Striped flea beetles Phyllotreta striolata. Photo by D. Pageau, AAFC Normandin, Que.
Flea beetles have been causing economic damage to cruciferous crops for decades and continue to be a problem pest for canola growers and crucifer vegetable crops across Canada. In a December 2003 Top Crop Manager article, researchers noted: “Even though flea beetles have been a problem for canola growers for many years, alternative methods have not been easily discovered.” At that time, the beginnings of an integrated flea beetle research program were underway.
Researchers have continued working on alternative flea beetle control methods over the past decade. And although some options show promise, over 90 per cent of canola grown in Western Canada today is treated with an insecticide for flea beetle control. Seed treatments are predominant, with an additional application of foliar insecticide when feeding damage encompasses 25 per cent of the leaf surface.
However, an integrated approach that could include host plant resistance is now becoming a promising option.
Flea beetles introduced to Canada
“The two most economically damaging species of flea beetles are both introduced species to Canada, the striped flea beetle and the crucifer flea beetle,” Julie Soroka, research scientist, entomology with Agriculture and Agri-Food Canada (AAFC) in Saskatoon, Sask., says. The striped flea beetle (Phyllotreta striolata) was first recorded in New York in 1776 and gradually spread throughout the continent. It was not considered a problem, as populations were not concentrated in any one area. The crucifer flea beetle (Phyllotreta cruciferae) was first recorded in 1921 in British Columbia, rapidly spreading its way east into the Prairies, causing significant damage to cabbages and other crucifer crops by the 1940s and 1950s. A third species native to Canada, the hop flea beetle (Psylliodes punctulata), is present in small numbers across Canada.
The crucifer flea beetle continued moving east to Manitoba’s Red River Valley and, by the 1960s and 1970s populations exploded, especially as canola acreage increased. “As canola acreage spread westward from Manitoba, the numbers of crucifer flea beetles also increased, pushing the striped flea beetles, which are more tolerant to cold and emerge earlier, to the northern edge of their range in the Peace River area,” Soroka says. “At that time, the hop flea beetle was actually the predominant species in the Peace. By the late 1990s, the crucifer flea beetle was the primary pest in over 70 to 80 per cent of the canola growing area in Western Canada.”
Soroka began a flea beetle monitoring program in 2001 that showed that flea beetle distribution remained relatively similar until about 2005. The situation changed between 2005 and 2007. “At that time, canola acreage was increasing, and in the Peace River [region] and other areas, growers started growing canola in shorter rotations,” Soroka notes. “This period also coincided with deregistration of the insecticide Lindane, which was equally effective on all three species. Since 2003, all seed treatments registered for control of flea beetles in Canada contain a neonicotinoid insecticide, which research has shown to be less effective on striped flea beetle populations.”
Between 2005 and 2007, Soroka started to notice striped flea beetle showing up more often in survey results. More intensive monitoring showed striped flea beetle had taken over the Peace Region and had moved as far south as the monitoring went. “Currently striped flea beetles have supplanted crucifer flea beetles as the principle flea beetle across most of the Prairies, except in the very southern regions and the Red River Valley area that has mixed populations,” Soroka says. “We believe this shift has been generated by several factors, including heavy use of neonicotinoid seed treatments. Until we can develop alternative strategies such as host plant resistance, growers for now are relying on chemical seed treatments for control of flea beetles.”
Hairy canola shows promise
As part of an integrated flea beetle management strategy, plant molecular breeders and entomologists began working on developing canola germplasm with protection against flea beetles in early 2002. “We initially introduced a trichome (or hair) gene from Arabidopsis thaliana, a close wild relative of canola, into the canola cultivar Westar,” Margaret Gruber, AAFC research scientist, explains. “This original transgenic experimental line of canola had a large density of hairs on the first three leaves, a few on the fourth leaf, but none on any other leaves or cotyledons. In laboratory and field studies led by Julie Soroka at Saskatoon and Lethbridge, crucifer flea beetles didn’t like feeding on hairy canola. Unfortunately the agronomics of that first experimental line, Hairy 1, were poor.”
In a second project, researchers developed a new line of hairy plants by depressing the activity of a second trichome gene within the Hairy 1 canola plant. The resulting line of plants, Hairy 2, had higher trichome density with longer hairs spread over the first 12 leaves and along parts of the stem. “This new plant line with expanded trichome density and coverage also had flea beetle resistance on the hairy leaves and non-hairy cotyledons, as well as some resistance to diamondback moth,” Gruber notes. “Although all cotyledons of canola are hairless, those from the hairy canola germplasm are resistant to flea beetles.”
The good news is the agronomics of the Hairy 2 canola line were so improved that the plants grew as well as the Westar control plants, and the seed yield was more variable (towards higher yield).
Gruber and her team also compared seed composition from the field trials of both lines. They found that all hairy canola plants had identical oil, protein, chlorophyll and glucosinolate levels as the control plants, except for some variability in two factors. Minor fatty acids in the seed oil of Hairy 1 plants were slightly lower, and three glucosinolates were more variable and slightly higher in Hairy 2 seeds, although still very close to the levels in the Westar control plant (in which the gene modifications had been undertaken). This meant transferring the enhanced trichome trait into a current commercial cultivar should result in seed quality within industry standards.
The information on these transgenic experimental hairy canola lines was shared with industry and plant breeders. Seed and genetic tools are also available from Agriculture Canada for others to transfer the trichome trait into their own elite B. napus germplasm.
One other study underway compared 1000 accessions of natural (non-transgenic) plant germplasm collected by Plant Gene Resources of Canada to evaluate hair density patterns within all of the Brassica species used in canola and mustard breeding. Researchers were looking to select plants with the most number of hairs spread evenly over the young leaves and stems, or plants with no hairs at all. If the coverage is patchy, insects will eat around the hairs.
“We are now looking across these different selected plants for real commonalities in gene expression patterns within hairy leaves compared with non-hairy leaves and cotyledons,” Gruber says. “For example, B. nigra (black mustard) is very hairy, although not as hairy as our hairy canola lines. B. villosa, a native species, is most hairy and averages 4000 hairs/cm2 on the leaves, but as with all of the other species, it has no hairs on the cotyledons.
“We have also found natural B. napus canola accessions with hairier leaves than in current cultivars. We are comparing expression intensities for all genes that influence hair density, growth or metabolites that influence insect fitness or attraction or avoidance of food. These plants and their genetic information are road maps that can be used in future breeding efforts to create non-transgenic host resistance to insect pests.”
In the field and lab trials of these projects, the hairy canola plants were resistant to both species of Phyllotreta flea beetles, in some cases surpassing the protection level provided by neonicotinoid seed treatments. “In our field trials in 2006, virtually all of the flea beetles were crucifer, but by 2012 at our Saskatoon field location, close to 80 per cent were striped flea beetles,” Soroka notes. “The results of our lab trials showed that striped flea beetles are as repelled by hairy canola as are crucifer flea beetles, unlike their insecticide responses. The hairy canola plants were also somewhat resistant to diamondback moth.”
Over the years several others were involved in the project, including Song Wang (former MSc student) and Ushan Alahakoon (former PhD student) who developed the two transgenic hairy canola lines. Gruber adds Peta Bonham-Smith, with the biology department at the University of Saskatchewan co-supervised both of the students, “and has collaborated with me from the beginning on hairy canola.” Visiting fellows Ali Taheri, Nagabushana Nayidu, and Zohreh Heydarian are still working on expressed gene analyses, and Larry Grenkow and Jennifer Holowachuk played major roles in field trials. Dwayne Hegedus is carrying the project forward. Funding outside of AAFC came from western Canadian canola growers groups, the Alberta Agricultural Institute, and the Saskatchewan Agricultural Development Fund.
Should hairy canola cultivars result from these efforts, growers could have alternatives to insecticides for control of flea beetles, and potentially other pests such as diamondback moth. Growers would then be able to implement an integrated flea beetle management program for canola. Researchers say the trichome trait will also be transferable to crucifer vegetable crops.