Red and Gray Seed Weevils Adult red seed weevil This is the most common damaging insect group on flowering sunflower. The red seed weevil is the more common of the two weevils. Both are easy to scout, identify and control. The red weevil adults are 0.1 to 0.12 inch long and the gray weevils are slightly larger. Both weevils have pronounced ‘snouts’ that make them recognizable. Life Cycle: Weevils usually emerge in late June and early July. Because of the late season in 2009 weevils emerged in later July. Once the plant is flowering and making pollen, the weevils feed on the pollen for several days before laying eggs in the developing seeds. The eggs hatch and the larvae consume part of the developing kernel. The larvae then burrow out of the seed and drop to the ground and prepare for overwintering. Damage: The red seed weevil female lays eggs in as many as 27 seeds. The hatched larvae consume a portion of the kernel before dropping out of the seed. The gray seed weevil female lays fewer eggs but lays them earlier in the R-3 or 4 stage. The gray seed weevil larvae generally consume the majority of the kernel before dropping out. The infested gray weevil seeds are generally blown out of the combine. However, that is not the case of for the red weevil. Damaged confection seeds are discounted and most contracts have a minimum percent of infestation allowable. This is not the case for oil-type sunflower; however, it is likely that the farmer is losing test weight and yield. Economic Thresholds: For confection sunflower one weevil per head is the economic threshold. NDSU has developed a formula which considers price, cost of treatment and plant population. The table below provides a guide for spraying threshold. (A PDF of the table is also available below in the "Additional Documents" section.) Economic Threshold (weevils per head) for Red Sunflower Seed Weevil Column headers represent Sunflower Plants per Acre (x 1000) COST OF SPRAY PER ACRE = $8 Market Price $/lb. 17 18 19 20 21 22 23 24 25 .10 7 6 6 6 6 6 5 5 5 .11 6 6 6 5 5 5 5 5 5 .12 6 5 5 5 5 5 5 4 4 .13 5 5 5 5 4 4 4 4 4 .14 5 5 4 4 4 4 4 4 4 .15 4 4 4 4 4 4 4 4 3 .16 4 4 4 4 4 4 3 3 3 .17 4 4 4 4 3 3 3 3 3 .18 4 4 3 3 3 3 3 3 3 .19 4 3 3 3 3 3 3 3 3 .20 3 3 3 3 3 3 3 3 3 .21 3 3 3 3 3 3 3 3 2 .22 3 3 3 3 3 3 2 2 2 .23 3 3 3 3 3 2 2 2 2 .24 3 3 3 3 2 2 2 2 2 .25 3 3 2 2 2 2 2 2 2 COST OF SPRAY PER ACRE = $10 Market Price $/lb. 17 18 19 20 21 22 23 24 25 .10 8 8 8 8 7 7 7 7 6 .11 8 7 7 7 7 6 6 6 6 .12 7 7 6 6 6 6 6 5 5 .13 6 6 6 6 6 5 5 5 5 .14 6 6 6 5 5 5 5 5 5 .15 6 5 5 5 5 5 5 4 4 .16 5 5 5 5 5 4 4 4 4 .17 5 5 5 4 4 4 4 4 4 .18 5 4 4 4 4 4 4 4 4 .19 4 4 4 4 4 4 4 3 3 .20 4 4 4 4 4 4 3 3 3 .21 4 4 4 4 3 3 3 3 3 .22 4 4 4 3 3 3 3 3 3 .23 4 4 3 3 3 3 3 3 3 .24 3 3 3 3 3 3 3 3 3 .25 3 3 3 3 3 3 3 3 3 COST OF SPRAY PER ACRE = $12 Market Price $/lb. 17 18 19 20 21 22 23 24 25 .10 10 10 9 9 9 8 8 8 8 .11 9 9 8 8 8 8 7 7 7 .12 8 8 8 8 7 7 7 7 6 .13 8 7 7 7 7 6 6 6 6 .14 7 7 7 6 6 6 6 6 5 .15 7 6 6 6 6 6 5 5 5 .16 6 6 6 6 5 5 5 5 5 .17 6 6 5 5 5 5 5 5 4 .18 6 5 5 5 5 5 5 4 4 .19 5 5 5 5 5 4 4 4 4 .20 5 5 5 5 4 4 4 4 4 .21 5 5 4 4 4 4 4 4 4 .22 5 4 4 4 4 4 4 4 3 .23 4 4 4 4 4 4 4 3 3 .24 4 4 4 4 4 4 3 3 3 .25 4 4 4 4 3 3 3 3 3 Scouting Method: Like most insects, field borders generally will have higher weevil numbers so it is important to move deeper into the field. Scouting should begin in the R-3 and R-4 (bud) stages and continue to R-5.7. At the bud stage the weevils tend to feed and hide among the bracts. An easy way to get them to emerge is spray a mosquito repellent containing Deet on the face of the heads. The same can be used when the buds have opened. Management: There are many effective insecticides labeled for the adult weevils. Since the red seed weevil adults need to consume pollen before laying eggs, spraying at early pollen shed when about 30% of the plants are at R-5.1 is recommended. It is recommended that confections be sprayed even earlier and two sprays may be warranted depending on populations. The timing of control of the red seed weevil is well suited for controlling both the banded and the sunflower moth. For the gray weevil spraying at early bud stage R-2 is recommended. However, gray weevil populations have historically been low and no control has been required. Common Mistakes: Waiting too long to scout and take control measures are historically the most common mistake with the red seed weevil. Research: The goal of research programs partially funded by the NSA is to develop hybrids that are resistant to the seed weevil complex. USDA ARS Sunflower Unit in conjunction with NDSU and SDSU have been field screening currently available sunflower accessions, interspecific crosses, and lines for those having reduced seed damage from larval feeding by the red sunflower seed weevil. The discovery of germplasm that has lower insect damage can provide the seed companies with breeding material to be incorporated into hybrids targeted to locations where specific insect problems occur. A long-term goal is to identify germplasm with resistance or tolerance to more than one insect pest. The NSA has provided a post-doctoral scientist to the USDA ARS Sunflower Unit for 2008-11 to accelerate this work. For further information, click on the links below (please note the files may take a while to download). Another resource about insects can be found in the Archive section of The Sunflower magazine. Additional Links NSA Crop Survey The Sunflower Crop Survey is conducted annually prior to harvest to gather data on yield and production practices, weeds, insects, diseases and bird damage. Additional Documents Economic Threshold for Red SF Seed Weevil (60 kb .pdf file) Download Adobe Acrobat Reader Table showing 2009 economic threshold (weevils per head) for red sunflower seed weevil by cost of spray and market price. High Plains Sunflower Production Handbook (1554 kb .pdf file) Download Adobe Acrobat Reader NDSU Extension Bulletin 25 - Revised 9/2007 (5592 kb .pdf file) Download Adobe Acrobat Reader
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