ISSUE 10 July 9, 1998
WHEAT MIDGE ACTIVITY REACHES TREATABLE LEVELS IN HOT SPOTS
Areas where overwintering populations of wheat midge larvae exceeded 500 larvae per square meter are experiencing threshold level numbers of adults. Evening scouting reports have indicated numbers of adults exceeding 1 midge per four wheat heads. Highlights of the July 7 reports are:
Northeast region - Cavalier County: midge numbers increasing; wheat on wheat fields reaching threshold numbers (1 per 2 to 1 per 5 wheat heads).
Nelson County: economic levels found throughout county. Towner County: midge at 1 per head found west of Cando.
Central region - Benson County: threshold levels observed near the Maddock area.
North Central region - Burke County: large numbers of midge found on sticky traps in fields, scouting is necessary to determine number on a per head basis. Bottineau County: In the Newburg area, economic levels in wheat on wheat fields. In western areas of county, midge counts in the 1 per 8 to 1per 20 wheat heads (more common).
Degree days (DD) are accumulating rapidly with the current warm weather, averaging 25 to 30 per day. Counties north and west of Foster county are still less than the 1600 DD and flights would be expected to continue for another 4 to 5 days. Areas still at risk, based on degree days include Cavalier, Towner, Rollette, Bottineau, Renville, and Burke Counties. Continue to scout for wheat midge in areas where degree days have just reached the 1600 level. Previous experiences have found that localized flushes of activity by wheat midge after 1600 DD do occur. However, the general experience is that midge numbers decline on a region wide basis when 1600 DD are reached.
In areas to the south and east, where degree days have reached 1700 to 1800, it is time to inspect developing wheat heads for the possible presence of the orange colored wheat midge maggots. They can be found feeding on the developing kernels. There is outward change in the appearance of the wheat head when it is infested by midge. The only way to determine if they are present is to rub the heads to dislodge the maggots, or pull back the glumes to expose the kernel.
CORN BORER SCOUTING IMPORTANT
Corn borer moth activity continues to increase around the eastern areas of the state. Moth captures in Richland County have had increasing moth captures during the past 7 days (40 to 50 moths per night). Field inspections in the area of these traps have found low number of egg masses present (less than 5% of the plants) and very few plants with shotholing of the leaves by smalllarvae. However, moths were detected in grassy margins around fields and occasional moths were detected in the fields.
Scouting to determine levels of egg laying, hatching time, and larval age will be critical during the next two weeks.
SUNFLOWER MIDGE FLIGHT UNDERWAY
Sunflower midge adults and larvae have been found on volunteer sunflowers in the Mapleton area, 15 miles west of Fargo. Sunflowers at risk will be in the bud stage. Unfortunately, there still is no responsive measures we can recommend that would provide consistent, economic control. Inspections of flowers in the next couple of weeks to determine if midge are present in the developing buds.
The adult sunflower midge, Contarinia schulzi Gagne!, is a tan-colored delicate fly 2 to 3 mm long which is a pest in cultivated sunflower. Females are slightly larger and more robust than the males and have a long telescoping ovipositor.
Larvae reach 2 mm in length, are creamy-white in color and appear somewhat flattened. Larvae often feed in clusters underneath the bracts surrounding the sunflower head and also in the center of the developing sunflower bud.
Adults from the overwintering generation emerge from the soil in early to late July, move to sunflower, and oviposit first generation eggs. Some larvae of the first generation complete development and emerge as adults from mid August to early September to produce a partial second generation. Each adult lives one to two days. Adult emergence occurs in abrupt peaks, and there may be multiple emergence peaks during these intervals. The relationship among midge emergence, rainfall, and soil moisture is not clear. However, it does seem that wet soil conditions favor larger emergence.
Eggs are usually laid in clusters on developing sunflower buds, either underneath bracts or in the center of the bud. Females prefer laying eggs on buds 1 to 2 inches in diameter but will oviposit on buds and heads of any size, as well as in leaf axils. On the second day, eggs hatch and larvae crawl to the base of the bracts or into the center of the bud to begin feeding. Larvae develop in the bud for 7 to 10 days after which they drop to the ground. Larvae complete development in the soil for a second generation or they overwinter in the soil where they form cocoons, to pupate and emerge the following summer. The impact of the partial second generation in mid August is thought to be minimal.
The abnormal shape and size of midge infested sunflower heads is a result of larval feeding activity. If damage is confined to scarring at the base of the bracts, there will be little if any economic loss. Damage becomes measurable when there is a crease in the face of the head or the petiole is noticeably thickened. More severe damage includes a reduction in head size, head folding, development of a central dead area or hole, and lack of seed production. There can be large variation in extent of damage from sunflower head to head within a field.
If midge damage is restricted to field margins or small portions of fields, economic losses for the field will be minimal. When midge populations are very heavy, damage will extend into the field and substantial economic losses can occur.
The sunflower midge can be found on wild sunflower throughout the northern Great Plains and south to Texas. It is most abundant in the areas of southern Manitoba, Minnesota, North Dakota, and South Dakota where cultivated sunflower has been grown for the longest period of time, particularly in the Red River Valley of the North. Sunflower midge can be found throughout much of the Red River Valley each year, yet the areas where populations are high enough to cause yield loss to sunflower can move annually.
In the past, midge distribution and abundance have been associated primarily with heavy soil types and high soil moisture conditions. These conditions allow more larval midges to survive the winter and permit a larger portion of soil inhabiting larvae to emerge the next summer than under drier conditions. However, midge damage is not restricted to areas of heavy soils or to wet, rainy seasons. Most recently, the range of the sunflower midge has expanded west, out of the Red River Valley.
Sunflower Midge Management
At present, the sunflower midge cannot be controlled by conventional insecticidal approaches. Larvae crawl into the bud soon after hatching and are protected from topically applied insecticides. Adult midge are much more exposed in the environment, but since they are present for only a day or two there is usually not enough time after observing them to initiate controls before they lay eggs. Sampling for midge is difficult because both adults and eggs are very small and present for such a short time. Because the plant is susceptible throughout the bud stage, and because there can be several emergence events of adult midges, correct timing of insecticides for adult midge control is very difficult.
Both foliar and systemic insecticides have been used in attempts to control the midge. Neither has given consistent, adequate control of adults or larvae. Multiple applications of insecticides have also proven inadequate and not economical. At this time, it is known that chemical insecticides do NOT consistently reduce losses, making it difficult to justify the cost of application.