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Midge and Seed Weevil Still Merit Vigilance

Saturday, March 1, 1997
filed under: Insects

Two old foes of sunflower merit careful attention from producers in 1997:

• The sunflower midge has resurfaced in a number of eastern North Dakota and western Minnesota fields within recent years and may cause problems again this season.

• While the red sunflower seed weevil has not been a major pest the past few seasons, it is too soon to consider the seed weevil gone for good.

Sunflower Midge — The sunflower midge caused considerable consternation in the Red River Valley of North Dakota, Minnesota and Manitoba during the early 1980s; but then faded as that area’s sunflower acreage declined. The return of the midge within recent years is probably due to an increase in Red River Valley sunflower acreage coupled with environmental conditions conducive to a large emergence of the overwintering population. (While those “conducive conditions” are not clearly understood, midge populations seem to be higher when when there are wet soils during late spring/early summer.)

Given its 1996 population, there should be plenty of midge waiting in the soil for conditions which favor their emergence. Unless those emergence requirements are not met, we should again expect the midge to reach damaging levels at some affected locations in 1997.

Past efforts to control the midge with insecticides have failed. Systemic insecticides didn’t work; nor did various tested rates, methods and timing of several foliar insecticides. Because midge eggs and larvae are hidden between the bracts and developing seeds, foliar insecticides probably don’t reach them. With systemics, the activity likely does not last long enough to protect the plants until late bud — which is the most susceptible stage and the one during which oviposition occurs.

While the adult midge also can be targeted for insecticidal control, the adults’ brief life-span requires that the spray be timed very accurately. At this point, we just don’t know enough about midge biology and flight patterns to achieve this critical timeliness.

Late planting may sufficiently delay sunflower development so that plants are not yet in the susceptible bud stage during peak midge populations. However, a late or second flush of midge may occur and coincide with the susceptible stage of a late-planted crop. So while late planting usually will minimize midge damage, it’s not guaranteed — and, as well, might not fit with the producer’s other agronomic or plant protection needs.

There is genetic resistance in sunflower to midge, but inherent testing difficulties have blocked its development. Screening breeding lines for resistance requires field testing and reliance upon natural midge populations to infest the test material. However, midge populations are not consistent from year to year or location to location, so there’s no way to ensure an adequate midge population for a given test.

A number of sunflower hybrids were evaluated for midge damage at three Red River Valley locations in 1996. Not all of the hybrids were planted at each location, but midge populations were high enough at each site to allow for damage ratings. Nine of the tested hybrids recorded either “low” or “very low” damage ratings at the sites.

Red Sunflower Seed Weevil — Until about 1993, the red seed weevil was the single most important insect pest of sunflower in the Dakotas and Minnesota. Fortunately, the weevil has not been a serious problem since then; but it could be poised for a comeback in some areas. Growers treated for seed weevil in several South Dakota locales in 1996, for example; plus, there were some extremely high seed weevil counts in portions of North Dakota’s Richland County (southeastern corner of the state).

Several new management options for the red sunflower seed weevil have been developed and are incorporated into a new NDSU Extension Service bulletin. These updates include a revision of the economic injury level, the incorporation of a sequential sampling program, and information about trap cropping.

The economic threshold has been revised to include not only yield loss, but also oil loss due to feeding by weevil larvae. New data also indicate that female weevils lay more eggs than previously thought. As a result, damage per weevil is higher and fewer weevils are needed to reach the economic threshold. Under the old formula (assuming an insecticide treatment cost of $8.00 per acre, sunflower at 12 cents per pound, and a per-acre plant population of 20,000), the economic threshold for oil-type sunflower was 12 weevils per plant. With the revised formula, that economic threshold is five (5) weevils per plant.

In terms of sampling method, the traditional seed weevil sampling was based on a fixed sample population size. The new sequential plan does not list a fixed sample size, but instead is designed to provide a rapid estimate of the weevil population. This more-accurate sequential method complements the revised economic threshold.

Though the concept of trap cropping is not new to seed weevil management, it can be a very valuable management practice. The working premise behind trap cropping is the red seed weevil’s strong attraction to blooming sunflower. This powerful attraction is used to concentrate the weevils within a small area of the field, i.e., the trap rows. If the weevils in the trap rows build into an economic population, they can be controlled by applying insecticides to only the trap rows, thereby greatly reducing the cost of insecticide treatment.

(Details on new economic thresholds, sequential sampling and trap cropping can be found in Sunflower Seed Weevil Management, NDSU Extension Service Bulletin E-817, available later this spring.)

Outlook for 1997 — Unless conditions are unfavorable for its development, the sunflower midge probably will produce damaging populations in localized areas in or near the Red River Valley. Growers who experienced midge problems in 1996 should consider late planting and/or planting of one of the hybrids which tested “low” or “very low” for midge damage in last season’s testing. (Producers can contact their county extension office or seed supplier for details on which hybrids have exhibited tolerance to midge.)

Monitoring for red sunflower seed weevil populations should be carefully conducted in 1997 in case economic levels do develop. Decisions regarding treatment should incorporate the new pest management tactics. If planting sunflower in an area where red seed weevil control was required last year, planting with a trap crop design is a recommended practice.
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