CROOKSTON, Minn. – Those attending the annual Crops and Soils Day at the Northwestern Research Outlook Center heard about insect challenges in crops. In regard to one of the insects – the wheat stem sawfly – growers were asked to help scout for this pest between now and harvest, since this is the peak time for damage from the sawfly.
Significant sawfly numbers started showing up last year in Polk County and caused major yield loss in the borders of some wheat fields, and an increased number of infected fields are expected this year, according to Jochum Wiersma, Extension agronomist at the Crookston site. Visual inspection by the combine driver, along with yield monitor data should help pinpoint if and where sawfly infestations are present and will help with planning to reduce next year’s problem.
“Sawfly is endemic here – it has always been here, but something has changed to where it is at economic levels right now,” Wiersma said. “When you guys are combining and see stems laying over, or the entire edge of a field is flatter than a pancake, get out and see if you find the frass, or the plug that resembles sawdust, on the inside of the wheat stem.
“I looked at a field last year, where the first 30 feet of the field was completely clipped off by the sawfly and the yield maps showed it made about 20 bushels per acre.”
The hatched larva feed inside the wheat stem, working their way down to the crown area, where they girdle the stem and cut it off like a razor cut, Wiersma explained. With any wind, those stems will start falling over.
Chemical control will not work against sawfly and the best way to control the pest is with variety selection and going to a solid stem wheat variety. This has been the practice in western North Dakota and Montana, where there is a yield drag associated with a solid stem wheat variety. They planted a solid stem variety at Crookston this year to check for yield drag; however, that plot received hail damage, so the results will be inconclusive this year, he noted.
Reducing small grain plant height – Wiersma also touched on using a plant growth regulator such as Palisade to reduce plant height and lessen the risk of crop lodging in small grain stands. As yields increase in small grains it is necessary to increase the straw strength and have shorter plant heights in order to support the larger heads associated with those higher yields.
He has been doing some research with Palisade and has found the most effective results are obtained later in the growth stage and at the higher rate, however it is expensive – around $15 per acre.
Palisade works in two ways to increase straw strength. First it actually shortens the straw length by limiting the elongation of the cells in the straw and secondly by composition – the plant will begin to favor compounds with longer strands, or lignin, and lignin is stiffer than cellulose.
Bugs and spuds and other crops – Extension entomologist Ian McRae focused on three insects during his presentation – potato beetles, armyworms and soybean aphids.
The NROC is starting to do more potato research, and part of that has to do with Colorado potato beetles, which McRae calls the ‘crowned prince of resistance’ to insecticides. They have become very resistant to neonicotinoid insecticides, which the area has relied upon heavily in the past for control. He outlined some of the other insecticides that can be used successfully at this time against the Colorado potato beetle. Growers are encouraged to contact him for approved insecticides and also to test to see if a population of CPB on their farm is show resistance to a certain insecticide.
Growers have also seen armyworms, especially in lodged grain.
“Armyworms are basically night feeders and providing them with lodged grain is like putting up a canopy over the buffet,” McRae said. “Now they can get down into that lodged grain and feed 24 hours a day. They don’t have to worry about birds and predation – that’s the bad news. The good news is they are susceptible to anything that is labeled against them, but the challenge is delivering that chemistry if they are under lodged grain.”
When spraying in this situation, he recommended good sprayer pressure and small droplet size to penetrate the canopy. Also, with the small grain harvest not too far in the future, growers need to consider the Pre-Harvest Interval (PHI) when deciding on an insecticide. That can be as low as 7 days for Malathion to up to 28 days for insecticides such as Lorsban Advanced, Cobalt Advanced and Stallion.
Finally, the incidence of soybean aphids had been very low so far this year in Crookston. Those numbers can change in a hurry and McRae encouraged growers to scout fields often. If control does become necessary, he said growers need to remember there was pyrethroid resistance last year and it was widespread across the valley.
There are four kinds of chemistry labeled for aphids, McRae noted: 1) – organophosphates (such as Lorsban); 2) – synthetic pyrethroids (which there is now widespread resistance to) 3) – neonicotinoids and 4) – products related to the neonicotinoids (such as Sivanto). Since one of the chemistry groups (the synthetic pyrethroids) is already out of the picture, he said growers should not do any spraying until they have reached the economic threshold of 250 aphids per plant and are building population, not starting to decline.
“If we have to go back and make another application later in the season, we are going to be limited as to what we can go back in with,” he said. “I would strongly recommend rotating your modes of action if you need to repeat an insecticide application.”