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Posted 9 September 2008. PMN Crop News.


A "Perfect Storm" of Wheat Stem Sawfly Caused Severe Crop Losses


Montana State University. extn.msu.montana.edu


Bozeman, Montana (August 26, 2008)--Many wheat growers have reported very heavy yield losses due to wheat stem sawfly this year. Some had thought that the prolonged cool weather earlier this year would lessen the pressure caused by this insect, but this is not the case.

 

George Wood, a grower southeast of Conrad, said he has had the worst sawfly stem-cutting in 20 years on his land and has swathed some of his winter wheat crop to prevent further losses, even though he planted a variety that is partially resistant to sawfly. His son is direct-combining winter wheat that fell after sawfly cutting and is having difficulty recovering the grain.

Their scenario has been common this year over a large part of Montana. Dan Picard, agent at the Montana State University Extension office in Pondera County, says that even swathed crops have been scattered by unusually strong winds. These winds help explain the extreme stem lodging that is due to the sawfly, but this is only part of the story this year.

Montana has had severe damage from wheat stem sawfly for decades. Insecticides are ineffective against the immature stages that develop inside stems, and the duration of the sawfly flight period limits the effectiveness of trying to spray adults.

In 2007, about 1.8 million acres of solid-stem wheat were planted. These varieties reduce lodging and kill some immature sawflies, as well. Solid-stem wheat is only planted in response to sawfly damage, because hollow stem cultivars are generally more agronomically desirable. However, losses due to feeding by the larvae still occur and even solid-stemmed varieties can be cut badly at times. Based on the acreage planted to solid-stem wheat this year and information from a 1996 survey, overall losses are likely to total more than $50 million in 2008.

Why is the problem so bad this year?

Basically, we have had a nearly "perfect storm" of conditions.

First, a cool spring slows down all components of the agricultural system, so the insect and crop remained matched with each other. Cool weather leads to slower developing crops, but also longer-lived females, which have the time to forage throughout a field rather than staying on the edges. They distribute their eggs more broadly, which spreads potential lodging over a larger area. Although it is a risky practice, late planting of crops can sometimes reduce infestation in a typical spring, but not when everything is delayed equally. There appears to have been little benefit from the cool, wet spring in areas affected by the wheat stem sawfly.

Recent research has shown that before emerging as adults, sawflies overwintering in the stubble are more vulnerable to heat. The low temperatures early this spring throughout Montana may have increased sawfly survival by decreasing mortality in stubble, and increased the number of sawfly larvae by extending the time females could lay eggs. The cool summer also led to delayed harvest, particularly in winter wheat, which prolonged larval feeding within the stems. The harvest delay also means that infested stems stood in the field longer, facing more days of wind and rain.

Cool, rainy weather also is believed to reduce the amount of pith that creates a solid stem in wheat. With less pith, there is greater sawfly survival and more stem cutting. This may be exaggerated in partially resistant varieties with moderate levels of pith, resulting in even more lodging.

Finally, multi-year weather also figures into this year's wheat stem sawfly problem. Several years of hot dry weather has reduced the population of parasitic wasps available to kill immature wheat stem sawflies. This is because there are two generations of parasitoid wasps per year. The second generation occurs very near harvest and has a greater impact on sawfly numbers the following year. But the timing is pretty precise, and, when hot dry conditions cause an early harvest, the second generation of these wasps is only partially successful.

The problem for the parasitic wasp is compounded when standing residue is reduced. These insects overwinter in the wheat stem after harvest. Modern varieties are short, and decreased height due to drought leaves very little standing residue. Thus, fewer parasitoid wasps survive the combine in dry years, while early harvest due to heat and drought appears to have no negative impact on the wheat stem sawfly.

Since cool weather helps parasitic wasps kill more sawflies and survive in greater numbers through to the next year, we hopefully will have more natural enemies of the sawfly next year.

What is being done to help growers?

The quality of solid-stem winter and spring wheat varieties continues to improve. However, cutting has been observed by some growers even in the spring wheat variety Choteau. This variety approaches the maximum possible level of stem solidness, indicating that this trait alone is not likely to provide complete control of the sawfly.

It is important to note that solid stem cultivars always decrease lodging significantly and kill some immature sawflies, but the level of performance varies across locations and years. Solid stem cultivars are important in management, but cannot effectively manage wheat stem sawflies as a single strategy. Information on the recent history of the level of stem solidness in wheat varieties can be obtained through County Extension Offices, Montana Agricultural Experiment Station research centers, and on the Internet.

The use of wheat varieties that vary in their attractiveness to egg-laying female sawflies has shown promise. When attractive varieties are companion-planted as traps alongside unattractive varieties, sawflies concentrate in the attractive trap-crop. This lets growers swath the attractive cultivar for hay at the end of the sawfly flight, killing the immature sawflies. A variation of this is to plant a trap crop that kills sawflies, such as attractive solid-stem wheat or another cereal that is not a suitable host for sawflies. Developing trap crops will be much easier, because attractiveness to female sawflies in wheat has recently been determined to be heritable and markers can be used to rapidly screen both attractive and unattractive cultivars. It is important to use the correct varieties when planting trap crops. The method does not work unless an attractive variety surrounds an unattractive one.

The type of tillage chosen is important. Heavily tilled fallow is tough on the sawfly's natural enemies but not on the sawfly. Research shows that heavily tilled fallow soil decreases numbers of beneficial insects, whereas lightly tilled or untilled fallow has no negative effect on them. However, tillage does not appreciably decrease the infestation by wheat stem sawfly the following year. It is believed that surviving sawflies can emerge from under the soil, but the fragile parasitic wasps cannot.

Because parasitic wasps overwinter in the stems remaining after harvest, cutting higher preserves the greatest number of them. This should be done each year in an area with a history of sawfly damage even if losses are no longer an immediate concern. This practice also will work with low to moderate levels of infestation, because overall lodging is not too severe and the crop can be successfully harvested. However, when a field is heavily infested and unrecoverable lodging is likely, parasitic wasps can still be conserved. Swathing the crop at a height of 8 to 10 inches before much lodging occurs will conserve more natural enemies than direct cutting at ground level. 'Scraping the ground' with the combine header removes all stems containing parasitic wasps while attempting to recover the lodged grain.

Parasitic wasps are being relocated through the combined efforts of research and Extension personnel in a state- and federally-sponsored project. In early summer, straw containing approximately 500 female parasitic wasps are placed in fields with a history of heavy losses. The adult parasitic wasps that emerge attack the sawfly larvae in the new locations. These releases are inoculations intended to begin population growth of the natural enemies to the level that they will decrease sawfly numbers a few years later. Material has been distributed to more than 50 fields across 15 counties statewide, and is coordinated by participating MSU Extension personnel. For more information, contact David Weaver at weaver@montana.edu.

There are other research projects aimed at different types of biological control, insect pheromones, optimal cropping practices, new forms of host plant resistance and evaluation of insecticides.

It is important to remember that the wheat stem sawfly causes such tremendous losses because it is not readily controlled using single approaches. Ongoing efforts to develop integrated strategies for management are the key to limiting future losses.


Contact:
David Weaver
MSU Department of Land Resources and Environmental Sciences
406-994-7604
weaver@montana.edu