Watch out for the wheat stem sawfly
By Kylene Scott
There are a lot of variables involved in growing a wheat crop. For years in northern areas of the High Plains, farmers of spring wheat have been aware of the wheat stem sawfly. The wasp-like creature lays its eggs in the stems of wheat, and when the resulting larvae mature, they often girdle the stem, causing the wheat to eventually lodge.
“Well, this has historically been a pest of spring wheat for a long time and almost back a century in the northern Plains,” J.P. Michaud, entomologist, Agricultural Research Center, Hays, Kan., said. “It has been present here in Kansas for a very long time as well, but it’s always stayed in the wild grasses.”
It has always been assumed that the insect wouldn’t hurt winter wheat because it matures too quickly to permit a complete development of the pest. But Michaud said the pest spreading now is a new variant that has evolved faster development.
“It began where it was restricted to spring wheat up north and for quite a number of years it has been attacking winter wheat, and now it’s been moving southward to where it’s causing problems in eastern Colorado, southwestern Nebraska, Wyoming and that area,” Michaud said. “We’re quite sure that it’s present in northwest Kansas. We’re going to do some sampling this spring to determine how widespread it is.”
But, until the sampling is done, Michaud is unsure of its scope.
“At this time it’s not causing any economic losses and in fact, for much of the southern areas of its range, it still seems to be experiencing a lot of mortality,” he said. “In other words there’s not a lot of wheat getting cut—even though there might be fairly high levels of infestation, in some spots a lot of those larvae are not surviving to emerge as adults, and if they don’t survive, then they don’t cut off the stems.”
When the pests don’t cut the wheat, it can easily go unnoticed by the farmer. It is really only going to be noticed when lodging occurs and a lot of the wheat is lying on the ground at harvest.
“They don’t always cut either. But this is where the real yield losses come—when all those larvae survive and cut off the wheat,” Michaud said. “If everything is lying on the ground you can’t combine it and that’s the real problem.”
The boring of the larvae in the stems also causes the number of seeds per spike to be reduced as well as reduced test weights from shriveled grains. Also, to some extent the protein content may be affected.
“These are the kind of yield losses that could easily go unnoticed but if the larvae aren’t cutting the stems, those other types of impact on yield are not going to be very noticeable,” Michaud said. “It’s quite conceivable that there are pockets of this pest already active in northwest Kansas, and certainly all around the borders there.”
The wheat stem sawfly is something farmers need to be aware of and watching for in their fields, but it is still not yet known how much impact it will have in Kansas.
“We don’t know yet how big of an economic problem it’s going to become,” Michaud said. “It might be a number of years before it becomes a significant management consideration, but the potential is there for this to be a very big problem in wheat production because it’s very difficult to manage.”
And that is where management gets tricky because insecticide won’t kill the larvae since they are housed inside the stem, and adults have to be hit physically by the chemicals.
“The adults don’t feed at all and so it reduces any exposure—they’re not going to eat any contaminated leaf surface, and they emerge over fairly extended periods so you can’t just get one spray on the wheat and hope to kill them all. It just wouldn’t be economic,” Michaud said.
Michaud said trials have been done to control them with chemicals, and it never worked. Management has to be multi-faceted and based on special varieties of wheat bred to have solid stems. If there is more pith in the stem and it makes it harder to tunnel, thus reducing larval survival.
“We really don’t have any solid stem varieties for Kansas. We don’t have that type of resistance in any commercial wheat lines presently,” Michaud said. “So that would be a big breeding initiative to try and get those traits into varieties for Kansas.”
Another primary strategy for management is crop rotation. Michaud said a lot of farmers are in the habit of planting wheat continuously, and this exacerbates pest problems of many kinds, including mites, wheat streak mosaic, and Hessian fly.
“Crop rotation is the primary strategy, so it would really spell the end of continuous wheat,” he said. “If you plant wheat right on top of the infested stubble, you can go from 5 percent infested in one year to 75 to 80 percent plants infested the next year. So it’s going to be another important deterrent for people not to plant wheat continuously.”
Interaction with the weather conditions is not known at this time. In 2012, spring came early and temperatures were warm, and it’s not been the same case in 2013.
“It’s going to be interesting to see, because last year what we saw up in Nebraska was in fact a very high mortality,” Michaud said. “You know the question is—warmer temperatures in the spring accelerate the development of the insect and the plant. And so who comes out with the net benefit is one thing we really don’t know yet.”
Michaud thinks that it seems like the early spring favored the plant over the insect in 2012 because a lot of the insects were found dead in the stems.
“So maybe this year it’s going to favor the insects since we’ve had a little bit more time to develop,” Michaud said. “The reason it’s hard to predict because both the plant and the insect have temperature thresholds, which they can start growing.”
It’s not a clear indication as to whether or not an early spring plays a role in the development of the wheat straw fly. Historical data may back things up, showing that the pest is evolving faster.
“(It) makes sense when you consider if it’s laying eggs—and winter wheat is such a huge acreage—and if these insects are laying eggs, even if they are one in a 1,000, their offspring are surviving. Those are the ones that are going to breed; those are going to be the ones that have the fast development for the next generation,” he said. “So there’s tremendously strong selection for evolving faster development in this insect.”
These specially evolved variants are expanding their range southward, completely separate from the populations that are already present in Kansas but stay in the wild grasses. The insects are very responsive to and quite specific on the types of grasses they want to attack. Chemical stimuli from the plant cause a response in the insects, and grass stimuli vary greatly from wheat.
“So ours probably don’t get attracted to the wheat at all because they respond to chemicals associated with their wild hosts,” Michaud said. “Where this pest population that’s expanding from the north orients specifically to wheat.”
A parasitoid can help control the straw fly, doing a pretty good job of it, but because of differences in biology, its effectiveness is limited because the second generation of the straw fly has to go look for food.
“The sawfly is down in the stubble, and so they are going to overwinter and the first generation of parasites comes out to find another host for this second generation. They don’t overwinter,” Michaud said. “They can’t overwinter in the sawfly because they need to have a second generation.”
Parasitoids also pupate higher up in the stems, and one of the ways to enhance biological control is to use a stripper header or at least raise the height of the combine as high as possible and just clip the heads to leave as much of the stalk as possible. That way the stalk is preserved and all of those parasites that are higher in the stalk are not destroyed.
For example, in North Dakota, if there is a 10 or 15 percent infestation in a field, it is recommended to swath the wheat and pick it up, Michaud said.
“And of course that takes completely different equipment,” he said. “You can’t even combine it—you mow it and swath it. So now you’re into higher harvesting costs because you can’t go across in one pass. You can’t pick it all up in one pass and you need different equipment—so it would not be popular among our wheat farmers.”
But, Michaud stressed, this plant is very important in the region where it is endemic—areas like North Dakota and Montana, where it causes $50 million a year in damage.
“It is probably the key pest of wheat of the north. So if it’s coming down here it’s something that people need to be aware of,” Michaud said. “And again, we don’t know yet. I don’t want to be an alarmist; we don’t know yet how much of a problem it’s going to become in Kansas or how long it’s going to take, but it’s coming.”
Kylene Scott can be reached by phone at 620-227-1804 or by email at firstname.lastname@example.org.