 Hello, my name is Jan Knotle. I'm the Extension Entomologist for North Dakota State University. Wheatstem soft flies are economically important insect pests of wheat, especially hard-bred springweed in North Dakota. In the northern Great Plains of North America, hard-bred springweed is one of the major crops to the tune of 13.4 million acres of annual production. Of that 13.4 million acres, about half of the acreage is grown in North Dakota each year. In order for springweed growers to maximize production efficiency and achieve maximized profits, insects, weeds, and diseases need to be managed. In the area of insects, few exclusively attack springweed. However, one insect, the wheatstem soft fly, has a history of causing damage and yield losses. The wheatstem soft fly originally attacked the native grasses of the northern Great Plains. Around the turn of the century, farmers in North Dakota, Montana, Manitoba, and Saskatchewan began to notice damage to their springweed crops. By the 1920s, the wheatstem soft fly became the northern Great Plains' most damaging springweed insect. Currently in the U.S., the soft fly is only a major concern in western and south-central parts of North Dakota and the central and western sections of Montana. Producers in eastern Wyoming and western South Dakota and Nebraska reported an increase in damage of winter wheat by the wheatstem soft fly. In the early years, producers tried a number of strategies to reduce the problem, including deep moldboard plowing, early removing of rye grasses for hay, trap crop planting, and early wheat harvesting. These strategies provided some control, yet for the most part producers have quit using them. Adult wheatstem soft fly is actually a small wasp. It is about three-quarters of an inch long with a shiny black body, yellow legs, and stripes on the abdomen. The adult has four smoke-colored wings that usually fold over the back when it rests. Adults begin to emerge in early June and emergence may continue into early July. Adult soft flies can be found resting on the leaves or stems of host plant, especially during windy conditions. The insect is most active when the temperature is 70 to 90 degrees with sunny skies and little wind. The soft fly is a weak flyer, so it usually seeks out grass stems near its emergent site. However, it can migrate up to a mile in search of a host. When the female locates a grass stem of suitable diameter, it deposits a single egg inside the stem. However, a stem often receives several eggs from a number of females. The soft fly eggs are kidney-shaped and about a sixteenth of an inch long and pale white in color. After about seven days, the egg hatches. The larvae is legless with a white body and a brown head capsule. It will go through at least four larval stages before development is complete. It is about a half an inch long when mature. If removed from the stem, the larvae assumes an S-shaped position. While developing, the larvae feeds on the inside part of the weed stem during the months of June and July. After the larva completes development, it moves downward in the stem and cuts a V-shaped notch about an inch above the soil surface. When coupled with the weight of the head may cause the stem to break, resulting in plant lodging. To determine if the lodging is a result of soft fly damage, split the stem lengthwise. The presence of dust-like debris means the lodging was due to the soft fly. After cutting the notch, the larvae plugs the stem with debris immediately below the notch. The resulting stub serves as an overwintering chamber. The soft fly overwinters in the larval stage. Next spring, diapause is broken and the larvae develops into a pupae for 20 to 30 days. Adults emerge from the pupa cocoon in mid to late June. The weed stem soft fly is only damaging in the larval stage. Adults do not harm the plant. The loss to the crop is due to the combination of soft fly larval feeding and lodging. Damage from the feeding inside the stem can reduce yields by 10 to 20%. Both the kernel number and weight is reduced, while protein percent drops slightly. The second type of loss results from the weeds lodging after the soft fly cuts a notch in the stem. Lodging rates vary depending on the subsequent weather conditions and losses can be extreme up to 100%. Once the stem is cut, high winds or thunderstorms will create excessive lodging. The soft fly only damages wheat. In North Dakota it attacks spring wheat, germ wheat, and winter wheat. The insect will deposit eggs and oats in barley, but in oats the larvae fails to develop. And in barley, the larvae seldom survives. Many crops are totally immune to soft fly, including sunflowers, corn, dry beans, flax, soft flour, canola, buckwheat, soybeans, cranberry, potato, pulse crops, and sugar beets. In the early 1950s, researchers developed a soft fly resistant variety of wheat. It was called rescue. Resistant varieties have a stem filled with pith, taking away the soft fly's habitat. The larva cannot survive to cut the stem. Since rescue, additional solid stem varieties have been released through the agricultural experiment stations of North Dakota, Montana, the USDA, and Canada. Examples of current varieties include Mott, Shoto, and AC Lillian. Post plant resistance using solid stem wheat varieties are the best pest management strategy for reducing soft fly damage. The newer solid stem varieties have comparable yield and grain quality to the hollow stem varieties. If a grower has consistent soft fly problems in certain fields, a resistant variety of spring wheat should be seeded or a non-host crop should be planted. If the grower does seed a non-resistant variety, damage is reduced if seeding is done after May 20th. But a late seeding may mean a reduction in yield. Weather conditions can influence the development of the pith in solid stem varieties. If the weather is cloudy and rainy during stem elongation, the stem may not completely fill with pith. Resistance to the soft fly is reduced. The most effective pest management strategy is to plant as early as possible to maximize yield of solid stem varieties. Several natural enemies attack wheat stem soft fly. These natural enemies can kill a high proportion of soft fly larvae, up to 80% in some years and locations where soft fly populations have been high. In North Dakota, the most important species in wheat is Braycon cephi, a parasitic wasp. Another species, Braycon lisogaster, attacks soft fly in more native grass habitats. Braycon cephi females are able to sense soft fly larvae feeding in the stem, and the females lay their eggs in the wheat stem near the soft fly larvae. Once the parasitic wasp larvae emerge, they begin feeding on the soft fly larvae and will kill them. Parasitic wasps have two generation per year. The first generation will cut a small circular hole in the stem when it emerges during mid-season. Parasitoids overwinter in the upper half of the wheat stem in cocoons. Parasitoids also are able to survive in solid stem wheat varieties, increasing the value of biological control as an integrated pest management strategy that is compatible with host plant resistance. Several cultural control practices used singly or in combination may help reduce or minimize wheat stem soft fly infestations. Swathing, tillage, delay planting, and crop rotation all have been recommended. Although each has an associated cost, swathing or using a stripper header are the only pest management practices that can be utilized in the current year of the infestation. Swathing sometimes is conducted on just the outer one or two swaths bordering the field if the infestation is heavy in the field edges only. Swathing prevents the soft fly larvae from cutting the stem and reduces yield losses due to lodging. The disadvantage of this technique are that it requires an additional field operation and swathing may adversely impact parasitic wasps that attack the soft fly larvae in the upper portions of the stems. If a producer decides to swath grain, use a high swathing height to conserve the parasitoids that attack wheat stem soft fly. Research from Montana State University has shown that taller residue, at least the lower one third of the plant is better for conserving the parasitoids. To determine if producers need to swath fields, sample wheat crops and determine the percent of plants infested by soft fly before harvest. The presence of wheat stem soft fly can be verified by splitting the stems and looking for the S-shaped larvae inside the stem. Another symptom of the soft fly feeding is the presence of the sawdust like frass inside the stem. If more than 15% of the stems are infested by soft flies, producers should swath or use a stripper header on the wheat crop. Producers should swath the soft fly infested wheat as soon as the kernel moisture drops below 40% to prevent infested stems from lodging. Stripper headers may be used for straight cutting the crop and improve parasitoid conservation by leaving the majority of the stem intact. Both fall and spring tillage have been used to expose overwintering soft fly larvae to cold and dry conditions to increase larval mortality. Tillage for soft fly control runs counter to the current, reduce no tillage recommendations. Current research has demonstrated that 10% larval survival in the field will lead to infestation levels as high as the previous season. Tillage practices will not cause great enough larval mortality to be effective. Also, tillage negatively impacts the parasitic wasp numbers. In a recent survey of tilled and no-till fields in Montana, 75% of the no-tilled fields had higher parasitoid numbers and less soft fly damage than the neighboring tilled fields. Swathing and tillage also add expenses such as fuel, tractor time, and labor cost. To laid planting after May 20th also has been suggested. A laid planted crop will not have reached the stem elongation stage when wheat stem soft fly females are ready to lay eggs, but lower yields are usually obtained because of the laid planting date. Also, calendar dates may not accurately reflect soft fly development. Wheat stem soft fly will not lay eggs into corn, legumes, or other broadleaf crops, so rotation with a non-host crop can reduce populations at least within a specific field. However, soft flies can fly considerable distances so reinfestation from nearby fields, grass borders, or conservation reserve program land is possible in subsequent years. Crop rotation also has disease and pest management and soil fertility benefits. Insecticides generally have not been effective against wheat stem soft fly. Possible reasons why insecticides are ineffective for pest management of wheat stem soft fly include 1. Emergence periods of adult is long, up to one month. 2. Short lifespan of the adult soft fly makes it difficult to target insecticide applications for adults to prevent overposition. 3. Adults spend little time feeding or inviting water, which may minimize oral exposure to insecticides. 4. Eggs larvae pupae are protected within the plant, making them inaccessible to foliar insecticides. In terms of economics, insecticides can often be too costly for production management practices and low value and large acreage crops. A recent field demonstration plot near Mott in southwestern North Dakota resulted in a net loss of $13.5 per acre when three applications of perrythritic insecticides were applied at the beginning, peak, and end of the wheat stem soft fly adult flight. Insecticides also disrupt and kill beneficial insects that naturally keep soft fly populations low. After more than 90 years, wheat stem soft fly continues to plague wheat producers in North Dakota and other wheat growing areas. Solid stem varieties provide the most reliable pest management strategy to prevent losses. Research at North Dakota State University continues to focus on the development of solid stem varieties with excellent yield potential and quality. Additional studies are planned to develop a degrading model for wheat stem soft fly and parasitoids and to continue evaluating alternative methods of integrated pest management. For more information on wheat stem soft fly, check out the following websites on your screen. Thank you and have a good day.