Integrated Pest Management of Wheat Stem Sawfly in North Dakota

Wheat stem sawfly, Cephus cinctus Norton (Hymenoptera: Cephidae), is widely distributed across North America, from California to the Mississippi River and from British Columbia to Manitoba. It has been reported from as far south as Kansas and New Mexico.

Many authorities consider it a native North American insect that adapted to wheat as European settlers began large-scale cultivation of cereal crops. Alternatively, some researchers have suggested that the wheat stem sawfly may have been introduced into North America inadvertently from northeastern Asia. Whatever its origins, wheat stem sawfly is the most serious insect pest of spring wheat and durum wheat in North Dakota.

Wheat stem sawfly first was reported as a pest of wheat in Saskatchewan and Manitoba in the late 1890s. In 1906, larvae were found attacking wheat in south-central North Dakota. By 1909, losses of up to 25 percent were reported around Minot and in the Red River Valley near Fargo.

The North Dakota infestation reached epidemic levels in 1916 but receded rapidly, and by the early 1920s, wheat stem sawfly was a pest of minor importance. During the 1940s, wheat stem sawfly again became a problem, with as much as 50 percent crop loss reported in northwestern North Dakota.

Sawfly populations have fluctuated across years and locations, although infestation levels and damage are greatest in western North Dakota. Recently, infestations of wheat stem sawfly have been observed in eastern North Dakota and even into northwestern Minnesota. Crop loss due to wheat stem sawfly often ranged from 10 to 25 percent. However, some spring wheat fields in western North Dakota with severe lodging, can have more than 80 percent crop lost due to wheat stem sawfly.

Egg: Wheat stem sawfly eggs are pale white, somewhat oval shaped and less than 1/16 inch in length (Figure 1). Eggs are laid singly in the lumen of a wheat stem and can be seen easily when a stem is split. At high infestation levels, multiple eggs may be laid in each stem; more than 10 eggs have been observed in a single stem.

Larva: Newly hatched larvae are colorless (Figure 2) and begin feeding immediately on the stem pith. They turn a greenish yellow as they ingest plant material and become yellow-white when mature (Figure 3). Larvae pass through four or five instars and reach a length of approximately ½ inch. The dark and heavily sclerotized head capsule of the wheat stem sawfly larva and its fairly large body size distinguish it from other larvae that may be found in cereal or grass stems. Larvae that have entered diapause will form a characteristic “S” shape when removed from the stem (Figure 3) in late fall or spring.

Pupa: The prepupa (Figure 4) is a short, quiescent instar between the end of the larval period and the pupal period and resembles a fully mature larva. The pupa (Figure 5) is a resting, inactive period and the intermediate stage between the larva and adult. The pupa of wheat stem sawfly is white and resembles the adult, with the limbs lying free but closely attached to the body.

Adult: Adult wheat stem sawflies are dark and slender, with yellow markings on the abdomen (Figure 6). Adults are less than 1 inch long, and females are generally larger than males. Females have short ovipositor sheaths that protrude just past the tip of the abdomen. Males lack this trait. Adults are not strong fliers but will move to nearby fields to mate and lay eggs (Figure 7). Wind also will aid the dispersion of adults longer distances to find wheat fields for oviposition. When adults are present, they are not difficult to see, and the best way to confirm their presence is by using a standard sweep net.

Wheat stem sawfly has a single generation per year. Although adults are relatively short-lived (seven to 10 days), adult emergence occurs during a long period of time, usually about three weeks but sometimes as long as one month.

First emergence is typically during mid- to late June in North Dakota. Female sawflies deposit eggs into the elongating stems of host plants in early summer (Figure 8), and developing larvae feed on stem tissue and move up and down the length of the stem. Although several eggs may be laid within a stem, only a single larva survives to maturity.

As the plant matures, and usually prior to harvest, the larva moves down to the base of the stem and chews a notch around the inside of the stem. The stem lumen is plugged with frass and sawdust below the notch, forming a chamber. The notch weakens the stem, which usually breaks (Figure 9), producing a stub that remains anchored in the ground (Figure 10).

The larva undergoes an obligatory diapause within this chamber and overwinters as a mature larva. The stub often is covered with debris or soil, and therefore is well-protected from excessive cold or dry conditions.

Diapause is broken as temperatures, moisture levels and/or photoperiods increase in the spring. Post-diapause development includes prepupal and pupal stages lasting approximately 20 to 30 days, depending on ambient temperatures. Adults emerge from the cocoon and use their mandibles to exit the stub through the plugged end.

Spring wheat, winter wheat and durum wheat are the main cereal crops attacked by wheat stem sawfly, although infestation in other small grains such as barley, triticale and spelt has been observed. Barley suffers very little damage, and cultivated oats does not support wheat stem sawfly, although females do lay eggs in oats.

Until recently in western North America, wheat stem sawfly primarily attacked spring wheat, while winter wheat suffered less damage because of its more advanced growth stage when sawflies emerge. Wheat stem sawfly has adapted to winter wheat, and winter wheat now sustains extensive damage in the northern Great Plains. Wheat stem sawfly also survives on a number of native and domesticated grasses, including species of Agropyron, Bromus, Elymus and Elytrigia, in addition to cereal crops.

Feeding by wheat stem sawfly larvae reduces the plant’s vascular efficiency and results in fewer kernels per head and lower kernel weight. This type of damage can reduce yield on infested stems by up to 20 percent. Sawfly feeding also can reduce the protein content of the grain. Grain loss also is caused by lodging after the inside of the stem is girdled (Figure 11).

In Montana and North Dakota, stem infestation levels of greater than 70 percent have been recorded, and losses of up to 80 percent due to lodging have been reported. Lodged plants can be harvested by placing the combine head on or very near ground level and using longer pickup fingers, but this practice can cause additional wear and tear on the machine.

Complete harvest of lodged plants is impossible. Yield loss still can range from 10 to 25 percent using this harvest practice.

Efforts to develop sawfly-resistant wheat cultivars with solid stems began in the 1930s. The first solid-stemmed cultivar (Rescue) was released in 1946, and this has been followed by the development and release of a number of solid-stemmed cultivars (Table 1).

Hard red winter (HRW) and hard white spring (HWS) solid-stemmed wheat cultivars, in addition to solid-stemmed hard red spring (HRS) cultivars, are available for use against wheat stem sawfly. Sawfly larvae suffer higher mortality and cause less damage in solid-stemmed versus hollow-stemmed cultivars (Figure 13).

In addition, the performance of some solid-stemmed cultivars has varied across years and locations. One cause of the variable performance is that the degree of stem solidness may be reduced by factors such as light, temperature, moisture, nutrient supply and plant spacing. Also, older solid-stemmed cultivars had lower yield potentials than hollow-stemmed cultivars.

The most recent HRS cultivars with resistance to wheat stem sawfly have consistently yielded on a par with the popular hollow-stemmed cultivars, especially when sawfly pressures are high. The most up-to-date information on wheat variety characteristics and performance can be found in the NDSU wheat guide at www.ndsu.edu/agriculture/ag-hub/ag-topics/crop-production/crops/wheat.

Solid-stemmed cultivars do not appear to adversely impact parasitism; parasitism levels often exceeded 50 percent in solid-stemmed cultivars. If 10 to 15 percent of the crop was cut by sawfly in the current growing season, a solid-stemmed variety of wheat is recommended for the following planting season.

Table 1. Released wheat cultivars with
resistance to wheat stem sawfly.

1 HRS = hard red spring wheat, HRW = hard red winter wheat,
HWS = hard white spring wheat

2 AC = Agriculture Canada, AP = AgriPro,
MAES = Montana Agricultural Experiment Station,
NDAES = North Dakota Agricultural Experiment Station,
WB = WestBred LLC

Several natural enemies attack wheat stem sawfly, although they are not always effective in keeping sawfly populations low. In North Dakota, the most important species in wheat is Bracon cephi, a parasitic wasp (Figure 14). Another species, Bracon lissogaster, attacks sawfly in more native grass habitats (Figure 15).

Bracon cephi females are able to sense sawfly larvae feeding in the stem, and the females lay their eggs in the wheat stem near the sawfly larvae. Once the parasitic wasp larvae emerge, they begin feeding on sawfly larvae and will kill them (Figure 16).

Parasitic wasps develop two generations per year. The first generation will cut a small circular hole in the stem when it emerges during midseason (Figure 17). Parasitoids overwinter in the upper half of the wheat stem in cocoons (Figure 18).

In some years and locations, these natural enemies can kill a high proportion of sawfly larvae (sometimes more than 80 percent). A survey conducted in western North Dakota during 2000-03 found that parasitism ranged from 7 to 88 percent and averaged 35 percent.

In general, parasitism was low where sawfly populations were low. Recent research indicates that parasitoids are able to survive in solid-stemmed wheat varieties, increasing the value of biological control as an IPM strategy that is compatible with host plant resistance. Growers are encouraged to practice conservation of parasitoids to help lower the severity of wheat stem sawfly infestation.

Insecticides have not been effective against the wheat stem sawfly. The egg, larval and pupal stages are well-protected inside the plant stem from insecticides.

Spraying for adults has not been successful because newly emerged adults can readily move in and out of wheat fields. The wheat stem sawfly emergence window is long about 1 month, and adults that emerge after spraying have reduced exposure to the insecticide. The adult does not feed on plant or drink water, which minimizes exposure to insecticides.

Overall, broad spectrum foliar insecticides are ineffective in controlling wheat stem sawfly; relatively costly for a lower-valued, large-acreage crop such as wheat; and damaging to beneficial parasitoid populations.

North Dakota State University Fargo, NorthDakota