by Heather Jones
It’s taken almost 30 years but Agriculture and Agri-Food Canada (AAFC) scientists have won a monumental battle against the Colorado potato beetle (CPB).
Dr. Benoit Bizimungu and Agnes Murphy, potato breeding research scientists at the Fredericton Research and Development Centre, recently developed two selections that are resistant to the major insect pest. Both varieties are available for industry to use in trials as part of an accelerated release program.
While only about seven millimetres wide and 10 millimetres long, the CPB has devastated crop yields since it arrived in New Brunswick around 1882.
Although its diet is strictly potatoes, tomatoes, eggplant, and some weeds, “the beetle is a really, really good defoliator,” said research scientist Dr. Helen Tai.
“It’s a very hungry pest and it will eat all the leaves off a plant so all that’s left is a plant with bare stems,” she said. “No leaves.” Without leaves to photosynthesize, no tubers can be produced.
The new resistant varieties were developed as a team effort. While she played an important role in the process, Tai said, “I stand on the shoulder of giants.”
Over the years, breeders and entomologists developed strategies and weapons to fight the voracious pest. They include biological controls (Bacillus thuringiensis), plastic-lined trenches around fields, an eight-row propane flamer, and crossing potatoes with CPB-resistant species.
A WALK ON THE WILD SIDE
The potato originated near the equator in South America, Tai explained. And it has a lot of wild Solanum relatives “that people don’t cultivate as a crop but they have a lot of really good characteristics such as the beetle resistance. We’ve also used that germplasm (to breed) for resistance to Late blight and resistance to other kinds of diseases.”
In the late 1970s and 1980s, AAFC breeder Dr. Henry De Jong brought wild Solanum species from South America to Fredericton to crossbreed with domestic varieties. Entomologist Dr. Yvan Pelletier and geneticist Dr. David De Koeyer began the quest to identify the CPB resistance.
Tai stressed that traditional methods were used in all the breeding programs. “Pollen from one plant was put on the flower of another plant. There are no GMOs (genetically modified organisms) in the potato breeding program at AAFC.”
Tuberization was another major obstacle for researchers.
“Plants are very, very sensitive to the length of the day,” said Tai. “This is a way that they control when they flower and when they make tubers.”
The researcher explained that North America and Europe have 16 hours of daylight during the summer. But “in South America their day lengths are only about 12 hours or less in their growing season.
“The wild Solanum species stopped making tubers here in the summertime,” said Tai, adding that scientists “had to do a lot of growing in growth chambers where they could control the day length, and could get the day length genes from the domesticated Solanum tuberosum transferred together with CPB resistance from the wild species.”
The biggest issue researchers confronted was determining which wild Solanum species were resistant to the pest.
“Dr. Pelletier’s team did some really wonderful work on that,” said Tai. “They planted them out in one of our fields here and determined by visual assessment which plants were being eaten by the beetle.”
From the collection of 30 or more varieties, “I think there were about nine of them that stood out,” she said. “From there the breeders started crossing some of the intermediary hybrids.”
But quantifying the amount of feeding done by the CPB evolved into a massive amount of work “because beetles are finicky eaters. Their level of feeding could be inconsistent from year to year.”
With all the feeding variations, researchers “got a statistician involved, my father actually, Dr. George Tai. He figured out a way that we could compare beetle feeding between years.”
That was about a decade ago when she became more heavily involved in the project. “We decided to look for a chemical signature produced by the leaves as a way to determine how much resistance there was to the beetle.”
Helen Tai built on work initiated by Dr. Russell King on chemicals naturally produced in leaves that repel CPB. Chemical analysis conducted on the leaves of the domesticated potato that the CPB liked was compared with wild species and intercrossed hybrids that the pest ignored.
“That’s when we discovered a characteristic chemical signature that was associated with the beetle resistance,” said Tai. “The plants that the beetles didn’t eat had this distinct chemical signature.”
Once the chemical signature was identified, she said researchers were able “to develop much better screening tools to increase the efficiencies to select for CPB resistance from our germplasm.”
AMERICAN IDOL IN THE POTATO FIELD
Bizimungu’s breeding team typically starts with 100,000 potato clones every year in the greenhouse. “From there, they have to sort through all of the clones that don’t make nice looking tubers,” said Tai. “They could be knobby, misshapen, poor in yield, all those things get screened at the farm.”
“From 100,000 clones, you may get two out of those crosses that are suitable for human consumption, for processing, or for other purposes. It’s like American Idol in the potato field. In the end, you’re going to get a couple of celebrity stars.”
Breeding new CPB-resistant varieties “opens the way to a new era where potato growers could reduce pesticide spray applications for insect control,” she said, adding that the insects “are already showing a resistance to the popular pesticides.” That chemical resistance, along with environmental concerns and production costs, should encourage commercial growers to investigate the new AAFC varieties.
Tai said there has been a lot of interest expressed by small farmers who don’t want to use pesticides. “One fellow was telling me that he was going through his field with a blowtorch to clean off the (CPB) eggs.”
Tai said the new varieties can be used in conjunction with integrated pest management. “That would be an excellent strategy for growers.”
She and the new AAFC entomologist Dr. Chandra Moffat will continue to focus on finding other solutions in the battle against Colorado potato beetles.