” Cox said.
It also is likely to be weakening the wood and making it susceptible to things like blister bark. For this reason, Cox said, researchers have not yet ruled out a link between black stem borers and blister bark. •