Manitoba potato growers may need to get more comfortable with leaf damage.
According to Tracy Shinners-Carnelley, vice-president of research and quality with Peak of the Market, growing insecticide resistance in Colorado potato beetles means it’s something they’ll likely see more of.
Much like flea beetles in canola, potato producers are being urged to hold off spraying at the first signs of defoliation.
“Potato plants can tolerate more defoliation than what we can tolerate actually looking and seeing,” Shinners-Carnelley said, although she noted there is no consistent visual guide (like those published by the canola sector) which helps determine when defoliation has hit an economic threshold.
Self-control over spraying is one of several points that experts hope will slow the creep of Colorado potato beetle insecticide resistance.
The industry has noted the slide in control provided by established chemistries with concern. In both the 2018 and 2019 provincial insect summary, provincial horticultural pest management specialist Vikram Bisht noted a flush of the insects in July and the later potato season. The report suggested that flush might be escapes due to increasing resistance to Admire, Titan or Actara neonics.
“Apparently, they are not working as well and there is a need for application of Delegate insecticide or some other insecticide as a foliar treatment somewhere in the late-June or mid-July area, when you get another flush,” Bisht said.
Some areas have reported multiple flushes, he noted. The provincial expert singled out the Winkler area as worst hit, although areas around Portage la Prairie and Carberry have started to see similar troubles.
Shinners-Carnelley is currently gathering data on the efficacy of different pesticides as part of a national research project. The project continues the work of a previous three-year study that she oversaw in the Winkler area, which tested the efficacy of Titan, Actara, Minecto Duo and non-neonic product Verimark (which showed limited control). The study found that established chemistries were giving shortened periods of control, and that foliar applications were needed after that control gave out.
Now, Shinners-Carnelley says foliar insecticides such as Delegate or Exirel are also seeing less response.
Both Shinners-Carnelley and Bisht cited a general lack of chemical options for Colorado potato beetle control. There are few new chemistries coming into play, Bisht noted, although he suggested that some biopesticides may start to dig a niche into the market.
Crop rotation does help slow resistance pressure, the two experts said, although simply moving potatoes to the next field over might just import the problem, since beetles can easily travel to the new field.
Some areas of North America are experimenting with plastic-covered ditches to stem that spread, Bisht noted.
“Crop rotation currently is the only option unless there is some breeding work done for resistance to this,” he said.
Both, however, also stressed the need to rotate chemistries.
“The farm really has to take a look and see what classes of chemistry they’ve used for foliar insecticides in the past and within their area,” Shinners-Carnelley said. “If you treated a field with a foliar insecticide in one year and then the next year you have another potato field on the other side of the road or the other side of the tree row, you need to realize that beetle population was the same beetle population that was in your field right next door the year before, and that population has been exposed to that foliar group of chemistry.”
Likewise, Bisht said, farmers should make sure they are rotating seed-treatment chemistries.
Shinners-Carnelley has also urged producers not to cut back on rates when they reach for seed treatments, in order to get the most out of each application.
2018 versus 2019
Last year offered a bit of a reprieve compared to the beetle pressure in 2018, Shinners-Carnelley noted, something she attributes to weather.
Both years showed extended emergence periods for the beetles, she said. In 2018, however, she said that warmer weather kick-started that beetle growth, exacerbating the problem.
“We had overwintering adults coming out and then those first adults were feeding and laying eggs, but then we continued to see adults (emerging), and then at the same time those first egg masses were hatching and we were starting to see larvae advancing through their stages. It got to the point where when you saw adults, we weren’t sure if they were wintering adults or if they were the new generation,” she said.
The result was both a blooming population, as well as a challenge for timing foliar applications, which are often targeted to certain stages in the insect’s life cycle, Shinners-Carnelley said.
In contrast, she added, last year’s cooler temperatures may have slowed that larval development, despite a similar extended emergence.
“We never really had a situation where there was a tremendous amount of beetle pressure and, for the most part, a lot of their stages seemed to be more consistent,” she said.
Looking to this year, Bisht warns that winter likely did little to beat back populations this year. Relatively mild weather likely allowed overwintering adults to survive into spring, he noted.