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Wheat Stem Sawfly | |
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Cephus cinctus
Insect Life Cycle
- Host plants
Wheat stem sawfly is native to North America and lives in grasses, mostly the wheatgrasses, Agropyron. Cultivated hosts include wheat, rye, triticale and some varieties of barley. Within the wheats, spring wheat is most heavily attacked in Alberta; winter wheat appears to have become a potential host in Alberta only within the last few years. In Montana, winter wheat has been severely infested and is perhaps the major host there. Winter wheat is occasionally attacked in southern Alberta in counties bordering Montana. The old varieties of durum wheat were resistant to wheat stem sawfly but some new varieties are susceptible. Oats and broad-leaved crops are immune. Female wasps will lay eggs in barley but the larvae don't live long enough to cause yield losses. Plant age is important to egg-laying females. Plants that have not reached the jointing (stem elongation) stage are not acceptable to females. Similarly, plants in the boot stage are immune to sawfly attack.
- Overwintering
The sawfly larva feeds within the stem and burrows down to or below ground level by the time the wheat heads begin to ripen. It then turns around, head upwards, and cuts through the stem about 2 cm above the ground, seals the end above itself, spins a cocoon in the stem and passes the winter as a larva in diapause (hibernation).
- Spring appearance
Overwintering larvae pupate within their cocoons in May; adults begin to emerge in early June from stubble fields and native grasses. As is common for many insects, males start to emerge first followed within a few days by females. A study in northern Montana showed peak male emergence occurred about June 15th and peak female emergence about June 25th. They are rather inactive insects that drift from plant to plant and spend most of their time resting on grass stems.
- Number of generations
Wheat stem sawfly has one generation per year.
- Natural enemies
Parasites - There are nine known parasites of wheat stem sawfly; only one species, however, provides significant control. Bracon cephi, a native braconid wasp, is one of the few insect parasites that can move from grass to crops with the sawfly. When weather conditions delay crop maturation and sawfly larval development, B. cephi can produce another generation, thus extending its control of the wheat stem sawfly population. Bracon lisogaster, a close relative of B. cephi, attacks sawfly larvae in the stems of grasses. It can significantly control wheat stem sawfly on native grasslands and roadsides.
Pathogens - Viruses cause disease in wheat stem sawfly and at times are an important natural control agent. Certain viruses are registered biocontrol agents in Canada for use on a limited number of pests (red-headed pine sawfly and douglas-fir tussock moth).
Damage Assessment
- Economic importance
The sawflies are all plant-eaters. Wheat stem sawfly is best known as a pest of wheat and has caused extensive losses to wheat in the northern Great Plains. Its history in Canada dates from 1895 when it damaged wheat near Moose Jaw, Saskatchewan and Souris, Manitoba. Wheat stem sawfly was slow to adapt to cereals but achieved pest status in the 1910s and 20s.
Changes in farming practices have affected the abundance of wheat stem sawfly. Tractor farming increased the relative abundance of wheat and decreased the proportion of oats grown. In addition, as strip farming gained acceptance, sawflies spread easily from stubble and native grasses to wheat. As stubble farming of wheat on wheat stubble increased, so did wheat stem sawfly. Heavy losses occurred primarily in the 1940s and 50s. In 1941 losses totalled 50 million bushels on the Canadian Prairies. Annual losses in Saskatchewan over the period 1926 - 1958 ranged from 1.4 per cent to 10.3 per cent of potential yield. The development of solid-stemmed wheat varieties greatly decreased the importance of this pest.
- Damage description
The sawfly larva bores down inside the stem and makes a discolored tunnel from about the top joint to the root. Sometimes, however, eggs are laid above the top node of the plant and tunnelling by larvae destroy sufficient vascular tissue so that the head turns white.
The greatest losses occur around the margins of fields. Wheat stem sawfly losses are of two types. Larvae feed within the stem of the plant and reduce both yield (a 5 - 15 per cent decrease in total seed weight) and quality of grain (from reduced protein and kernel weight). Larvae cut stems and cause stems to break in the wind, fall to the ground and become unharvestable. These effects of feeding by larvae usually go unnoticed until the plants are toppled by wind and the weight of maturing heads. Mature larvae chew part way through and all around the inside of the stem just before cocoon formation in late summer.
- Sampling and monitoring methods
Determine percentage of plants cut by sawfly per square metre prior to harvest.
- Economic threshold
Control methods are required if 10-15 per cent of the crop in the previous year was cut by sawfly.
Management Strategy
- Effects of weather
Most fluctuations in populations are caused indirectly by weather. The effects of rainfall or drought on the primary food plants, grasses, can be important in determining the size of infestations in the secondary food plants, cereals. When the weather is rainy in the fall or spring, the numbers of large, head-bearing stems of native grasses are adequate for sawfly populations. But when there is a drought, the numbers of grass stems suitable for attack are few and the sawfly concentrates its attack on wheat instead of on grasses. Since only one sawfly will emerge per stem, a shortage of suitable oviposition sites will result in higher mortality due to cannibalism. When rain promotes an abundance of suitable stems, the proportion of stems receiving more than one egg is low and sawfly populations increase. During warm, sunny, windless weather, especially after rain, the sawflies disperse widely. Their attack is otherwise concentrated near the field margins in fewer plants.
Drought conditions can reduce infestations the following year by killing plants that have larvae inside them. Drought in the spring can cause overwintered larvae to re-enter diapause but the influence of this on population size is not clear. Abundant rainfall tends to produce outbreaks of stem rust disease, which is harmful to sawfly larvae. In one instance only 18 per cent of larvae survived in heavily rusted wheat. A population decline in 1955 was attributed to high rainfall and severe rust infestations that killed the larvae in 1954. Doubtless, various fungi cause mortality in the larval population during wet years. Hail, too, can reduce infestations. In one instance there was a small infestation after a severe hailstorm but severe infestations persisted outside the area of hail damage.
Populations build up and damage often occurs at field edges where suitable grass and crop hosts coincide. Wheat stem sawfly is a weak flier and will not take flight readily during cool, rainy or windy weather. Sunny calm weather during the egg-laying period will promote dispersal of wheat stem sawfly.
Weather also affects populations of the parasite, Bracon cephi. The parasites are inactive when the grass or crop is wet but become active when conditions are drier. Wet conditions will delay the maturation of host plants. Wet weather may also delay larval development since the larvae apparently require that stem moisture content be under 50 per cent before they will cut the stems. This extended larval period may also prolong exposure to parasite attack.
- Cultural practices
A number of practices reduce losses caused by this insect. More than any other practice, the use of resistant varieties has reduced sawfly damage. Sawfly populations have increased in recent years, perhaps because of a neglect (or reluctance) to use resistant varieties, because of a decline in resistance properties of the new varieties, or because of the tendency to use fewer tillage operations. A resurgence in sawfly populations recently led to losses estimated at over $5 million in 1990.
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For more information about the content of this document, contact Scott Meers.
This document is maintained by Shelley Barkley.
This information published to the web on November 22, 2001.
Last Reviewed/Revised on April 3, 2007.
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